Moisture concerns adding rigid foam insulation to interior of 100 year old home
Hi,
I’m located in eastern Canada, climate zone 6. We have a 100 year old duplex and are remodelling the kitchen on the first floor. We had to gut all the lathe and plaster. Our builder proposed that while we have the walls open we can add some insulation. I brought up the moisture concerns I have heard about but he seemed confident that if we use the right insulation it should be OK and I gave him the go ahead. But now I’ve been doing more research and I’m not sure if it was the right call.
Our walls are these thick horizontal or vertical wood planks. I believe there may be strapping/sheathing on the exterior of that (though not 100% sure) and then brick. There is no vapour barrier on the exterior. The insulation he is adding is “isofoil isofoam”. These are 2″ rigid foam panels. I’m not sure but I think it’s XPS foam with a foil vapour barrier to the inside. From what I’ve read this might be an OK approach since the vapour barrier is only on the inside. But I’m also worried because he didn’t seem to provide for any sort of air gap between the wood planks and the rigid foam panels. And with the couple inches of space that I will be losing I’m wondering if its worth adding at all now.
Note that we recently had the (flat) roof and flashing re-done and the window and its flashing there are fairly new as well. Of course there is no guarantee that there won’t be water penetration from the exterior in the future.
Thank you.
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Replies
The product name including "is0" means that the sheets are polyisocyanurate, one of the foam insulation products that are somewhat less environmentally damaging than other foams. You are right to be concerned; brick walls in a cold, wet climate are vulnerable to getting wet, freezing and spalling if heat flow through the building is slowed too much. I think you'll be ok with 2" of foam, as that will slow heat flow but will likely allow enough heat flow to dry the brick. The foil facer is ok, as you don't have another vapor retarder in the assembly. Take extra care to make everything as airtight as you can get it; up to half of all heat loss is via air leaks.
Thank you for your reply.
Interestingly their data sheet says that "ISOFOIL is a rigid insulation panel of expanded polystyrene with a laminated reflective vapour barrier membrane". Source: https://isolofoam.com/wp-content/uploads/2019/10/Isolofoam_ISOFOIL_TDS_ON-ATL_EN_2016-08_1.7.pdf
So it sounds like it is XPS and not polyisocyanurate?
You mentioned the brick, but I'm wondering if I should be worried about the wood which will be between these rigid panels and the brick. Will it be able to dry to the exterior sufficiently?
Expanded polystyrene is EPS; extruded polystyrene is XPS. EPS is similar to polyiso in its carbon emissions-per-R-value, it just has a bit less R per inch than polyiso.
Whether the wood framing can dry adequately to the exterior is hard to answer. How large are the roof overhangs? One story or two? What is the exposure--are you on a cliff facing the ocean, in a suburban setting or something else?
In Canadian climates, drying is almost always toward the exterior, so the only real change that adding the insulation would introduce is having enough heat flowing through the wall to dry the wood and brick. If the brick rarely gets wet from storms and if there's a gap between the brick and the wood, my guess is you'll be ok.
Its a 2 story semi-detached duplex with a flat roof (so practically no overhang) in an urban setting. The insulation would only be installed on the first floor currently.
I think there is a small gap between the wood and the brick, but not sure.
Would it be important to ensure there is a gap between the rigid foam and the wood to allow for any condensation that forms on the foam to dry without touching the wood?