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:
- Conduction thermal bypasses, also known as thermal bridging, results from a lack of insulation.
- 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.
- 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.