Fiberglass Insulation around blocking
Monte_Main
| Posted in General Questions on
We are remodeling a very small bathroom, stripping down to the studs (1950s 2×4 construction in climate zone 5A – Ohio). The pedestal sink is against an outside wall. The sink calls for 2×6 blocking in the wall. There will also be 2x blocking for shelving. The house had no insulation, we are adding fiberglass bats in the outside wall and air sealing. How do you put insulation behind the blocking? Do I use 2″ polyiso behind the blocking? (R-13 for 2″) If so, should I use spray foam to hold the polysio in place? Or just the pressure of the fiberglass against it? Any recommendations for spray foam to use?
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Replies
amainiii,
You can use foam board behind the blocking, or you can just cut a chunk out of the batts to fit around it. Considering that the studs are a complete thermal bridge every 16", the reduced insulation at the blocking is pretty incidental.
The usual way to do this is to just cut/notch the batts to fit behind the blocking, similar to what you'd do to get around an electrical box or other obstruction that isn't the full depth of the cavity. With some care, you can get a good fit here, and do a good job insulating.
If you have a number of things to cut around, you may want to consider using mineral wool instead of fiberglass. Mineral wool is easier to cut and fit into place, because it's a bit stiffer and has a more solid-like "form" to it. This is especially helpful to DIYers that may not have lots of experience fluffing fiberglass batts to get a good fit. If you're only insulating a relatively small area, the cost premium for mineral wool won't be very much in absolute dollars terms, and you get the added bonus of some extra R value in the same space too.
Bill
Yes to mineral wool, for all the reasons Bill stated. Much easier to work with than fiberglass.