Replace, clean, or encapsulate old sheathing, and ok to seal both sides?
Hot humid southeast here.
I’m starting a big renovation on a 1950s house and plan to do (2) 3/4″ rigid foam with air gap on exterior with hardie planks. On the interior, I plan to do ~2″ of CC SPF to help air/bug seal and strengthen the old sheathing.
The sheathing is old 1-by pine on the diagonal. There appears to be 70 years of cockroach activity 😬 among the tar paper and sheathing behind the abestos siding. Why do people say 50’s houses are well built. 🤨
Question: should I just replace sheathing and clean up 2x4s? Or is encapsulating it with CC SPF a good option?
Perhaps with the money saved from keeping the old sheathing should I upgrade to rockwool comfortboard so the sheathing can breath?
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A full fill of open cell foam does a better job of air sealing than 2" of closed cell, and uses less polymer than 1" of closed cell, and it allows the assembly to dry toward the interior. Diagonal plank sheathing is pretty structural (way better than horizontal)- just not air tight.
Save the high R/inch budget for exterior continuous sheathing under your new siding. A 3/4" layer of foil faced polyiso on a 2x4/R13 studwall would meet code min in US climate zone 3, and would beat code in zones 1 & 2.
Dana, thanks for the quick reply. However, you didn't address my concern of the roach poop. 💩 The attached picture doesn't show the worst of it (mostly along top plate). Sorry, I should've been more specific about my concerns of the existing sheathing causing allergen problems if it is drying to the inside.
Hi Jarrett,
Drying shouldn't cause anything airborne to get into the home. That would be the result of air infiltration, which you prevent by air sealing.
I don't know the precise size of a molecule of cockroach poop allergen (I'm not even sure it is an allergen, but let's assume). But the smallest of allergens have a particle size of around 2 microns (20 microns for a typical dust mite allergen particle). A molecule of water vapor is about 2.75 angstroms. That is, a water molecule is 0.000275 microns.
So, I think it's safe to say, "drying to the inside" has absolutely zero relevance to the ability of an allergen molecule ten-thousand-times larger to pass through the same foam.
Hi Jarrett,
Unless your sheathing is damaged, I don't think you need to replace it. Another option for you would be to use a fully adhered water and air barrier on the exterior of your sheathing, beneath the polyiso foam that Dana suggested. Do a good job of integrating the WRB with flashings and you'll have solid water management and air sealing. Then you can use any number of fibrous insulations in the stud bays. Pay close attention to air-sealing the drywall and you have nice wall assembly without closed cell spray foam (most of which is not-so-environmentally-friendly).
I added pictures above to show that the 💩 particles are already *in* the home on the 2x4s and back of the sheathing.
You guys keep missing my concern. I might as well use dirty diapers as the fibrous insulation in the stud bays.
Hi Jarrett.
I am no expert when it comes to this type of remediation, so, I'll say it like this. If it were my house, and I knew that I had solved the roach problem, I'd do my best to clean up the feces, and be comfortable insulating and closing up the wall without doing anything special to encapsulate the staining. I would be doing everything I could to air seal all six sides of the stud bays and in that way no (or little) airborne concerns would be reaching the interior. Your comfort level (and sensitivity, as Jon points out below) may be different and if that's the case, you may want to consult an expert of replace the sheathing.
Cleaning out the cavities and perhaps applying insecticide vs spray foam depends on how sensitive the occupants are.
I'd use unfaced EPS (moderately permeable) on the exterior - mineral wool is expensive and more perms than you want.
A blast with a steam cleaner would remove most of the particulate matter, a light spray down with a bleach mixture would sterilize it and remove most of the staining.
Air sealing it with open cell foam would pretty much lock it into place anyway. With no air moving through the wall (only water vapor and other tiny molecules) there isn't much stuck to or soaked into the studs & sheathing that will make it into the indoor air.
Wow, great community here! You guys rock.
Current plan:
* Clean and sterilize stud bays (mainly for peace of mind)
* Repair bottom of sheathing with plywood as needed (see pictures)
* Bora care on studs, sheathing, band joist
* Diatomaceous Earth puffed into any cracks and crevices
* Follow GBA 9-00301 for insulating band joist with spray foam in crawlspace
* Follow GBA 9-00401 for exterior insulation and WRB
* Consider prosoco foundation -> WRB
* Follow GBA 9-00603 & 9-00605 for windows (also airgap for exterior siding)
* Add bug screen at top and bottom of air gap
* Stud cavities ~2.5" CC SPF (already have DIY experience with CC)
* Vented or open soffit
* No vented attic, CC SPF directly under roof decking (still need a thermal break somehow)
* Connect attic insulation to exterior rigid foam for complete air barrier
* Encapsulate crawlspace, 10+ mil over dirt, rigid foam on walls (no termite gap, going to rely on bora care)
BTW, my design is adding a half story but it uses shed dormers to get the 8' ceiling height.