Wall design in renovation – with moisture issues
Sorry in advance for the long question. it seems important to establish the full context because i’ve been unable to match my circumstances to the thousands of articles i’ve read here and other places.
Zone 5a/6a border (Hudson valley, ny) 1959 slab on grade, standard 2×4 framing single story house.
It’s cut deeply into the hill, and the entire back of the house is 2 feet away from a 3-5 ft high retaining wall. ugh.
The house is environmentally almost a walk in basement. high humidity levels, and, no surprise, mold. solving the moisture will hopefully help the mold issues.
An important consideration: this house is occupied by a tenant. i dont see an airtight drywall approach as being viable, besides the expense and finicky detailing, when its all done and then a tenant comes in and pokes holes in the walls hanging pictures or whatever…oh oh oh. this isn’t luxury housing and i’m aiming for affordable rent. Passivhaus is nice but on a $750/mo rental sometimes good enough has to be good enough.
Here’s the good part: while redoing the concrete that separates the house from the retaining wall, i smelled mold and opened up the siding to find the sill plate is rotted to compost, basically across the entire back of the house. i see this as a great opportunity to completely rebuild the wall with better insulation and moisture management. i am highly sensitive to not repeating the mistakes of the past (last course of siding buried in concrete and covered in asphalt so that moisture would not ever escape, etc).
I hired a ‘green’ contractor for a consultation. he went on about poly sheeting behind the drywall, and spray foam. when it came out that this is not my primary residence and its not eligible for expensive subsidised upgrades he kind of checked out.
OK: i need a wall assembly that will be able to dry in what is a very moist environment. i have read about rainscreen gaps and this seems absolutely necessary here. i have read about the risks of cold sheathing, and i get it. but i dont know what to do about it. the recommendations i read here seem to be placing rigid foam over the sheathing. will that not then make it difficult to impossible for the sheathing to dry to the exterior, especially if i am using this as an air barrier (taped seams etc)? i seemed to read something that one can place the vented rainscreen gap between the sheathing and the foam boards but then the foam is no longer an air barrier and i would think the sheathing is no longer sufficiently insulated. and if i omit the foamboard, code seems to call for class I vapour retarder on the interior. what to do?
For the record: current sheathing is pine boards, only the bottom couple of courses need replacing, was thinking of plywood for that. intending to go with densepack cellulose for insulating the stud cavity. considering adding crossbracing on the interior (‘mooney wall’) to reduce bridging and add a couple more inches of cellulose. its the back of the house and basically invisible and inaccessible (and nearly below grade) and i can re-side it with anything i want to.
So to sum up my question: i need a cross-section design of a wall i can construct for this situation. i have looked at dozens on this site and others, and either didn’t see it or saw it and didn’t recongnise it.
Thanks in advance for your advice and insight.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part