Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Box Sill Insulation - To Foam or Not To Foam

Jeff Aderholdt - Monday, December 24, 2012

In recent years, there have been a lot of insulation advancements. New techniques have been developed and employed to make buildings tighter and more energy efficient. The results have been mixed. (See Flash and Batt - Risky in Cold Climates).

One of the most problematic areas in platform construction is how to insulate is the box sill, or band sill. This is the area at the end of the floor joist that is exposed to the outside and needs to be insulated. Several factors make this area difficult to insulate well. For one, a lot of framing meets at the box sill. This means that there is a lot of air leak potential. Another factor is we are not dealing with a confined space. This makes it difficult to create an air tight seal at the inside, where it needs to control convection, and thereby control moisture.

The usual insulating technique used to be to use fiberglass batting. A lot of us used kraft or foil-faced batting neatly installed and fastened to the floor joists. As we started air-tightening the rest of the structure, indoor winter humidity started to rise. This made condensation behind the batting more noticeable. Since we could never make the fiberglass facing airtight, the recommendation was to use un-faced fiberglass instead. The reasoning was since moisture is going to get behind the batting anyways, we want to allow the moisture to easily get out. Though installing fiberglass in the box sill was simple, many of us were never satisfied with the quality. Then came spray foam.

Soon two-part spray foam became readily available. Several manufacturers produced small portable foaming kits. This meant that you didn't need to purchase expensive equipment to do small foaming jobs, like the box sill. Spray foam became the preferred material for box sill insulation, but not without drawbacks. Spray foam is temperature sensitive. Both the foam components and the surface of the material where the foam would be installed needed to be within a very limited temperature range. This is a problem in climates like northern Wisconsin where most of the winter is too cold. The biggest deterrent to using foam was the cost. Foam is expensive! Very, very expensive! Unfortunately, in a competitive market, where too many people look only at the price, it many times is a tough sell to convince the homeowner of the value of the added cost. At NTC, we started looking for a better way. With the cooperation of some very progressive builders we found it!

In the last few years, dense-pack cellulose has become a more desirable insulation option. (See Dense-pack Cellulose - Cellulose reborn). R-19 fiberglass for exterior walls had long been the mainstay. As energy codes stiffened, more insulators opted to use R-21 batting instead. This moved the price difference between dense-pack cellulose and fiberglass batting so close that the move to dense-pack was an easy choice. Since we were already dense-packing the rest of the wall areas, could we dense-pack the box sill too? To make this work, we would need to create isolated, airtight pockets. To do this, is would mean extra work for the builder.

We presented the idea to a couple of our regular builders. We explained that it would mean some added work and materials for them, but the end result would be comparable performance to spray foam. The reduced insulating cost should more than compensate for any added expense. John at Wahlberg Construction was willing to give it a try. We had a plan and a project. Let's see how it works!

How we did it.

The first step was to isolate the areas of the box sill so they could be made airtight. For the ends of the building, that wasn't a problem as the floor joist created the barrier from the rest of the floor system. For the other areas, the builder installed OSB barriers. The bottom of these were set flush with the bottom of the floor joists so the installed ceiling would pinch and seal the cavity. They were also set in far enough to allow access for insulating. This was done prior to any wiring or mechanicals being installed as it is easier to run a wire through a piece of wood than it is to install a piece of wood around a wire.


Step 2. After wiring and mechanicals were installed, we needed to seal any penetrations and potential air leaks. We did this by using a foaming gun with minimal expanding foam.





Step 3. We prepared the areas for insulating. We installed insulating fabric and stapled it to withstand the pressure of blowing.

Step 4. Blow in the insulation. This is the same technique used to install dense-pack cellulose in the walls.




Step 5. Poly over the finished insulation. It's now ready for the ceiling material to be installed.

Some may wonder about the lack of vapor barrier over the OSB. Remember that the main way to stop moisture migration is to stop the air flow. Sealing the penetrations in the barriers creates the primary air seal. Added to that is the virtual air seal created by the dense-pack itself.

Update (11/11/2014)

Recently, instead of using OSB to isolate areas, we have been using XPS foam board.  It installs easier and is inherently a vapor barrier.






For a fraction of the cost of spray foam we have insulated the box sill with a result that rivals the performance of spray foam. Some aspects, such as the ability to flex as the structure expands and contracts, may even out perform spray foam. Though this has not completely eliminated our use of spray foam, it has greatly reduced it. With the reduction of spray foam comes a reduction in cost, giving the home owner more performance for less money. This is now our preferred way of insulating the box sill.

At NTC we never stop looking for ways to improve the product quality that our clients receive. Call us and see what we can do for you.

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