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.
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Replies
Kikiyut,
The first step is to do everything you can to dry out the narrow alley between the back wall of your house and the retaining wall. The best way to proceed would be to drop the grade at least a foot, if possible. Grade the soil at this level to drain one direction or both -- but not toward the back wall of the house. Then install a layer of landscape fabric, some 4-inch perforated drain pipes leading to daylight, and about 6 inches of clean crushed stone.
Then install a gutter on the back eave of your roof, and connect the gutter to a conductor pipe that carries the roof water far from your foundation.
Your new sill plate will be pressure treated. You might even consider using pressure-treated plywood for your wall sheathing.
It's safe to install rigid foam on the exterior side of your wall sheathing, as long as your wall can dry to the interior. More information here:
How to Install Rigid Foam Sheathing
Calculating the Minimum Thickness of Rigid Foam Sheathing
Rotted sill plates are as likely to be from moisture wicking up the foundation as it is to be from air leaks or vapor diffusion into the wall. You can protect that from recurring with an EPDM (membrane roof material) or metal capillary break between the top of the foundation and the sill plate, but it's always a good idea to deal with the moisture sources too.
Even when building into a hillside there is benefit to grading the surface slope away from the foundation, but you may need to dig in a shallowly buried EPDM "skirt" sloping away from the foundation, as well as a shallow surface drain to move water around and away from the foundation. Ideally there would also be a drain at the footing for pulling water away too.
On the wall stackup, if you're leaving most of the plank sheathing in place, and can't extend the thickness of the wall toward the exterior, by at least an inch, a Mooney Wall approach to thermally breaking the framing on the interior works. On the exterior you'll want to use a crinkled type housewrap (for a better capillary break), detailed as an air barrier, and backventilate the siding, with rainscreen/air-gap to enhance drying of both the siding and sheathing. Vinyl siding is inherently back-vented, but if using fiber-cement or wood shiplap/clapboard, even thin furring strips of 1/4" ply or OSB tacked to the studs to establish the gap works, and offer a huge improvement in the drying rate toward the exterior. With rainscreened siding and 5" of dense packed cellulose the wall is pretty resilient to wintertime moisture diffusion in a 5A climate, and you won't need vapor barriers on the interior.
If you take the exterior foam approach, the thickness and type of foam matter. If you have only an inch, going with unfaced EPS would not appreciably slow the drying toward the exterior into the rainscreen gap. XPS would slow it some, but would still have reasonable drying rates. Foil faced polyiso would be a true vapor barrier, but has sufficient R value to protect the sheathing, provided you stay with the original 2x4 construction (no Mooney-extension.) If you have a couple of inches it's generally better to go with the polyiso (even with the Mooney wall), but at a minimal inch you get better overall drying going with EPS, provided you have the rainscreen gap vented at both top & bottom where practical.
While you have it stripped to the sheathing, take a careful look at all window & door flashing, (which could easily have been a large part of the problem if absent or mis-installed) and be sure to lap it correctly to any housewrap/felt layer on the exterior.
thanks martin for the super fast reply and the wholistic view. it was during the repouring of the concrete for the "alley" that i discovered the rot in the house. its a fearsome steep hill and going down further in the rock to put in a drainage pipe was abandoned for merely gravel and solid concrete rather than the cracked and broken layers there before. i've already connected the gutters to pipes that carry the water around front of the house where it speeds off down the hill. cutting down the weed trees that shaded the alley and filled it with leaves has helped a lot too.
ok, pressure treated sill over sill gasket and termite guard, check.
my main concern is that the sheathing won't adequately dry through 5 inches of cellulose into a humid room. i can buy my tenants dehumidifiers and a mini-split with a "dry" setting, but i can't make them use it. i guess if its well insulated the sheathing won't get cold enough to get very moist?
i am planning to use #15 felt rather than house wrap as the wrb. ok?
and siding: suggestions? cheap and durable. ugly is ok, its the back of the house, essentially buried in a trench. aluminum has been suggested (and then would i need to construct a rainscreen gap or would the space behind the AL siding be enough?
thanks again, i have read far too much and lack the wisdom of experience to put it all together.
Kikiyut,
There is no reason that your exterior sheathing should be wet. If it's wet, you have a problem.
You want to avoid splashback that wets the siding near grade. This is accomplished by installing gutters at the roof eaves -- which you have done -- and by dropping your grade as low as possible, so that the lowest wooden components of your house are at least 8 inches above grade. Twelve inches is even better.
Exterior rigid foam will keep your sheathing warm and dry.
#15 asphalt felt will work fine as a WRB. However, it won't work as an air barrier. Every wall needs an air barrier. One way to create an exterior air barrier is by taping the seams of your sheathing with high quality tape.
Almost any type of siding will work. If you want cheap, install vinyl siding. If you want to go up a step from vinyl, install fiber-cement siding.
dana,
our description of the particulars of what foams do what mirrors what i already had read but it was great to have it presented in the context of what i am trying to do. i was kind of hoping to get away with only repairing one wall, and skip the foam, which it seems is theorietically possible, but it makes more sense to bite the bullet and put on 2in of poly iso while i have it all open.
thanks again to both martin and dana for the thoughtful and detailed responses.
sorry y'all i am staggeringly bad at formulating my questions.
as i read the discussion of foam boards i find myself increasingly uncomfortable with the idea of wrapping this cottage in expensive petroleum products from the dow chemical corporation- especially after reading this evening in two places on this site that "physics trumps chemistry".
is it physically possible to construct a wall that i can stuff with densepack cellulose that will not rot or mold the way the old one has, without expensive brown foamboards? (and, i am open to being told "no!")
i find myself drawn to riversong's descriptions of his buildings' systems, but logistically and financially airtight drywall approach isn't going to happen here. is drywall and caulk a good enough air barrier for such a wall? or will i need poly? if so which side?
i'm trying to find a real world balance between spending all my money on foam boarding and airsealing ONE ROOM in a house with single pane windows, gaps under the exterior doors, bad bathroom ventilation, condensation on icy cold floors in winter etc etc. the absent sill plates are at the top of the list, and while i have the walls open it makes sense to do the best job i can in putting them back together.
thanks again.
Kikiyut,
I just want to say that I am surprised and very happy that you are putting this much thought and care and concern into a rental house. You will end up making it a much better house and the tenant will be very fortunate that you worked on the house.
I'm not in the building field, just interested in making my house and the houses for my family much more energy efficient and comfortable. I've been reading GBA for about 3.5 years. So keep that in mind when you read my advice.
I think the unique features of your building site, the proximity to the hill and less than ideal drainage (that you have already improved) actually make foam a good choice. I say that because the backside of the house on the hill is such a reservoir of moisture, I think you actually reduce the risk of moisture issues by using the foam. Since cellulose is hygroscopic, I think without the foam, it could end up functioning as a moisture reserve itself.
You could use a mineral wool product on the exterior. I'll be using Roxul Comfortboard on a house we'll be building soon. It is insect resistant, fire resistant and still has some vapor permeability (haven't looked that number up though), but I doubt you would save any money by using it. I would guess it might actually cost more and installation is trickier.
Just thought of another option - rather than use cellulose on that wall, why not use a mineral wool insulation? The regular mineral wool insulation is not hygroscopic. Make the air barrier at the level of the sheathing at least on that troublesome side of the house. You're not going to have a solid continuous air barrier in a 1959 home unless you totally gut the home.
I also like the idea of a Mooney wall. You could use a smart vapor retarder over the studs, air seal at that level with the vapor retarder (specific brands are Membrain and Intello Plus) before you put the second layer of insulation in. Then it doesn't matter what the tenants do to the drywall.
I get what you're saying about how Robert Riversong builds homes, the idea has a lot of appeal, but this building setting is very different from what Riversong would choose for a new build.
If you keep the sheathing from getting wetted excessively from the exterior (= rainscreen + crinkle-type house wrap- sorry, but it's mostly polyolefin, but it's a superior capillary break than felt, and allows an order of magnitude faster drying), and stop water from wicking up from the foundation into the foundation sill & beyond (= a solid capillary break between the concrete & wood, either copper or EPDM, pick your poison) a dense-packed Mooney Wall will perform well both from a moisture & thermal point of view, without the need for a lo-perm interior side vapor retarder (latex paint 'll do ya.)
Using mineral wool would guarantee that the wintertime moisture drives would be harbored in the sheathing, whereas with cellulose it's shared.
A key part of getting away with a Riversong style exterior side stackup is the amount of roof eave/rake overhang of his designs (minimizing the splash back) and using copper flashing as the capillary break between foundation & sill. Both are essential. But he doesn't use a rainscreen, and you have that option, which will work pretty well at any roof overhang depth, even in wind-driven temperate rain forest environments like Vancouver B.C. (where 9mm rainscreens are now required by code.)
You'd be nuts to put poly sheeting anywhere in this assembly. With a rainscreen and dense pack the amount of air transported moisture into the assembly is miniscule, and maintaining good drying capacity in both directions offers more resilience than hazard.