Exterior insulation over siding w/o sheathing
Hi all –
New to the forum, so apologies if this is already out there in an article or another thread (I looked but couldn’t find anything specific).
Bottom line first: can I add exterior insulation to a home with “structural siding”, i.e. no sheathing?
I’ve got a four square in Richmond, VA (CZ4) built in 1920. Today, as I was cleaning up the basement rim joist in preparation for the froth-pak I just picked up, I found a small section of rot near a corner. The mud sill is a bit compromised, but it’s mostly the siding. I went outside and started poking at the siding and a couple of square feet started crumbling.
Sorting that out isn’t a huge deal, but I was surprised to see that the siding is proving my shear strength. I know it’s a thing that was done, but didn’t think my home is that vintage. I’d been planning to eventually add exterior insulation when I reside and replace windows.
Now I’m not so sure. Can I just “re-sheath” with OSB over the old siding, apply a WRB, rain screen, and reside? Seems logical, but that’s a lot of added weight.
The plaster walls are in pretty decent shape, so I don’t want to tear them down to studs. I’m also a bit hesitant to dense-pack the cavities – Richmond can get fairly wet and very humid.
Input welcome, and thanks very much!
-Kevin
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Replies
What do you currently have for siding? Some things, like T1-11 can act as structural sheathing and you can put rigid insulation over them if you want. Other things, like vinyl siding, need to be removed before you put up rigid foam.
You’ll want to fix that rot before you go insulate everything. It might just be your exterior siding degraded and let the weather in, but you might have an issue with water wicking from the foundation in which case you’ll need to add a capillary break.
You don’t want to seal in the rot until you fix the cause because you might end up making it worse.
Bill
Thanks for the quick reply!
I purchased the house at the end of last year and it seems that there were some drainage issues at a few spots around the brick foundation in past years. I immediately rerouted the downspouts and added a drain tile about 24-30" below grade (didn't want to go much deeper and risk destabilizing the foundation wall). The basement has been completely dry since I arrived and the rotted wood did not appear to be moist at all. So hopefully it was just some poor maintenance and not a wicking issue. I found a couple of small old roots reaching up the wall cavity, so it seems like some bad landscaping conspired to prevent drying.
Regarding the siding: I think it's referred to as novelty shiplap with a double cove profile. Example is attached. Roughly 3/4" at the thickest point and 1/2" at the thin point of the profile.
Would a competent builder/remodeller be able to pull off the old siding in sections and replace with modern sheathing, so as to avoid removing the shear strength from too much of a wall at one time?
Make sure you have the grade sloped away from the house and make sure the ground is sufficiently lower than the siding too. Sometimes people add topsoil near the house for gardens and that can cause problems if it brings the level of finished grade up too close to the wood parts of the home.
I don’t think that style of siding is adding much shear strength. Normally you need diagonal bracing or large panels (like an uncut sheet of plywood) to provide racking resistance. I don’t see any reason a good builder wouldn’t be able to remove what you have, but you’re almost guaranteed to have at least a few of the original pieces get broken or at least damaged in the process.
Bill