Insulation and air barrier
I’m starting a major remodel on a 1972 ranch home. SInce I am changing room locations and door and window locations most of the drywall on the exterior will be compromised. Using the DIRTFS (do it right the first time) theory I am just going to remove all of the drywall and replace the insulation with rock wool. I just finished reading about vapor barriers,and air barriers here and there is blood seeping from my ears.
It seems that you are saying more water comes into walls from the exterior then from the interior. So I have a strange question. Since my walls will be opened up, and I will be able to see the exterior ply sheathing from the inside why wouldn’t you spray a vapor retardant paint on the inside of the wall and then insulate. Could you not then keep the outside of the exterior sheathing dry with polyiso or a combination of a rain shield with polyiso.
Please stop rolling on the floor and laughing. I’m new.
Dan
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Replies
If you put a vapor retardent paint on the interior side of the sheathing it limits the ability of the sheathing to dry into the cavity, making it even more susceptible to exterior moisture drives, and less resilient against bulk water incursions.
Polyiso with foil facers is a true vapor barrier, which would block all drying toward the exterior. Polyiso with asphalted paper or fiberglass facers would allow some drying toward the exterior, but it would be extremely slow. A rainscreen is a good capillary break between the siding and the interior layers, but it's not a drying path for the sheathing if there is a layer of polyiso between the sheathing and the air gap.
The most important thing for keeping the sheathing dry is to allow a fairly free drying path in one direction or the other (or both). The next most important thing is to keep it's average wintertime temperature warmer than the dew point of your conditioned space air (presumed to be 38-40F.) The average temperature of the sheathing is a function of both your climate, and the ratio of exterior insulation to cavity insulation. The more exterior-R you have, the warmer (=drier) it will be. The climate clearly matters, and the MINIMUM exterior R-values necessary to be somewhat protective when the interior side vapor retarder is standard interior paints (with a reasonably free drying path toward the interior) is spelled out in the IRC chaper 7:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
Those are minimums. More is clearly better. Less is a bad idea, but can sometimes be mitigated with smart vapor retarders.
Dan,
There are two ways you can help keep your wall sheathing dry.
Number one is to make sure that you have a ventilated rainscreen gap between your siding and your sheathing (or between your siding and your WRB). This gap limits the entry of wind-driven rain, and the gap also helps damp sheathing dry faster. If your house doesn't have that gap, you would need to remove your siding to add the gap.
Number two is to install an adequate thickness of rigid foam insulation (or mineral wool insulation) on the exterior side of your wall sheathing. Warm sheathing is dry sheathing. The rigid foam is especially helpful in winter, the season when sheathing tends to accumulate moisture. This retrofit job also requires you to remove your siding.
If you are only working from the interior, your options are limited. If you are opening your walls from the interior, the best way to protect your sheathing from moisture is to install a layer of closed-cell spray foam on the interior side of the sheathing, using the flash-and-batt method.
Here is a link to an article with more information on this approach: Flash-and-Batt Insulation.