Rigid Insulation Retrofit and Vapor Barriers
I’ve spent hours researching my new wall detail and I’m down to one question, should I tape the 2″ XPS Rigid Foam or not.
Background:
I am in Rochester NY (Climate Zone 5)
I have an existing home where I am taking the exterior down to the studs. I’m adding 1/2″ OSB to the exterior (none now), 2″ of XPS Rigid, Housewrap, 1×4 strapping and then a mix of cement siding and vinyl. The existing walls are 2×4 with R-11 Kraft Faced.
Should I tape the seams of the Rigid to create a secondary solid drainage plan? My only concern is that this will not allow the wall to dry to the outside at all (I know 2″ of XPS is not very permeable, but I figured some vapor could escape through the untapped seams). I’m concerned about the drying as I’m planning on the wall to dry to the the inside but the existing batt insulation is kraft faced and this may limit the drying potential?
On a side note, this site has some great information. Thanks in Advance.
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Replies
Matt,
Tape the seams to improve the airtightness of your wall. Your wall is designed to dry outwards from the exterior face of the rigid foam, and to dry inwards from the interior face of the rigid foam.
Most XPS is not rated for use as a drain-plane/weather resistant barrier but some products are, provided you follow the manufacturer's instructions. But detailing it as an air barrier (as well as your detailing your OSB for air tightness) counts.
If you're not pulling the kraft faced batts adding 2" of XPS to the exterior gives it very little drying capacity, but some, in either direction. Going with unfaced EPS at 2.5" rather than 2" XPS for the same R value, you would still have excellent drying capacity toward the exterior- at least 4x that of 2" XPS. (Also, XPS has over 100x the lifecycle greenhouse gas potential of EPS, due to the HFC blowing agents used. EPS is blown with pentane, which has only 7X CO2 greenhouse gas potential, compared to the typical HFC134a used for blowing XPS which is over 1400x CO2.)
It's worth drilling the OSB and blowing cellulose into the cavities compressing the batts to achieve higher air-tigthness and a more complete cavity fill before installing the housewrap, plugging the fill-holes with 1-part foam. It sometimes takes 3-4 holes in a stud bay to get a decent average density but it's difficult to weasel in a "dense packing" hose to crank it up to 3+ lbs per cubic foot. But even 2.6-2.8 density cellulose is a huge improvement in air retardency over R11 batts, and when blown under pressure the fibers find and plug all the leak points, even if not fully dense-packed.
Using air-leaks that thermally bypass the rigid foam to relive moisture is pretty silly, and may introduce enough summertime air in to create mold conditions on the kraft facers. Air-tight is good, control the moisture flow via diffusion only whenever you can.
If you are going to all that trouble, pull out the fiberglass and have cellulose blown in before the OSB goes up.