Insulation for exterior walls with no sheathing


Old 12-10-12, 12:46 PM
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Insulation for exterior walls with no sheathing

We're renovating a house built in the 1850's that has cedar siding nailed directly to the studs. No Sheathing.. no tar paper.. no insulation.. nothing. Taking the wood siding off is not an option. We have completely stripped the interior down to the studs. All the studs look in good shape, no wood rot or water damage. So our question is what would be the best way to insulate?

Without the sheathing we are pretty worried about moisture. Since it's just the wood siding there is a bit of rainwater seepage. We have been researching this for quite a while and have seen various problems with everything. We have been told.. by various people.. that cellulose couldn't handle the rainwater seepage, closed cell foam spray would trap the water next to the siding, causing it to rot, and open cell would eventually get saturated and we would have the same problem.

Is there any real solution to this? I have read somewhere that someone attached housewrap to each stud, leaving a 1/2 in gap for the siding and then spray foamed that. Is this a real possibility? We are tempted to see if this works at all, as we have house wrap and open cell foam left over from another project.. I spent over 6 weeks straight hand scraping that cedar siding and do not want to see it rot.. Feeling a little overwhelmed..
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Old 12-10-12, 01:15 PM
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Welcome to the forums! If the outer siding is intact, you can easily insulate it from the inside. I would use Roxul insulation. Better insulative qualities than fiberglas, IMO. Once it is installed, you can apply a vapor barrier over the insulation and finish the wall with your sheetrock, paneling, etc.
With your situation as it is, and has been for years, obviously, the only way to properly install a house wrap is over sheathing from the outside, so stop gap measures won't work here. You have what you have. If you properly caulk joints and corners, properly prime and paint the siding, you should have no water infiltration. Roxul is water repellent, so it won't absorb water, and that is a plus. It is also flame retardant and the heat transfer is almost non existent. Pricier than fiberglas, sure it is, but the trade off is worth it.
Old 12-10-12, 02:03 PM
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My first choice would be two-pound density closed cell polyisocyanurate. That stuff is mighty pricey though and not DIY friendly.

Second choice would be what Chandler is recommending, Roxul brand mineral fiber. It is also expensive but not as bad as the polyiso. The Roxul IS DIY friendly. I use an electric carving knife to cut it.

If you want additional insulation I would consider foil-faced polyiso boards over the studs and then drywall glued to the polyiso. Be sure to tape the seams in the polyiso with foil tape before adding the drywall.
Old 12-10-12, 02:31 PM
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With the existing empty cavities, wind driven moisture has had an easy path into those walls. Once you complete as Larry and Furd have stated, the cavity and inside walls will essentially be air sealed. You can't blow air into a straw if the other end is sealed.

Address any obvious leaks where water can simply run in and as long as it is well sealed and the wood can dry to the outside you should be all set. Ditto on the Roxul.

Old 12-11-12, 12:26 PM
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Join Date: Dec 2012
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Thanks for all of the advise! We have never heard of the mineral wool insulation before. Looks like the perfect solution to our problem. I think we are going to do everything you all have suggested. It is a bit more expensive, but well worth it!
Old 12-11-12, 07:43 PM
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In the 1980s I renovated a house built in 1908, and faced the same situation - the only difference being that the siding was poplar. Here are a few tips I picked up from that project - and later...
  • Before you install the insulation, seal the sill against moisture coming coming up from the ground/basement/foundation - a few cans of spray foam should work nicely for that; the whole trick here is to keep the interior of the wall as dry and still as possible;
  • Use the mineral wool batts;
  • If you have double-hung sash windows with counterweights that ride in channels, try to get some insulation, even if it's just a thin piece of XPS, behind the weights - against the outside window trim; if nothing else, air-seal the joint between the outside trim and the weight channel with caulk on the top and both sides;
  • After the insulation is installed, apply a moisture barrier over it - all mineral wool batts are unfaced. Buy a piece that's long enough to cover the wall from end to end and wide enough to lap out onto the ceiling and the floor. The ceiling lap can stay in place through the installation of the new ceiling, if you're replacing that too; the floor lap can be trimmed off after the wall is finished and the base re-installed, just prior to installing the trim where the base meets the floor; and the windows can be uncovered as you re-install the interior trim around them - seal the poly to the inner window casing (where the pulleys and the access panel for the weights are) and trim it back just enough not to show beyond the trim.

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