Replies

1. 
   walta100 | May 08, 2020 04:57pm | #1

   Going from the current R2 of the wood to an R9 of the wood and 1 inch of foam is a huge improvement.

   I am guessing that is about a 40-50 % change in heat loss from walls.

   If possible the best bang for your bucks is to blow the walls full of cellulose insulation before siding. The no go problem would be knob and tube wiring.

   You may want to skip the foam and just do the cellulose and a house wrap being very careful around all windows, doors and pipes that water is flashed to the outside and can't get behind the house wrap.

2. 
   Insulationnoob | May 11, 2020 08:38am | #2

   Thanks for the advice, I've been thinking going the cellulose route as well! will do a little more research on it!

3. Expert Member
   BILL WICHERS | May 11, 2020 10:48am | #3

   Note that if you have the walls open anyway, it’s not that difficult to replace any old knob and tube wiring. I wouldn’t use “I’d need to rewire it” as a reason to avoid upgrading your insulation if you already have the wall open.

   Bill

4. Expert Member
   Dana Dorsett | May 11, 2020 11:29am | #4

   >"...want to add 1 inch of XPS (R-5) insulation..."

   Read the fine print. An inch of XPS is only warranteed to R4.5 @ 20 years, and at full depletion of it's HFC blowing agents would drop to R4.2, the same as EPS of similar density.

   Those HFCs are all extremely powerful greenhouse gases, making XPS by far the LEAST green insulation in common use in North America. The predominant component in the mix is HFC134a, with a CO2e of ~1400x CO2 @ 100years. See:

   https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

   A much greener solution would be 1" polyisocyanurate, (R6-ish) or graphite loaded EPS, either of which is blown with much lower impact hydrocarbons, predominantly variants of pentane with a CO2e of ~7x CO2 @ 100 years. In the case of EPS most of the blowing agent leaves the foam and is recaptured at the factory, not vented to the atmosphere. With polyiso some of the blowing agent is retained for some time, especially with foil-faced goods, but in neither case does the blowing agent affect the long term performance of the foam.

   An inch of EPS is and about 3x as vapor permeable as XPS, and even more vapor permeable than standard interior latex paint, and would offer superior drying capacity toward the exterior compared to foil faced polyiso. There are vapor permeable versions of polyiso, but those are still more vapor tight than unfaced EPS (graphite loaded or standard).

   In zone 6 an inch of EPS or polyiso isn't sufficient for dew point control at the structural sheathing layer, but filling the stud bays with cellulose and using "vapor barrier latex" primer on the interior would be sufficient to protect from wintertime moisture accumulation without a polyethylene vapor barrier. The cellulose buffers and distributes a major fraction the seasonal moisture burden, keeping the moisture content of the sheathing and studs lower.

   A typical uninsulated 2x4 studwall comes in at about R4-R5 "whole wall R" after factoring in the thermal bridging of the framing, and adding in the R-values of the interior & exterior finish layers & air films (including the air films inside the empty cavity). An inch of foam would roughly double the performance of the wall, provided the wall is truly air tight, but if it leaks a lot of air it won't be quite as effective. For a 1940s vintage house it's worth it to use a fully adhered weather resistant barrier (Vycor, Blueskin, et al) on the plank sheathing for air tightness. This becomes even more important if not dense packing the cavities with fiber insulation.

   A fiber-insulated 2x4 studwall (no foam) comes in at about R10 "whole wall R". Adding the inch of foam would bring that up to about R15, which would be about a 2/3 reduction in heat loss from the walls, and will have a noticeable comfort improvement from the higher wintertime temperatures of the interior facing wall surface.