Permeable structural exterior sheathing?
Hello Everyone
I have a 120 year old home with original wood lap siding that I’m remodeling. The home has been gutted to the studs. I’m planning on doing blown damp cellulose for insulation inside the home. I’m also planning on redoing the siding with something like LP Smartside or Hardie board, but do not want to remove the original siding if I can prevent it.
My question is what, if anything, can I put between the original lap siding and the new siding that will help add some R value but will not trap moisture against the original siding? Is it feasible to add another layer of insulation to the exterior between the original siding and new siding? I’d like to create an air barrier from the outside if possible, and am concerned about moisture getting trapped against the original wood siding and causing issues.
I live in northern KY. I’d probably be fine with the insulation added from the blow cellulose, but would like to take advantage from the outside if possible if I’m going to be redoing the siding. Thanks in advance for any advice.
RS
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Replies
RS,
Installing foam over the original siding is not ideal, for a number of reasons. Think of all the air spaces that will be running behind the foam. It will be difficult to air seal and each one of those air spaces will be diminishing the foam's effectiveness. Do you know if your house has sheathing behind the siding?
Thanks for the reply Brian. I haven't removed the existing vinyl siding yet. I have peeled the siding back in a few areas, and can see a layer of very thin foam (about 1/8 inch) sitting on what appears to be some sort of asphalt shingle material. Right below that is the original wood siding. There does not appear to be any sheathing like OSB or plywood.
Ron
In US climate zone 4A northern KY you can install vapor-impermeable insulation on the exterior without risk of excessive moisture build up, as long as the interior side finish is between 1-10 perms (a Class-III vapor retarder), such as standard latex paint on wallboard. Even foil faced polyisocyanurate board is not a problem, as long as the window flashing details and drain plane are attended to.
Is the original siding shiplap or wideboard plank, with the plank surfaces co-planar, or is it clapboards, overlapping each other with a slight angle? If it's structural and reasonably flat, leave it!
Dana,
Thanks for the reply. The original siding is overlapping and not flat on the exterior. The siding company I had come take a look suggested placing 1/4in fanfold over the original siding exterior to obtain a smooth surface to place the new siding over. There will be an air gap between the back of the original siding and insulation within the stud cavity, so the siding can dry to the interior, but may have trouble drying to the exterior if fan fold is right adjacent. But maybe not. This my question. I tend to want to plan for worst case scenario if possible. Thanks again.
Again, in your climate zone it never needs to dry toward the exterior as long as it can dry toward the interior. You're trying to fix a problem that doesn't really exist, assuming the bulk water(flashing, drain plane, WRB) is being managed reasonably.
1/4" fan fold won't do much for R-value, but the air gap will provide an additional capillary break and a drying path (and perhaps a thermal bypass of the ~R1 of the fan-fold itself), keeping any liquid water that blows past the new siding from ever reaching the old siding.
A 2x4 /R13 +R1 fan fold wall does not meet current IRC code minimum for zone 4. (it misses by quite a bit, in fact.) If there is a structural sheathing layer under that old siding the performance of continuous 3/4"-1" polyiso taking up that layer of the stackup would be enough to bring it up to code min, and would keep the sheathing warm enough to never worry about wintertime moisture condensation.
Another way to bring it up to code performance (on a U-factor basis) without stripping the siding would be to install 1" (R6) polyiso edge strips on the interior side of the framing and install 4.5" of cellulose (R17-ish) instead of 3.5", long-nailing/screwing the wallboard through the 1" foam. With the R-values of the extra layer of siding, additional air-films and the R1 of the fan fold it would duck under U0.060 (=R16.6 "whole assembly").