Improving R-value of 2×4 wall in renovation with interior foam board
Hi gang,
Doing a renovation in MA, USA, and I need a little help… There’s a 2×4 wall that needs insulating, and I’m not redoing the exterior of the building. It would be easiest for me to get the wall dense-packed with cellulose, then go over the whole thing with 2″ foil faced foam… but I have a couple questions…
1] Is that legit from a ‘dew point’ perspective.. I mean you want the foam on the outside of an assembly like that, right? Or is that such a winning combination, with such a small amount of humid air moving through it, that one need not worry about condensation?
2] How do you go about drywalling that? Aren’t you supposed to have the drywall tight up against the insulation these days? The easy thing would be to install 1 x 3s over the foam and drywall that. does anyone drywall with 3 1/2″ drywall screws? That sounds insane. But I’ve done crazier things.
3] I frame out a deep wall for the cellulosers to insulate, but I am too lazy.. maybe this would actually be easier than dealing with the foamboard / strapping / drywalling conundrum?
Thanks in advance!
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Replies
Eric,
You can proceed with either approach: you can either build a double-stud wall if you don't mind losing the interior space -- packing the space between the studs with cellulose -- or you can install an interior layer of foil-faced polyisocyanurate.
If you use foil-faced foam, and you decide to install interior 1x3 or 1x4 strapping, you'll get an extra R-2 from the air space with foil facing on one side.
It's possible to install drywall directly over the foam if you want (although some drywallers don't like using long screws, and don't like the somewhat squishy feel of the foam). I've done it.
It's also possible to install interior furring strips (with the added advantage of the extra R-value if you have foil-faced foam).
Of course, you have to know where to set your electrical boxes -- so you need to make a decision.
Interior foam won't cause any moisture problems in your climate, although it makes it hard to do a good job of insulating rim joist areas and partition intersections -- both of which are better insulated if the rigid foam is on the exterior of the building.
Martin has given you the best advice. One difference I have with the outsulation folks is that inner insulation is not difficult. Partitions are easily blown with cellulose in one bay adjacent to the exterior wall as well as it is easy to insulate the interior of the rim joist.
Exterior walls need to be cold or warm. It's the in between temperatures that grow mold and induce rot along with all the other points made here at GBA.
Build your walls cold or warm and build in an air barrier and at least one direction of permeability. That's it in a nutshell.
Martin said..."It's possible to install drywall directly over the foam if you want (although some drywallers don't like using long screws, and don't like the somewhat squishy feel of the foam). I've done it."
I have seen mold between the sandwich caused by improper ventilation techniques. If your not managing the RH in your home 24/7 I advise against this practice, especially in wet rooms. (kitchens and bathrooms)
Eric,
I'm a hardcore DIYer not a pro. I want to help you avoid my mistakes :) I did a single wall of a room once during a window re-sizing (before I knew better). I cut and cobbled poly-iso into the bays (tedious, not recommended) and put one thin layer of polyiso in-board of the studs. This was a silly way to do it and not recommended, btw. But the lesson for me that I want to pass on to you is to think about how you are going to insulate things like framing next to windows and three studs corners. If you don't insulate these things, your extra insulation will get 'short-circuited'. If you do insulate those areas, it would be best to think about how you are going to affix drywall to them. If your drywaller likes corner bead, they could end up cursing you.