2015-04-16-thermal-bridging

Thermal Bridging and Air Sealing

DonanEngineering Articles, Featured, Forensic Engineering

In the process of building our first house, my wife has called me “the builder’s worst nightmare.”  And in some ways, she’s correct.  In other ways, I’m my own worst nightmare.  However, I’m only using the knowledge I have gained as a forensic engineer to limit thermal bridging and air leakage.

In older conventional construction (such as our current house which was built in 1997), it was common to have fiberglass batt insulated 2×4 walls with OSB or plywood wall sheathing.  Without adequate air sealing and protection against thermal bridging, it is common to see wall studs (from the interior and exterior) and cold spots utilizing a thermal imaging camera (Figure 1 through 3).

Figure 1: Wall studs visible from exterior.

Figure 1: Wall studs visible from exterior.

Figure 2: Wall studs visible from interior.

Figure 2: Wall studs visible from interior.

Figure 3: Cold spots at corners and wall-to-ceiling interfaces.

Figure 3: Cold spots at corners and wall-to-ceiling interfaces.

Cold spots occur at locations of thermal bridging and air leakage, and the best way to limit air leaks into a building is to seal, seal, and seal.  Cold spots are common at the top and bottom of walls, at the corners as gaps are typically present between the framing and insulation, and at exterior penetrations.  In older construction, it was common practice to essentially fill the corners with framing such that no insulation was present.  If sufficient interior moisture is present, one may see water stains at locations of cold spots due to condensation.

So what exactly is thermal bridging?  Thermal bridging is driven by heat transfer through the building’s materials.  When interior and exterior temperatures are similar, thermal bridging will be minimal as there is little-to-no variation in temperature.  However, when the interior and exterior temperatures vary significantly (such as winter here in Wisconsin), heat gains will occur through the materials.  The wood framing in a typical house has a thermal conductivity of approximately 0.15 while the in-filled insulation has a thermal conductivity of 0.04 or less.  The studs are visible using infrared imaging in a conventionally constructed home as heat travels through the wood framing, thermally bridging the exterior and interior, more readily than through the insulation.

In today’s construction, it has become common to use foam board on the exterior of the home (which type of foam board is up for debate as there is not a clear cut answer; ask a local representative and he or she will most likely tell you to use their product).  The foam board can be used as the wall sheathing alone (provided there is sufficient bracing) or over wooden wall sheathing.  Not only does the foam board increase the overall insulation value, its use reduces or eliminates thermal bridging through the wall studs.

One of the issues I noticed during the course of construction on my own home is that there are gaps where the sheathing (be it foam or wood) butts to one another or is attached to the framing (Figure 4).  While there is a building wrap outside the wall framing of our new home, I spent a lot of time covering any gaps with tape and “picture framed” the wall cavities using a few cases of caulk to further mitigate air leakage (a side note: pay the extra money and get a good caulk gun!).  Essentially, one caulks or spray foams the perimeter of the sheets of sheathing.  One obvious spot in my house is the intersection of the 1-inch foam, and ½-inch wood sheathing under ½ inch foam (Figure 5).  In my house, the ½-inch wood sheathing acts as the bracing.

Figure 4: Gaps where sheathing butts together.

Figure 4: Gaps where sheathing butts together.

Figure 5: “Picture framing” of wall cavities using caulk.

Figure 5: “Picture framing” of wall cavities using caulk.

There are the obvious air leak locations in a house including recessed lights, exhaust vents, plumbing vents, electrical and plumbing penetrations in top plates, electrical boxes in exterior walls, rim joists, around windows, etc.  However, there are several locations that you can only see when on the floor or up in the air (like as I’m running a few thousand feet of low voltage wires in the attic).  During a pre-insulation energy evaluation through Focus On Energy, our consultant pointed out air leakage locations such as the foundation wall-to-sill plate interface (even with a sill gasket) and subfloor-to-bottom plate interface (small gaps with a lot of lineal feet at these locations is a large area), the top plate where the truss clips are used, the intersection of top plates over built-up interior walls, the intersection of wood framing in soffits, and gaps between the drywall and housing of “air tight” recessed lights (Figures 6 through 9).  The consultant also made the comment, “If you have to think about it, seal it.”

Figure 6: Gap will occur between top plate and drywall due to truss clip.

Figure 6: Gap will occur between top plate and drywall due to truss clip.

Figure 7: Gap between top plates of intersecting 2x4 walls.

Figure 7: Gap between top plates of intersecting 2×4 walls.

Figure 8: Gaps between drywall and housing of air-tight recessed lights.

Figure 8: Gaps between drywall and housing of air-tight recessed lights.

Figure 9: Gap between bottom plate and subfloor filled with caulk.

Figure 9: Gap between bottom plate and subfloor filled with caulk.

In my case, the gap between the foundation wall and sill plate will be addressed as the rim joists will be spray foamed, the gap between the subfloor and bottom plate will be caulked, and I will be implementing the “air tight drywall” method through the use of gaskets.  Any gaps between intersecting pieces of wood framing will also be sealed.  Air sealing of the “air tight” recessed lights is relatively easy once the drywall is up.  Just caulk the gap between the recessed light and the drywall, or use spray foam prior to installing attic insulation.  While you’ll see the recommendation to install air-tight boxes over older non-insulation contact or non-air tight recessed lights, I was informed by the consultant that it is not necessary for the new insulation contact, air-tight recessed lights if they are sealed as stated.  Another way to limit air leakage is to use the “air tight drywall” method.  This method uses the drywall as an air barrier by caulking or gasketing the drywall to the wall framing (Figures 10 and 11).  Due to all this air sealing, my house will be very “tight” and indoor air quality becomes a concern.  So, we will also be utilizing a heat recovery ventilator (HRV) for whole-home ventilation in lieu of exhaust-only ventilation.

Figure 10: Gaskets installed for “Air Tight Drywall.”

Figure 10: Gaskets installed for “Air Tight Drywall.”

Figure 11: Gaskets installed for “Air Tight Drywall.”

Figure 11: Gaskets installed for “Air Tight Drywall.”

So, the last step is to perform the blower door test and determine how all of this air sealing performed.  The blower door test puts the house in a negative pressure environment and finds sources of air leakage.  Since our house is still under construction, we do not have the results of the blower test; however, I’ve been told by my builder and the energy consultant that it will pass with flying colors.

Now, you’re reading this and probably thinking to yourself, this seems excessive (you’re not alone, my wife thinks the same!).  While you’re right to think it may be excessive, do you remember looking at your last energy bill and thinking, there’s got to be a way to reduce this cost?  Since I have a feeling that energy prices will continue to go up and I really do not want to significantly change my energy use, it was reasonable to me to be more efficient on how my house utilizes energy.  Since caulk and gaskets are relatively cheap and I wasn’t paying for labor (since I was installing it), I made the house as air tight as possible during the course of construction to limit the conditioning of leaked exterior air.  Basically, why keep heating and cooling exterior air that leaks into the house?  In my research on building this house, I recall a comment from Mr. Joe Nagan in an online video through the Energy Center of Wisconsin about those that say you are building the house “too tight.”  He asks the question, “How do you build the home just leaky enough to dissipate all the moisture so that the building materials wouldn’t have a problem.” [1] With air-tight building practices and managed ventilation, a house will be comfortable from both an air leakage and indoor air quality perspective.

 

[1] Nagan, Joe, Psychometrics: Dew Point for Dummies, Energy Center of Wisconsin.