Floor Above Garage
Fully Aligned Air Barriers = Whole-House Draft Barrier
In cold climates, avoid plumbing in floors over garages. Plumbing pipes should not be run through floors over garages that are in unconditioned space because the pipes could freeze. Builders sometimes try other solutions that can lead to mold problems. For example, as shown in the photo, the builder has actually installed a supply duct in the floor cavity to heat this space and prevent the pipes from freezing. This duct will pressurize that space with warm, humid air, and when that warm, humid air comes in contact with cold surfaces, it can lead to condensation, which in turn can lead to mold-related problems. It is always best practice in cold climates to avoid putting plumbing fixtures in the building cavities over garages.
In cold climates, plumbing pipes should not be located in the building cavities over the garage because of the potential for pipes to freeze. This photo shows how a builder has tried to solve the problem by installing a supply duct to heat the space. However, when such warm, humid air meets a cold surface, it can condensate, resulting in mold.
Exhibit 2: DOE Zero Energy Ready Home Target Home. Certified under ENERGY STAR Qualified Homes Version 3. Infiltration (ACH50): Zones 1-2: 3; Zones 3-4: 2.5; Zones 5-7: 2; Zone 8: 1.5. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.
International Energy Conservation Code (IECC) Climate Regions
An attached garage presents some serious air sealing challenges. Typically, the garage is a source of car exhaust and other pollutants such as stored chemicals, paint, fertilizer, and gasoline for lawn mowers. This means that any air leaks between the garage and the living space can create an immediate indoor air quality concern. In addition, garages are unconditioned and exposed to ambient air temperature and wind conditions. This makes insulation between the garage and the living space as important as any insulation in an exterior wall, but this often gets overlooked. The key issue with insulation in a floor over a garage is the location of the air barrier, which is often confused, resulting in increased air infiltration and wind washing that can render the insulation ineffective (EPA 2008).
Air Barrier Alignment
All insulation in building cavities must be in continuous contact with the air barrier. In an attached garage with living space above it, the subfloor above the garage is typically the air barrier. This means the insulation in the floor over a garage must be in contact with the bottom of the subfloor.
When the insulation is not in contact with the subfloor it creates a thermal short circuit (as depicted below). Unconditioned air that infiltrates through exterior framing can pass over the top of poorly installed insulation. The moving air scavenges away heat from the floor system, and the insulation becomes ineffective.
The solution to eliminating these thermal short circuits is two-fold: First, completely air seal the floor system, paying particular attention to sealing the band joist area to limit infiltration. Second, create a continuous air barrier that is in "full alignment" with the insulation (EPA 2008).
Figure 2 - Infiltration through band joist. Platform framing presents many opportunities for air infiltration at the band joist. Combined with misaligned insulation, the thermal performance of the floor system shown at right has been seriously compromised.
Detailing the Floor above a Garage
- Install a continuous air barrier. In most cases, this air barrier will be plywood or OSB floor sheathing. Install the subfloor sheathing panels according to APA Sturd-I-Floor recommendations, which includes the following (APA 2011):
- Install subfloor panels sized for the on-center framing (16-, 20-, or 24-inches on-center).
- Stagger subfloor panel end joints.
- Use tongue-and-groove subfloor panels or install blocking below subfloor panel edges.
- Apply construction adhesive between subfloor panel and framing members.
Figure 3 - Subfloor as air barrier. Typically the subfloor of living space over a garage serves as the air barrier. To ensure a continuous air seal, the gaps between panels, as well as gaps in the band joist and wall plates require focused air sealing attention.
To make the subfloor an air barrier, two additional requirements are needed:
Seal the band joist.
Seal any penetrations through the wall plates into the floor system, as well as any penetrations through the subfloor itself.
2. Seal the band joist and penetrations. The band joist area along the garage perimeter requires focused air sealing attention. Specifically, the open ends of the joists must be blocked off. Joists that run from the garage ceiling across the wall separating the garage from the living space must be blocked and rigorously air sealed to prevent infiltration of airborne pollutants (EPA 2008).
It is not enough to stuff insulation batts into the band joist; fibrous insulation won't block air. Use blocking made of plywood or OSB (as shown at left), sheathing, cardboard, or rigid foam. The edges of the blocking must be sealed with a one- or two-part foam sealant, or caulk. Foam sealant will work better (and last longer) than caulk.
I-joists present a challenge for air sealing. The blocking must fit tightly against the joist webbing. This can be done with filler strips as shown in the photo above. Or, the corners of the blocking may be cut to fit around the I-joist flanges, as shown in the illustration below.
A good alternative to rigid blocking for sealing the band joist is spray foam insulation. Spray foam will fill small voids and cracks, and effectively stop air flow, while also providing a relatively high R-value per inch (DOE 2012).
3. Install insulation. Floor insulation must fit tightly between the floor joists. Avoid any gaps, compressions or voids.
Be sure the insulation is fully "aligned" with the air barrier. In most cases, this means the insulation must be in full contact with the subfloor. There can be no gaps between the insulation and the sheathing above it (EPA 2011). As shown in the photo below, batts should be fully lofted (not crammed or compressed into the space). Batts should be cut lengthwise to fit narrow joist bays, and split to fit neatly around electrical wiring running across joist bays.
Figure 7 - Correctly insulated floor. This insulation fills the joist cavity, so it is in contact with the bottom of subfloor of the living space above. Once the ceiling drywall is installed, the insulation will also be well supported. Both details are critical to thermal performance of the floor system.
Blown-in fiberglass or cellulose that fills the entire joist cavity may be a good option for insulating the floor above a garage. Particularly when floor trusses are used, blown-in cellulose can be an effective way to fill the voids around web members. When blown-in insulation is used, however, it must be installed to the correct density to prevent settling, and the drywall must be sufficient to hold the weight. Check with the insulation manufacturer and installer to learn if the joist depth, joist spacing, and the weight of the insulation are compatible. In some cases, netting installed before the drywall may help hold the insulation in place (EPA 2008).
Spray foam insulation is also a good choice for insulating the entire floor (not just the band joist) above a garage. It provides high R-value and a continuous air barrier in one labor-saving application. To qualify as an air barrier, open-cell or closed-cell foam must have a finished thickness greater or equal to 5.5 inches or 1.5 inches, respectively (EPA 2011).
4. Support insulation. Once installed, the insulation must be well supported to ensure it remains in contact with the subfloor above it. If the insulation fills the floor cavity, ceiling drywall will support it. If ceiling drywall will not be installed, or if batt insulation does not fill the entire joist cavity, insulation stays must be installed to keep the insulation in continuous contact with subfloor above.
Blower Door Testing
Conducted as part of whole-house energy performance testing, may help indicate whether air leakage at the floor has been successfully sealed.
An infrared camera also may be used to determine air leakage at the floor over a garage, if a sufficient temperature difference exists between the unconditioned garage and the conditioned space above the garage. For best results, scan twice. First, scan under static conditions before blower door testing has been conducted. This will allow the technician to evaluate the integrity of insulation behind the drywall if the garage ceiling has been finished. Conduct a second scan with the blower door running in depressurization mode and the door between the house and the garage open. This will demonstrate the integrity of the air barrier, showing where air leakage has infiltrated the framing and seeped through insulation.
With the blower door pressurizing, the garage door closed and the door to the house open, check for air leaks in the floor and near the rim joist with a smoke pencil. A smoke trail moving away from the smoke pencil indicates a leak to the unconditioned space that should be sealed.
Fully Aligned Air Barriers
- Install a continuous rigid air barrier or other supporting material to separate the garage from the conditioned space.
- Seal all seams, gaps, and holes of the air barrier with caulk or foam and complete before insulation installation.
- Install insulation without misalignments, compressions, gaps, or voids in all floors above garage.
- Install supports for insulation to remain in contact with the air barrier. Examples of supports include staves for batt insulation or netting for blown-in insulation.
ENERGY STAR Notes:
ENERGY STAR highly recommends using a rigid air barrier, but it is not a requirement.
An air barrier is defined as any durable solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage. ENERGY STAR recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness >= 5.5 inches or 1.5 inches, respectively, to qualify as an air barrier unless the manufacturer indicates otherwise. If flexible air barriers such as house wrap are used, they shall be fully sealed at all seams and edges and supported using fasteners with caps or heads >= 1 inch in diameter unless otherwise indicated by the manufacturer. Flexible air barriers shall not be made of kraft paper, paper-based products, or other materials that are easily torn. If polyethylene is used, its thickness shall be >= 6 mil.
Examples of supports necessary for permanent contact include staves for batt insulation or netting for blown-in insulation. Batts that completely fill a cavity enclosed on all six sides may be used to meet this requirement without the need for supports, even though some compression will occur due to the excess insulation, as long as the compressed value meets or exceeds the required insulation level. Specifically, the following batts may be used in six-sided floor cavities: R-19 batts in 2x6 cavities, R-30 batts in 2x8 cavities, R-38 batts in 2x10 cavities, and R-49 batts in 2x12 cavities. For example, in a home that requires R-19 floor insulation, an R-30 batt may be used in a six-sided 2x8 floor cavity.
Fully-aligned air barriers may be installed at the exterior surface of the floor cavity in all climate zones if the insulation is installed in contact with this exterior air barrier and the perimeter rim and band joists of the floor cavity are also sealed and insulated to comply with the fully-aligned air barrier requirements for walls.
Thermal Enclosure Checklist, Fully-Aligned Air Barriers. A complete air barrier shall be provided that is fully aligned with the insulation at interior surface of floors in all climate zones, including supports to ensure permanent contact and blocking at exposed edge. Examples of supports necessary for permanent contact include staves for batt insulation or netting for blown-in insulation. Alternatively, batts that completely fill floor cavities enclosed on all six sides may be used to meet Items 2.2 and 3.2, even when compression occurs due to excess insulation, as long as the R-value of the batts has been appropriately assessed based on manufacturer guidance and the only defect preventing the insulation from achieving the required installation grade is the compression caused by the excess insulation. Fully-aligned air barriers may be installed at the exterior surface of the floor cavity in all climate zones if the insulation is installed in contact with this exterior air barrier and the perimeter rim and band joists of the floor cavity are also sealed and insulated to comply with the fully-aligned air barrier requirements for walls.
Exhibit 2: DOE Zero Energy Ready Home Target Home. Certified under ENERGY STAR Qualified Homes Version 3. Insulation levels shall meet or exceed the 2012 IECC (Table R402.1.1) and achieve Grade 1 installation, per RESNET standards. Infiltration (ACH50): Zones 1-2: 3; Zones 3-4: 2.5; Zones 5-7: 2; Zone 8: 1.5. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol. Steel-frame ceilings, walls, and floors shall meet the insulation requirements of the 2012 IECC – Table 402.2.6.
Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria, Floors: Air barrier is installed at any exposed edge of insulation.* Table 402.4.2, Air barrier and thermal barrier: Air permeable insulation is not used as a sealing material.*
Table N1102.4.2 Air Barrier and Insulation Inspection Component Criteria, Floors: Air barrier is installed at any exposed edge of insulation.* Table N1102.4.2, Air barrier and thermal barrier: Air permeable insulation is not used as a sealing material.*
Table R402.4.1.1 Air Barrier and Insulation Installation, Floors: Insulation in floors (including above garage and cantilevered floors) is installed to maintain permanent contact with underside of subfloor decking. A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material. Junction of foundation and wall sill plates, wall top plate and top of wall, sill plate and rim-band, and rim band and subfloor are sealed. Corners, headers, and rim joists making up the thermal envelope are insulated.*
Table N118.104.22.168 Air Barrier and Insulation Installation, Floors: Insulation in floors (including above garage and cantilevered floors) is installed to maintain permanent contact with underside of subfloor decking. A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material. Junction of foundation and wall sill plates, wall top plate and top of wall, sill plate and rim-band, and rim band and subfloor are sealed. Corners, headers, and rim joists making up the thermal envelope are insulated.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.