Exterior foam over interior FG w/ foil face?
sapwood
| Posted in Energy, Heating & Insulation on
I’ve been planning on installing exterior 2″ XPS over existing siding then adding a rainscreen. The existing 2×4 walls have foil faced fiberglass insulation already. Ideally I would remove the foil faced stuff (and this is possible) but removing the sheetrock is problamatic. Foil facing isn’t the ideal vapor barrier or retarder but will it be enough of one to cause a problem… that is, retard drying to the interior should that be necessary?
Alternatively, I could use EPS foam for a better drying time to the exterior.
I’m in the PNW… lots of rain in the fall and winter. Hot dry days in July and August. June and September aren’t bad either.
Replies
Will the foil facing on fiberglass batts cause problems?
Sapwood,
In most cases, installing exterior foam on a house like yours will not cause moisture problems, as long as you get your flashing details right. Including a ventilated rainscreen gap will greatly lower the chance that any moisture will enter from the exterior.
It's fortunate that you have foil-faced batts rather than polyethylene, because the foil-faced batts allow more drying to the interior.
Of course, if your existing siding shows any signs of water problems or rot, you shouldn't proceed with your plan unitl you investigate whether your existing wall has a water-entry problem. All existing flashing errors must be repaired.
Martin,
Thank you for responding. I've got definite water entry problems from the exterior. They aren't severe and can be remedied. This remodel will entail moving and replacing nearly all of our windows which will then require quite a bit of siding repair. Fortunately (I guess) we dislike the siding and will take this opportunity to cover it over with new stuff. I love the rainscreen approach and did this on a new shop a few months ago. We will be rainscreening the house too.
If you don't think the foil facing will cause me any long term harm, leaving it in place along with the drywall covering it, will save me a lot of heartache.
I've read several articles that you've written regarding exterior foam and window placement. Those have been a big help. Thanks.