Replies

1. GBA Editor
   Martin Holladay | May 05, 2015 11:50am | #1

   Tracy,
   In most cases, insulating your basement wall will not cause your sill plate to rot. However, it may tend to increase the risk of that happening, so this is a judgment call.

   Factors that lower the risk:
   1. If your sill plate is now solid, with no signs of being punky.
   2. If you have no bushes on the exterior of your house that limit air flow and block sunlight.
   3. If the soil around your house is relatively dry rather than damp.

   Factors that raise the risk are the opposite: the sill is now punky, your house is surrounded by bushes, and your soil is muddy or damp.

   A few other points:

   1. If your concrete blocks are hollow and unfilled, you should shove some wads of fiberglass batts down the voids and cap each block with mortar or concrete.

   2. The 2012 IRC (building code) requires a minimum of R-10 insulation for basements in your climate zone. Your plan to install R-5 insulation is less than the code minimum.

2. 
   going_green78 | May 05, 2015 12:11pm | #2

   Thanks for the quick response Martin. The sill plate is solid and shows no sign of rot and I have just regraded the exterior to make sure water is draining away from the house. I have planted a few small bushes around the house and just put down rock instead of mulch. All of the downspouts are piped away from the house a minimum of 12 ft.

   I would love to fill in the block, but the sill plate covers the entire top of the block. You can only see they are unfilled by looking through the crack between the plate and top of the block. Is there any way to fill the blocks in a situation like this?

3. GBA Editor
   Martin Holladay | May 05, 2015 12:26pm | #3

   Tracy,
   Filling the cores of the blocks limits convective loops (air movement in circles) as well as air infiltration. If you don't have easy access to the cores, the next-best approach is to make sure that you have sealed the crack between your wooden sill plate and the CMU wall with high-quality caulk.

4. Expert Member
   Dana Dorsett | May 05, 2015 03:03pm | #4

   It's expensive, but filling the blocks with non-expanding injection foam (CoreFill 500, TriPolymer, etc.) can be done by drilling a 1" hole in one block of each vertical core cavity, which will also raise it's effective R-value from about R1.5 to R3(or more, depending on the size of the blocks.)

   With the blocks empty the capillary draw is far less than a poured concrete wall, and there is also a cavity for the block to dry into. Filling it with fiberglass increases the R-value slightly (less than injection foam), but increases the moisture wicking somewhat, though not enough to cause a problem.

   In termite zones it's sometimes important to fill at least the top block with concrete to keep the core from being used as an insect superhighway, but sheet metal slipped between the foundation & sill is just as effective.

   Using 1.5" of foil-faced polyisocynurate foam would get you to the code-minimum performance, as long as you maintain the 3/4" trapped air space between the facer and your wall gypsum. It's a lot greener too, since it's blown with a much more benign blowing agent. It has a higher kindling temp, and chars in place rather than melting the way polystyrene does. The labeled R value would be R9-R9.5-ish, but with the air gap & foil it adds a legitimate ~R1 to the performance. Foil facers are also easier to air seal reliably using a temperature rated foil tape (eg Nashua 324a, found in most box stores), and CMU foundations are a lot leakier than they look from an air-tightness point of view.

   With polyiso it's good to keep the bottom edge off the slab though, since unlike polystyrene, it will wick moisture over time if in contact with a damp concrete surface. Even 1/4" of air space between the bottom edge and the concrete is a sufficient capillary break.