Title:
Attic Insulation with Desiccant
Kind Code:
A1


Abstract:
A method of reducing the amount of cooling energy required to cool a building is provided. The method includes disposing a porous insulating material substantially covering the ceiling in the attic space of the building to a substantial depth. The porous insulating material includes a desiccant. The method further includes permitting the desiccant-bearing porous insulating material to adsorb water moisture from the attic space and then permitting the adsorbed water moisture to desorb from the desiccant-bearing porous insulating material into the enclosed room of the building, whereby the temperature of the desiccant-bearing porous insulating material is reduced, resulting in a reduction in the amount of cooling energy required to cool the building.



Inventors:
Toas, Murray S. (Norristown, PA, US)
Application Number:
11/694220
Publication Date:
10/02/2008
Filing Date:
03/30/2007
Assignee:
CertainTeed Corporation
Primary Class:
Other Classes:
52/95, 52/407.3, 62/259.1
International Classes:
E04D13/16
View Patent Images:
Related US Applications:
20050126100Seamsaver for drywallJune, 2005Dienner et al.
20060213131Roof clipSeptember, 2006Butzer
20070283661Engineered structural boardDecember, 2007Daniels
20080105387MOVABLE PARTITIONS, COMPONENTS FOR MOVABLE PARTITIONS AND RELATED METHODSMay, 2008Coleman et al.
20060277863Hopboard roof platformDecember, 2006Henning Sr. et al.
20080163808Room Arrangement, Ship, Building and Method for Constructing a Room ArrangementJuly, 2008Kordelin
20050247000Interlocking self-aligning cladding panel for floors, walls, ceilings, or the likeNovember, 2005Zhu
20070180780Roof anchoring systemAugust, 2007Foglia
20070101667Storm panel bracket systemMay, 2007Sweet Jr. et al.
20080034703Bracket for joining spanning membersFebruary, 2008Sensenig
20090031669Method for marketing reclaimed wood as flooringFebruary, 2009Stanton



Primary Examiner:
AHMAD, CHARISSA L
Attorney, Agent or Firm:
DUANE MORRIS, LLP;IP DEPARTMENT (30 SOUTH 17TH STREET, PHILADELPHIA, PA, 19103-4196, US)
Claims:
What is claimed is:

1. A method of reducing the amount of cooling energy required to cool a building, said building having an enclosed room partially defined by an outer wall, a horizontal upper wall plate, and an attic space disposed above said upper wall plate, said attic space defined by a ceiling of said room and a roof of said building; said method comprising: a) disposing a porous insulating material substantially covering the ceiling in said attic space to a substantial depth, said porous insulating material including a desiccant; b) permitting said desiccant-bearing porous insulating material to adsorb water moisture from said attic space; and c) permitting said adsorbed water moisture to desorb from said desiccant-bearing porous insulating material into said enclosed room, whereby the temperature of said desiccant-bearing porous insulating material is reduced resulting in a reduction in the amount of cooling energy required to cool said building.

2. The method of claim 1 wherein said porous insulating material comprises inorganic or cellulosic fibers.

3. The method of claim 1 wherein said permitting step (c) comprises desorbing said adsorbed water moisture from said desiccant via convective mass transfer.

4. The method of claim 1 wherein said building is located in a cooling dominated climate.

5. The method of claim 1 wherein said attic space has a relative humidity which is higher than the relative humidity of said room.

6. The method of claim 1 wherein said desiccant comprises a silica gel.

7. The method of claim 6 wherein said silica gel is disposed within an aqueous slurry precursor of said desiccant.

8. The method of claim 6 wherein said silica gel is disposed as a dry powder.

9. The method of claim 1 wherein said building experiences an exterior temperature greater than 72° F.

10. The method of claim 1 wherein said building comprises spaced-apart attic floor joists disposed above said upper wall plate, and spaced-apart roof rafters disposed below and supporting said roof of said building.

11. The method of claim 1 wherein said desiccant-bearing porous insulating material is disposed at least between said attic floor joists.

12. The method of claim 1 wherein said permitting step (b) reduces a relative humidity level in said attic space.

13. The method of claim 1 wherein said outer wall is defined by a plurality of substantially parallel studs, an outer sheathing, and an inner drywall layer, said desiccant-bearing porous insulating material being further disposed between said outer sheathing and said inner drywall layer.

14. The method of claim 1 wherein said building is air-conditioned.

15. A building insulating material comprising: a) randomly oriented fibers made of a cellulosic or inorganic composition disposed in a substantial thickness to provide insulation for a building, and b) a desiccant in an amount sufficient to enable said insulating material to adsorb enough water moisture to reduce the temperature of said insulating material by at least 1° F.

17. The building insulating material of claim 1 wherein said desiccant consists of one or more of the following ingredients: montmorillonite clay, silica gel, synthetic zeolite (molecular sieve), calcium oxide (CaO), calcium sulfate (CaSO4), carbon molecular sieve, activated alumina, or activated carbon, or sodium polyacrylate.

18. The building insulating material of claim 15 wherein said desiccant comprises at least 5 volume % based upon the volume of said randomly oriented fibers.

19. The building insulating material of claim 15 wherein said cellulosic composition comprises recycled paper fibers and said inorganic fibers comprise rotary glass fibers or textile glass fibers.

20. The building insulating material of claim 15 wherein said desiccant comprises a uniform mixture of silica gel.

21. A method of manufacturing an insulating material comprising: a) forming glass fibers from molten streams of glass; b) combining the glass fibers with a desiccant; and c) consolidating the fibers and desiccant onto a conveyor.

22. The method of claim 21 wherein said combining step (b) combines said desiccant with said glass fibers when said glass fibers are still hot.

23. The method of claim 21 wherein said combining step (b) disposes said desiccant in a dry powderous form by blowing said dry powderous desiccant into an upper area of a forming section of a rotary glass plant.

24. The method of claim 21 wherein said combining step (b) comprises adding said desiccant to a water slurry and spraying said water slurry containing said desiccant onto a plurality of hot mineral fibers in a forming section of a rotary glass plant.

25. The method of manufacturing an insulation material comprising: a) forming cellulosic fibers from a paper source; and b) admixing a fire retardant and desiccant onto said cellulosic fibers.

26. The method of claim 25 wherein said forming step (a) comprises forming said cellulosic fibers from a recycled paper source.

27. The method of claim 25 wherein said admixing step (b) comprises spraying a solution containing said fire retardant and said desiccant onto said cellulosic fibers.

28. A method of making an insulation material comprising: a) forming cellulosic fibers from a paper source; b) admixing a fire retardant with said cellulosic fibers; and c) adding a desiccant to said fire retardant-containing cellulosic fibers in a blowing machine hopper before said fire retardant-containing cellulosic fibers are blown into an attic cavity, wall cavity or crawl space of a building.

Description:

FIELD OF THE INVENTION

The present invention relates to insulation products, such as batt and loose-fill insulation, and board products such as duct liner and duct boards, having better thermal properties for cooling dominated climates.

BACKGROUND OF THE INVENTION

Most residential construction includes an attic space between the ceiling and the roof deck. The structure that supports the roof and provides the ceiling plane is often constructed with pre-assembled wood trusses. The structure can also be built on-site using traditional ceiling joists and roof rafters. Properly insulating the attic is essential to reducing home energy consumption (“building load”). Thermally isolating the attic from the rest of the house also increases the comfort of the living space below in both winter and summer.

Attic ventilation serves two purposes: prevention of moisture condensation in the winter and attic cooling in the summer. Ventilation during the heating season removes moisture-laden air from the attic before it condenses. Summer-time venting allows cooler air to flush heat out of the attic space. Typically, a good ventilation system has 50 percent of the required ventilation area high on the roof and 50 percent in the eave area. With properly spaced vents, the attic will have good circulation. Batt insulation is often a good bet for long-term thermal performance. Loose-fill insulation may also be used, including cellulosic and glass fiber loose-fill insulation. Loose-fill insulation should be installed at the same thickness throughout the attic. Voids and low spots behind framing should be eliminated.

Despite the many attempts to properly insulate attics, the attic space of most buildings is perceived as a source of a nuisance. In winter, moisture condensation on the attic ceiling encourages mildew growth. In summer, the heat build-up in the attic space increases the cooling load. Generally during the night, the high attic air relative humidity is caused by an air exchange with the outdoor environment. The wood framing materials, generally having a lower air relative humidity at the surface, adsorb moisture. During the daytime the attic air relative humidity is reduced due to the heat gain caused by solar heat. There is a higher air or relative humidity at the surface of the wood framing materials which results in moisture being desorbed by wood attic framing materials. The attic space during the daytime will typically be elevated above the outdoor environment.

In modern residences, the challenge of achieving a continuous air infiltration barrier and thermal insulation barrier at the interior ceiling level is especially difficult. The air barrier, used to isolate the living space from the attic, is usually the taped drywall ceiling, while the thermal barrier is the insulation placed on top of the drywall. Typically, the ceiling is not a single horizontal plane, but a series of horizontal planes, vertical planes (knee walls), and sloped planes, all intersecting to create the ceiling.

Current building codes across the United States require attic ventilation. Lstiburek, “Vented and Sealed Attics in Hot Climates” ASHRAE SYMPOSIUM ON ATTICS AND CATHEDRAL CEILINGS, TORONTO, (JUNE, 1997). In cold climates, the primary purpose of the attic ventilation is to maintain a cold roof temperature to avoid ice damage created by melting snow, and to vent moisture that moves from the conditioned space to the attic. Id., p. 3 Melted snow, in this case, is caused by heat loss from the conditioned space. When water from melted snow runs out over the unheated eave portion of the house, it freezes and expands, often driving its way back up the roof and between shingles. Id.

In hot climates, the primary purpose of attic ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. Id., p. 4 Roof shingle temperatures will be higher during no-wind conditions leading to a higher heat load on the attic. Id. Therefore, the greatest need for attic ventilation is when there is a little wind pressure to force air in and out of the attic.

Building heating and cooling loads from roofs for all single-family detached buildings were measured to be 446 trillion BTUs for heating and 128 trillion BTUs for cooling in the U.S. in 1999. Y. J. Huang et al. “Residential Heating and Cooling Loads Components Analysis,” LBNL-44636, Lawrence Berkeley National Laboratory, Berkley, Calif., (1999). Compared to the net heating and cooling loads at 2,958,814 trillion BTUs, the percent of roof loads are 15.1 percent and 15.8 percent, respectively. Therefore, roof load reduction can greatly reduce the total building loads. Although there are many ways to reduce roof loads, the most common way is to add more roof insulation. Due to limited attic space, adding more roof insulation may not always be a feasible way to reduce the total building loads in many instances.

Accordingly, there remains a need to reduce roof loads, thereby building loads by alternative means.

SUMMARY OF THE INVENTION

A method of reducing the amount of cooling energy required to cool a building is provided. The building generally includes an enclosed room partially defined by an outer wall, a horizontal upper wall plate, and an attic space exposed above the upper wall plate. The attic space is defined by a ceiling of the room and a roof of the building. The method includes the steps of (1) disposing a porous insulating material substantially covering the ceiling in said attic space to a substantial depth, the porous insulating material including a desiccant, (2) permitting the desiccant-bearing fibrous insulating material to adsorb water moisture from the attic space, and (3) permitting the adsorbed water moisture to desorb from the desiccant-bearing fibrous insulating material into the enclosed room, whereby the temperature of the desiccant-bearing fibrous insulating material is reduced resulting in a reduction in the amount of cooling energy required to cool the building.

In the more preferred embodiments of the present invention, the desiccant is a silica gel which is added to loose-fill or batt insulation used in attics in climates dominated by a cooling. As a result of the heat energy in the attic expended in evaporating the moisture in the silica gel in the insulation in the summer cooling season, the heat flow from the attic into the living space is reduced. The simulated, calculated net energy savings in a 1500 square-foot house in Miami with a 20 percent silica gel content (by volume) in R30 insulation can potentially reduce net annual energy costs by about $50.00.

The silica gel, or other desiccant, such as montmorillonite clay, synthetic zeolite (molecular sieve), calcium oxide (CaO), calcium sulfate (CaSO4), carbon molecular sieve, activated alumina, or activated carbon, or sodium polyacrylate, for example, may be added to the preferred mineral fiber or cellulose insulation during the manufacturing process. For blanket mineral fiber insulation, the silica gel, for example, may be added to the forming section as a dry powder blown into the upper area of the forming section of a rotary glass plant (similar to the manner that admix materials are typically added today) and/or mixed with the mineral fibers and binder in the forming section as an applied coating or ingredient, for example. The silica gel in a water slurry form may also be added to mineral fiber insulation in the forming section by spraying the slurry onto the hot mineral fibers. For mineral fiber loose-fill insulation, the silica gel may be added as a dry powder to the mineral fibers in the forming section or to the mineral fibers in the blowing machine hopper before the insulation is blown into the attic or wall. For cellulose insulation, the silica gel may be added to the cellulose insulation in the manufacturing process at the time the fire retardant, for example, is added to the shredded cellulose or to the mineral fibers in the blowing machine hopper before the insulation is blown into the attic.

In computer simulation models, it was determined that negative heat flux flowing from the living space into the attic increases due to higher thermal conductivity of the present invention's desiccant insulation mixture containing fiberglass and silica gel in the winter climate. Additionally, negative moisture fluxes flowing from the living space into the attic were reduced to a very small amount due to the high level of moisture in the desiccant insulation mixture in the winter. These results were compared to a fiberglass only insulation control in the attic space during the same climate conditions. In the summer months, the fiberglass insulation with silica gel resulted in a reduced positive heat flux flowing from the attic into the living area, in the model, since heat energy in the attic was being used to evaporate moisture from the desiccant. It also resulted in an increase in the positive moisture flux flowing from the attic into the living area due to extra moisture from evaporation from the desiccant into the living area. In the cooling dominated climate of Miami, the addition of desiccant to the attic insulation reduced the total roof energy load in the computer models of this invention. In heated dominated climates like Baltimore, Minneapolis, and San Francisco, the addition of desiccant to the insulation increased, rather than decreased, the total roof energy load in computer simulations.

Accordingly, the present invention is designed to assist “cooling dominated climates,” such as Miami, where the average monthly temperature is above 45° F. year round, as opposed to “heat dominated climates,” which typically experience between 4500 and 8000 heating degree days. In heated dominated climates, the dry air is outside and the moist air is inside. In the Southern United States where most cooling dominated climate constructions are located, the conditions are exactly the opposite. The air conditioners create dry air inside the dwelling, while the outside air is very humid most of the year.

In further embodiments of the present invention, a building insulation material is provided which includes randomly oriented fibers made of a cellulosic or inorganic composition disposed in a substantial thickness to provide insulation for a building, and a desiccant in an amount sufficient to enable said insulating material to absorb enough water moisture to reduce the temperature of said insulating material by at least 1° F.

In a further embodiment of the present invention, a method of manufacturing an insulating material is provided which includes the steps of forming glass fibers from molten streams of glass, combining glass fibers with a desiccant, and consolidating the fibers and desiccant on a conveyor.

In still a further embodiment of the present invention, a method of manufacturing cellulosic insulation material is provided which includes the step of forming cellulosic fibers from a paper source and admixing a fire retardant and desiccant onto the cellulosic fibers.

The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the disclosure, in which:

FIG. 1 is a partial cross-sectional side elevational view of an eave area of a building; and

FIG. 2 is a bar chart illustrating the reduced (or additional) annual electricity expense in U.S. dollars for a 10% and 20% mixing ratio of silica gel by volume in cellulosic ceiling insulation in four locations in the United States, developed through computer modeling.

DETAILED DESCRIPTION OF THE INVENTION

One of several alternative methods, proposed by this invention, is to change roof insulation characteristics by mixing desiccant material with a porous insulation material, such as fiber glass or cellulose insulation. In general, there are two main advantages to this process. The first is to add more thermal mass to the ceiling insulation when the desiccant adsorbs moisture which can lead to possible load shifting from peak to off-peak hours. The second is to reduce attic air moisture levels. The disadvantage with this method of adding a desiccant is that the thermal resistance of the insulation decreases somewhat due to the higher thermal conductivity of the desiccant compared to fiber glass or other insulation.

In addition to the general advantages and disadvantages of the desiccant insulation mixture mentioned above, one of the most important features of using the ceiling mixture is to add more moisture adsorption capacity. The moisture adsorption from the attic and desorption to the living space from the convective mass transfer at the ceiling surface can change the temperature of the ceiling insulation. Therefore, building energy loads can be changed due to the temperature change of the ceiling insulation, resulting in a change in the amount of heat that flows between the attic and the living space. The change may have a benefit in reducing cooling energy and conversely requiring more heating energy.

The desiccant can be added in amounts of at least about 5% v/v, preferably at least about 10-20% v/v (by volume of insulation, such as glass fiber or cellulosic insulation). By adding desiccant, such as silica gel for example, into the ceiling cellulose insulation, thermal conductivity increases 31% and 62% for the mixtures, with mixing ratios by volume of 10% and 20% desiccant, compared to ceiling insulation containing only cellulose, so that ceiling heat fluxes through the insulation increase due to the increased thermal conductivity. However, since silica gel is able to absorb a large amount of moisture from its surroundings and store it, the moisture level in the mixture insulation increases. When the moisture level in the attic space is lower than indoor living area humidity levels, the moisture flows from the indoor living area to the attic space. This is generally true in winter with an attic insulated with cellulose only. When the silica gel moisture adsorber is added to the insulation in the attic, the moisture primarily adsorbs and desorbs from the same place, the attic zone. Moisture flux from the attic into the living room space is reduced to a very low level in winter, due to the increased moisture level in the mixture. Therefore, moisture flow in an attic with a silica gel-cellulose mixture insulation does not interfere with the direction of the heat transfer, so that heating energy use increases due to the increased thermal conductivity of the silica gel-cellulose mixture ceiling insulation.

When the moisture level in the attic is higher than the indoor humidity level, which is generally true in summer, moisture evaporates from the silica gel-cellulose mixture insulation absorbed from the relatively wet attic space and is released and flows into the living zone. The moisture evaporation absorbs heat and causes the ceiling temperature in the attic insulated with the silica gel moisture to be lower compared to the cellulose only attic. As long as the amount of heat flux flowing from the attic into the living space reduced by the moisture evaporation is larger than the amount of heat flow increased by the insulation mixture's increased thermal conductivity, the total heat flux flowing into the living space from the mixture ceiling insulation is less than the cellulose insulation. Therefore, cooling energy use is expected to decrease with the silica gel-cellulose mixture insulation.

As shown in the figures, and particularly FIGS. 1 and 2 thereof, this invention is directed to a method of reducing the amount of cooling energy required to cool a building 100. The building 100 includes an enclosed room 24 partially defined by an outer wall 11, a horizontal upper wall plate 12, and an attic space 25 disposed above the upper wall plate 12. The attic space 25 is defined by a ceiling 20 of the room 24 and a roof of the building 100. The method includes disposing a porous insulating material 10, preferably not a closed cell insulation structure like extruded polystyrene foam boards or spray polyurethane foam, substantially covering the ceiling 20 in the attic space 25 to a substantial depth. The porous insulating material 10 includes a desiccant. The method also includes permitting the desiccant-bearing porous insulating material 10 to adsorb water moisture from the attic space 25. The next step in the preferred method includes permitting the adsorbed water moisture to desorb from the desiccant-bearing porous insulating material 10 and into the enclosed room 24. By this process, the temperature of the desiccant-bearing porous insulating material 10 is reduced resulting in a reduction in the amount of cooling energy required to cool the building 100. As in typical constructions, the building 100 of FIG. 1 includes an eave having a soffit 22 and an attic air vent 18 for permitting air flow 15 from the soffit up to the attic 25, and preferably out a vent fan or roof vent (not shown). The flowing air 15 travels through a space defined by the attic vent 18 and a pair of rafters 17 which support the roof.

In the preferred embodiments of this invention, the porous insulating material 10 preferably comprises inorganic or cellulosic fibers. Typical inorganic fibers include mineral wool or rotary glass fibers or textile glass fibers, and typical cellulosic fibers include recycled paper fibers which are treated with a fire-resistant additive prior to use.

In the preferred methods of this invention, water moisture desorbs from the desiccant-bearing porous insulating material through the ceiling 20 and into the enclosed living area room 24 by convective mass transfer. These preferred methods work best in cooling dominated climates such as those where the average monthly temperature is above 45° F. year round. In cooling dominated climates like southern Florida, for example, the relative humidity in the attic space 25 is typically higher than the relative humidity in the living space room 24, and the buildings typically experience an exterior temperature greater than 72° F. for a significant portion of the year. In such buildings, the typical construction includes spaced-apart attic floor joists disposed above an upper wall plate 12, and spaced-apart roof rafters 17 disposed below and supporting a roof of the building 100. In the preferred embodiments of this invention, the desiccant-bearing porous insulating material 10 is disposed at least between the attic floor joists. Additionally, the desiccant-bearing porous insulating material 10 of this invention can be disposed between joists supporting a cathedral ceiling in which there is relatively little attic space between the ceiling 20 and the roof. In either case, where there is a substantial attic, or barely enough room for an attic vent 18 to permit flowing air 15 to pass through, the desiccant-bearing porous insulating material 10 of this invention is capable of reducing a relative humidity level in the attic space or the area located between the ceiling 20 and the roof. Alternatively, in building construction, where there is an outer wall defined by a plurality of substantially parallel studs, an outer sheathing, and an inner drywall layer, the desiccant-bearing porous insulating material 10 can further be disposed between the outer sheathing and the inner drywall layer.

In a further preferred embodiment of the desiccant-bearing porous insulating material 10, randomly oriented fibers made of cellulosic or inorganic composition are disposed in a substantial thickness to provide insulation for a building 100. A desiccant in an amount sufficient to enable the insulating material to absorb enough water moisture to reduce the temperature of the insulating material 10 by a least 1° F. is added to the randomly oriented fibers.

Preferably, the reduction in temperature also occurs at the ceiling 20 when the adsorbed water is desorbed into the living space of room 24 by convective mass transfer. Such building insulation material 10 can be disposed in the form of a blanket, loose fill, or batt insulation product. In the preferred embodiment of this invention, the desiccant is a highly porous solid absorbent material that absorbs moisture from its surroundings and has a large moisture storage capacity. Preferred desiccants of this invention include one or more of the following ingredients: montmorillonite clay, silica gel, synthetic zeolite (molecular sieve), calcium oxide (CaO), calcium sulfate (CaSO4), carbon molecular sieve, activated alumina, or activated carbon, or sodium polyacrylate. The preferred desiccant is silica gel. The silica gels of this invention can include, for example, POLYLAM Products, Corporation white beaded and powdered silica and Grays Davidson Syloid® and Sylox® powders and Ludox® colloidal silica dispersions.

The preferred desiccant generally comprises at least about 1 wt. % and preferably at least about 5 wt. % based upon the dry weight of the randomly oriented fibers or other porous insulation. The desiccant can be provided in a uniform mixture of silica gel either in a dry form or added to water to form a slurry which is introduced into the randomly oriented fibers. In dry form the fibers and the desiccant can be consolidated onto a conveyer such as those typically used in the process of collecting rotary fibers in a glass batt manufacturing process. When the desiccant is added to the fibers, the glass fibers can still be hot or they can be cooled to ambient temperature. In dry form the desiccant can be blown into an upper area of the forming suction of a rotary glass manufacturing plant. Alternatively, the desiccant can be added to a water slurry and sprayed onto the hot mineral fibers in a forming section of a rotary glass plant.

The preferred methods of adding desiccant onto cellulosic fibers can include spraying the cellulosic fibers, or dipping them into a liquid desiccant or adding both desiccant and fire retardant to the cellulosic fibers at the same time or in subsequent steps. In one method, the desiccant is added to the fire retardant-containing cellulosic fibers in a blowing machine hopper before the fire retardant-containing cellulosic fibers are blown into an attic cavity, wall cavity or crawl space of the building.

Computer Simulation Example

A computer modeling study (Lixing Gu “Contract Report: Examine Potential Energy Savings Using Ceiling Insulation Mixed with Desiccant” Dec. 19, 2005 (unpublished)) was performed to determine the effects of desiccant-insulation mixtures on heat fluxes and building loads. The objective was to determine, through the use of building simulations, the potential energy savings by adding different concentrations of desiccant material to cellulosic ceiling-attic insulation and to examine ceiling peak load shifting due to the added thermal mass in the ceiling insulation. In addition, the attic humidity level due to moisture adsorption and desorption from added desiccant was modeled.

To evaluate the effects of sealed attics, in both heat dominated and cooling dominated climates, the computer modeling study was conducted for the Miami, Baltimore, Minneapolis and San Francisco climates. The computer model utilized was the FSEC 3.0 program (Florida Solar Energy Center, 1992, “FSEC 3.0 User's Manual,” FSEC-GP-47-92; the program does not simulate liquid water transfer). This one-dimensional finite element program calculates combined heat and mass transfer, including conduction, convection and radiation heat transfer and lumped moisture modeling by the Effective Penetration Depth Method (Kerestecioglu, A., M. Swami; 1990, “Theoretical and Computational Investigation of Simultaneous Heat and Moisture Transfer in Buildings: Effective Penetration Depth Theory.” ASHRAE Winter Meeting, Atlanta, Ga.

After determining the simulation tool to use, a prototype house with slab-on-grade was selected as a residential building in the present study. The prototype house was used in two ASHRAE research projects, 852-RP1 and 1165-RP2 3, all of which are hereby incorporated by reference in their entirely. It is a single story “L: shaped ranch style home with an open living plan, slab-on-grade concrete floor, and 139 m2 (1500 ft2) of conditioned space. The garage area is 42.4 m2 ((456 ft2) and attic volumes are 107 m3 (3769 ft3) over the living zone and 56.5 m3 (1996 ft3) over the garage. The attic areas are modeled as separate spaces with a plywood wall separating them. The house aspect ratio is 1:1:6 with the longer axis running east to west. 1 Gu, L., J. E. Cummings, M. V. Swami & P. W. Fairey, “Comparison of Duct System Computer Models That Could Provide Input to the Thermal Distribution Standard Method of Test (SPC152P),” Symposium of 1998 ASHRAE Winter Annual Meeting, San Francisco, 1998.2 Gu, L., J. E. Cummings, M. V. Swami & P. W. Fairey 2003, “System Interactions in Forced-Air Heating and Cooling Systems, Part I: Equipment Efficiency Factors,” CH-03-7-1 (RP-1165), ASHRAE Transactions 109 (1)3 Gu, L., J. E. Cummings, M. V. Swami & P. W. Fairey 2003, “System Interactions in Forced-Air Heating and Cooling Systems, Part II: Continuous Fan Operation,” KC-03-01 (RP-1165), ASHRAE Transactions 109 (2)

Envelope constructions are listed as follows:

Exterior frame wall: 12.7 mm (½″) gypsum drywall on the inside, R-19 batt insulation, and 11 mm ( 7/16″) masonry exterior siding with a solar absorptance of 0.75 and a far-infrared emissivity of 0.9.

Roof: 12.7 mm (½″) plywood exposed to the attic and 6.4 mm (¼″) shingles on the exterior with solar absorptance of 0.85 and emissivity of 0.9. The roof slope is 5:12.

Floor: 0.1 m (4″) concrete with 12.7 mm (½″) carpet on the interior.

Ceiling: 12.7 mm (½″) gypsum drywall and R-30 insulation mixed with 10-20% desiccant.

The heating and cooling thermostat set points are 22.2° C. (72°) and 25.6° C. (78° F.), respectively.

HVAC system: Cooling: SEER 10 Air conditioner with 0.75 sensible heating ratio; Heating: Electric heater

The selected desiccant material was silica gel. For the purpose of this example, it was assumed that the desiccant material was added into the fiber glass or cellulose insulation without changing the volume of the insulation, and that both individual materials were perfectly mixed. Using the above assumptions, it is possible to generate the effective material properties of the mixture based on volume rate and weight ratio. According to Table 4 in the 2005 ASHRAE Handbook of Fundamentals, the thermal conductivity and density of R-30 fiber glass blanket and batt are 0.475 (W/m.K) and 19.2 (kg/m3 average), respectively. In order to use a full set of thermal properties required by the simulation program, loose fill cellulosic insulation was selected to replace the fiber glass batt. There is a difference of density between loose fill cellulosic insulation and fiber glass batt. A large density difference could cause load shifting through thermal capacity. However, since both fiber glass and cellulose insulation are light, the thermal impact of load shifting from density difference can be considered to be negligible. For example, thermal resistance of fiber glass insulation is used in most simulation programs and density and specific heat are not required, such as DOE-2 and EnergyPlus. The main reason to eliminate inputs of density and specific heat is that thermal capacity impact of insulation material on thermal performance is negligible. In addition, those simulations with fiber glass are generally assumed to hold true for cellulose insulation. Therefore the calculated and simulated results for cellulose insulation in this study can be assumed to be similar to results for fiber glass insulation. The conclusions obtained from cellulose simulations can be applied to fiber glass ceiling insulation.

Thermal Properties

The following table lists the thermal properties of cellulose insulation and silica gel:

TABLE 1
Thermal properties of cellulose insulation and silica gel
Thermal
conductivityDensitySpecific heat
kρCp
W/m · Kkg/m{circumflex over ( )}3J/kg · K
Silica Gel0.144721821
Cellulose0.04665712

The effective thermal properties of mixed ceiling insulation with silica gel may be obtained based on volume ratio. The following formula is used to calculate the effective thermal properties:


Pe=Pfiber+Pdesi×ratio (3)

where

    • Pe=Effective properties, including thermal conductivity, density and specific heat
    • P fiber=Thermal properties of cellulose insulation
    • P desi=Thermal properties of desiccant
    • ratio=Volume ratio

After adding silica gel in the air space of the cellulose insulation, the air space is reduced. This may be equivalent to compressing the insulation slightly. Although the density of the cellulose insulation remains the same, the thermal conductivity may be increased slightly due to compression. In order to simplify the calculation, the thermal properties of the cellulose insulation are assumed to be the same as one without compression.

The effective thermal properties and percent property changes used in the present study are listed in the following table:

TABLE 2
Effective thermal properties of the silica gel insulation mixtures
KρCp% k% Cp
10% silica0.0604137.1769.322431.30110.928.05
gel mixing
20% silica0.0748209.2787.132962.61221.8510.55
gel mixing

Thermal conductivity, density and specific heat are 31%, 111% and 8% increases for the 10% silica gel mixture of ceiling insulation, and 62%, 222% and 11% increases for the 20% silica gel mixture of ceiling insulation, as compared to the cellulose only insulation.

Moisture Properties

The main interest of moisture properties of the mixture in this study is moisture adsorption and desorption, which can be expressed by a sorption curve (moisture content vs. relative humidity). Although sorption curves of cellulose and silica gel are known, the sorption curve of the mixture is not known. However, the sorption curve of the mixture can be estimated from the known moisture properties of cellulose and silica gel and the effective bulk density of the mixture found in Table 2 above.

TABLE 3
Summary of annual simulation results in Miami
Mixing ratioPercent difference
MiamiBase10%20%10%20%
LivingT_aver25.2325.1925.16−0.16−0.28
RH aver50.9353.6357.905.3013.69
AtticT_aver30.1830.5030.531.051.18
RH aver45.5036.6235.82−19.51−21.26
Energy use (kWh)Heating322.92378.56403.7117.2325.02
Cooling7060.06J6839.236483.14−3.13−8.17
Total7382.967217.796886.85−2.3−6.7
Ceiling heat fluxCold1.465.2012.07255.96725.33
(Kw/m2)Hot13.169.134.61−30.62−64.99
Ceiling moisture fluxdry0.2170.04310.00167−80.11−99.23
(Kg/s · m2)wet0.2740.4691.03071.40276.41

Table 3 presents a summary of annual simulation results. The first two columns are item descriptions. The third column provides annual results from the base case with cellulose only ceiling insulation. The fourth and fifth columns shows annual results from the mixture ceiling insulation with silica gel mixing ratios of 10% and 20%, respectively. The last two columns provide percent differences of mixture ceiling insulation with 10% and 20% silica gel mixing ratios, compared to the base case. The negative and positive signs represent the decrease and increase, respectively, as compared to the base case.

The items listed in Table 3 above, consist of indoor temperature (T aver.) and relative humidity (RH) in living and attic zones averaged over a period of a year, heating and cooling energy use (KWh), and the total energy use as a sum of heating and cooling energy. Annual ceiling heat (KW/m2) and moisture (Kg/s.m2) fluxes between the living zone and the drywall ceiling surface exposed to the living zone are also listed in Table 3. The ‘cold’ defined in the ceiling heat flux represents the fact that the drywall ceiling surface temperature is lower than living zone air temperature and occurs during a heating season and partly in a swing season (no heating and cooling), while the ‘hot’ indicates that drywall ceiling surface temperature is higher than the living zone temperature and occurs during a cooling season and partly in a swing season (no heating and cooling). The ‘dry’ defined in the ceiling moisture flux shows that humidity ratio of drywall ceiling surface exposed to the living space is lower than the living zone air moisture level, while the ‘wet’ presents the fact that ceiling surface humidity is higher than the living zone condition.

Compared to the base case of cellulose insulation only, the average living area air temperatures in Miami with the silica gel-cellulose mixture insulation are slightly lower—within 0.3%. The difference may be negligible. However, the annual average living zone relative humidity levels with the silica gel-cellulose mixture insulation increase 5.3% and 13.7% for silica gel mixing ratios of 10% and 20%, respectively. The main reason is that the higher moisture content in the mixture insulation absorbed from the attic causes a higher positive ceiling moisture flux flowing into the living space, so that the indoor relative humidity level is higher, as compared to the base case. Annual average attic temperatures with the mixture insulation are slightly higher than the base case within 1.2%. But, attic relative humidity levels with the mixture insulation are much lower than the base case, 20% less with 10% mixing ratio and 21% for the 20% mixing ratio.

The following Table 4 lists annual heating and cooling energy use in four locations and three types of ceiling insulations (base R30 insulation only, R30 with 10% silica gel, and R30 with 20% silica gel) through computer modeled building simulations.

TABLE 4
Annual Heating and Cooling Energy Use in
Four American Cities
Heating (kWh)4Cooling (kWh)Total (Heating + Cooling)
LocationBase10%20%Base10%20%Base10%20%
Miami323379404706068396483738372186887
Baltimore45306472664934317155453224728149348
Minneapolis259292747329014318271183262472774429197
San Francisco14752158251754115010041149021592517582
4The attic temperature in cold climates is well below the freezing point of water in winter, and the sorption performance may not be valid at very low temperatures.

Simulation results, also graphically depicted in the bar chart of FIG. 2, show that annual cooling energy use decreases, especially in “cooling dominated climates,” such as Miami, while annual heating energy use increases with an increase of ceiling-attic insulation mixing ratio of silica gel. It can be concluded that a desiccant included in the attic insulation will reduce the attic humidity levels, which may increase the life span of roofing materials, such as shingles and plywood.

Conclusions

A desiccant included in the attic insulation may reduce attic humidity levels, which may increase the life span of roofing materials, such as shingles and plywood.

A desiccant included in the attic insulation may increase ceiling moisture levels, which may reduce the life span of ceiling gypsum drywall. However, the amount of moisture increase may not be high enough to cause a negative effect on the drywall.

A desiccant included in the attic insulation may increase heating energy consumption, so that the ceiling mixture use may not be useful for heating dominant climates.

A desiccant included in the attic insulation may decrease cooling energy consumption, so that the ceiling mixture use is probably suitable for cooling dominant climates

Peak load shifting is believed to be insignificant when a desiccant is added to the attic insulation. The main reason is that the added desiccant mass is not large enough to make it become a heavy mass.

A desiccant included in the wall cavities may have similar impact with a desiccant included in the attic insulation on cooling and heating energy use. Since the wall cooling load is a smaller portion of the total cooling loads, compared to roof cooling load, the impact of cooling energy use would probably be smaller. However, the wall heating load is larger. The impact on heating load would consequently be larger.

It should be pointed out that although the cellulose ceiling insulation was used in simulations, the conclusions obtained from the cellulose insulation can be applied to the fiber glass blanket, loose fill and batt, because the difference in thermal performance between cellulose and fiber glass insulation is considered to be negligible.

Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.