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Retrofit insulation in 1970 rambler

jmalarke | Posted in General Questions on

My current home is a rambler built in 1970.  I’ve been working on air sealing, new windows, and insulation upgrades, and now it is time for the exterior walls.  A few details:
Climate Zone 7 (Fargo, ND)
1000 square feet over a full basement plus a 250 square foot addition over a well insulated, sealed, crawlspace.  Hot water baseboard heat with a 85% efficient natural gas boiler runs me about $500 a year.  Two mini-splits for AC.

The exterior walls consist of(inside to outside):

1/2 drywall
4 mill poly
Foil faced R11 fiberglass insulation
“Drywall” exterior sheathing (non structural, has let-in bracing)
1/2″ polyiso insulation
Vinyl Siding

The walls have been this way for more than 15 years. Based on my reading, it looks like I’ve got (at least) two vapor barriers.  I’ve had the walls open in a couple of places, and there is no sign of moisture, rot, mold or anything due to trapped moisture.  I pulled out some fiberglass(from the top) in one cavity during another project and it seems pretty easy to get most of the fiberglass out, but the foil facing is much harder.

I’ve considered a few options:

1. Dense-packed cellulose (relatively inexpensive, some air infiltration and noise reduction benefits)

2. Foam (Injection, slow-rise, ?) of some type.  (expensive)

3. Tear off interior drywall and reinsulate with ? (my wife would probably kill me)

4. Leave it alone.  House is reasonably comfortable as is.  I would like to to do something for fresh air exchange, but I’ll save that for another post down the road.

5. Tear off the siding, polyiso and sheathing, insulate with ?, then reinstall.  If I went this route, I would want to install R10 (or higher?) foam insulation on the outside, but then I need to figure out what to do about the siding, since the pieces would be slightly too short to just reinstall.  The windows would also need some effort. If I go this route, I would tackle it myself, so labor isn’t really a factor, just materials.

My big concern is that if I modify this situation without going with option 5,  the vapor barriers could lead to moisture problems. 

Any advice or options I haven’t considered?


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