Floor Above Unconditioned Basement or Vented Crawlspace

Scope

Fully Aligned Air Barriers

 

1. Install a continuous rigid air barrier or other supporting material to separate the exterior from the conditioned space.*
2. Seal all seams, gaps, and holes of the air barrier with caulk or foam. 
3. Install insulation without misalignments, compressions, gaps, or voids in floors above the unconditioned basement or unconditioned crawlspace. 
4. Install supports for insulation to remain in contact with the air barrier. Examples include metal support rods for batt insulation or netting for blown insulation.

   
*ENERGY STAR highly recommends using a rigid air barrier, but it is not a requirement.

Notes:

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.

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Description

Floors can account for one fourth to one third of the building enclosure’s surface area. When defects in the air barrier and insulation system exist, heat flow through floors over unconditioned basements or vented crawlspaces can have a big impact on thermal comfort, as well as have a significant impact on space conditioning costs.

 

Note: With respect to basements, emphasis needs to be placed on "unconditioned." In regions where basements are common, there are often many reasons for the basement to be more connected to the living space than to outdoors, and this space should be treated as inside the conditioned space, even if it is not directly heated or cooled. Many basements contain HVAC equipment and plumbing supply lines, which could freeze in cold weather if the basement space is outside the thermal boundary. In these situations, it is not desirable, nor is it typically cost-effective, to air seal and insulate the floor. However, if HVAC equipment is not located in the basement, and plumbing lines can be protected, it might be reasonable to establish the thermal and pressure boundary in the floor above the basement, rather than in the foundation walls.

 

Types of thermal defects. Thermal defects can be classified into three main areas:

1. Conduction thermal bypasses, also known as thermal bridging, results from a lack of insulation.
2. Air leakage thermal bypasses are caused by lack of an effective air barrier. In this case, infiltration causes a loss of heating and/or cooling between the conditioned and non-conditioned spaces.
3. Misalignment of the air barrier and thermal barrier results in air spaces where convection currents may flow through the insulation, reducing the effective R-value of the insulation (EPA 2011).

 

To prevent these thermal defects, insulation should be installed that is fully aligned with the air barrier. One method for doing this is described below:

1.  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): 

 

a.   Install subfloor panels sized for the on-center framing (16-, 20-, or 24-inches on-center).
b.   Stagger subfloor panel end joints.
c.   Use tongue-and-groove subfloor panels or install blocking below the subfloor panel edges.
d.   Apply construction adhesive between subfloor panel and framing members.

 

 

Figure 1 - 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:

e.   Seal the band joist.
f.   Seal any penetrations through the wall plates into the floor system, as well as any penetrations through the subfloor itself.

 

2. Seal penetrations. The trick to maintaining a continuous air barrier will be in limiting penetrations through the floor sheathing. Any hole through the subfloor will compromise this air barrier. If possible, avoid plumbing pipes, electrical wiring, HVAC ducts and other mechanical penetrations.

 

 

Figure 2 - All mechanical and plumbing penetrations through the subfloor need to be air sealed 

 

 

 

If mechanical penetrations are necessary, they must be sealed. Common sheathing materials, installed in conjunction with foam sealants, work well for sealing large penetrations, such as the one shown in the photo. Smaller pipe and wire penetrations can be sealed with a foam sealant or caulk (EPA 2011).

 

In addition to sealing mechanical and plumbing penetrations, the rim joist must be adequately sealed to limit infiltration through the perimeter framing above the foundation. (The term "rim joist" refers to the perimeter floor flaming above the foundation and is differentiated from a "band joist," which typically refers to the perimeter floor framing between stories of a building.)
Best practice calls for using a rigid insulation material, such as the rigid foam board used in the photo below. Note: The edges of the rigid insulation must be sealed with a foam sealant or compatible caulk.

 

 

Figure 3 - An effective air barrier for the rim joist is rigid foam board sealed with foam or caulk 

 

 

Another good alternative to sealing the rim joist is spray foam insulation, which will seal and insulate in one step. However, 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).

 

Stuffing batts into the rim joist is not sufficient for either insulation or air sealing. Fiberglass is not an air blocking material, and in the photo below, no attempt has been made to air seal the rim joist behind the insulation. Moreover, the R-value of the batts is reduced by the gaps and compression, so it is not even serving as a very good insulation.

 

 

Figure 4 - Batt insulation is not an effective air barrier  

 

 

3.  Install insulation. Floor insulation should fit tightly between the floor joists. Avoid any gaps, compressions, or voids.

 

Also, ensure that 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).

 

In the photo below, the insulation is not in alignment with the subfloor, nor is it in full contact with the joists. As a result, this insulation contributes nothing to the thermal performance of the crawlspace floor.

 

 

Figure 5 - Without wire stays or metal support rods, this batt insulation will not stay fully aligned with the subfloor air barrier. In addition, it has not been sized to fit tightly between the floor joists.  

 

 

4. Support insulation. Once installed, the insulation must be well supported so it stays in contact with the subfloor above it.

 

In a floor system, the insulation is constantly subject to gravity. Without metal support rods or other insulation stays, batts will fall out of the joist cavity, as shown in the photo above.

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Ensuring Success

Blower door testing, conducted as part of whole-house energy performance testing, may help indicate whether air leakage at through the floor has been successfully sealed.

 

Infrared imaging. An infrared camera may also be used to determine air leakage through the floor over a garage if a sufficient temperature difference exists between the outdoors and the conditioned space above the garage. For best results, scan twice – first 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.

 

The infrared image below shows the rim joist area. Typically the darker areas indicate the areas that are the coldest, and represent the greatest amount of heat loss. Here, where the insulation meets the subfloor and the I-joists, you can see dark purple spots. These spots indicate heat loss or air leakage. These areas are where you want to be sure you air seal before you insulate.

 

 

Through this thermograph from an infrared camera, heat loss or air leakage is visible as dark purple spots, which clearly display a lack of air sealing  

 

 

Diagnostic smoke. 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 outdoors that should be sealed.

 

 

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Climate

In cold climates, it is critical that plumbing lines running through the floor, or inside an unconditioned basement or crawlspace, are protected from freezing. This may require insulating the pipes.

 

In very cold climates, plumbing supply lines should be relocated into conditioned areas of the building.

 

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3. Exhibit 2: DOE Challenge Home Target Home. Infiltration (ACH50): Zones 1-2: 3; Zones 3-4: 2.5; Zones 5-7: 2; Zone 8: 1.5.

 

International Energy Conservation Code (IECC) Climate Regions

 

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Compliance

ENERGY STAR Version 3, (Rev. 6)

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.

 

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3. Exhibit 2: DOE Challenge Home Target Home. 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.

2009 IECC

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.*

 

2009 IRC

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.*

 

2012 IECC

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.* 

 

2012 IRC

Table N1102.4.1.1 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.* 

 

*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.

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