1 1/2 story house built in 1904 i think. Upper bedrooms are made up of about a 24 inch high exterior wall then vaulted ceiling. I thought the rafters were 2x6's but they are 2x4's. Anyhow they currently have batts of fiberglass between them which is in contact with the roof deck. Roof deck is not osb or plywood but the old school 1x's with a gap between them then tar paper and asphalt shingles. When i thought they were 2x6 rafters i was contemplating pulling the insulation batts and putting in 1.5 to 2 inches of closed cell spray foam then shoving the batts back in place and sheetrocking back over it. Now i am unsure what to do. Looking for advice on this, hopeing for a simple solution without a huge cost outlay. The shingles will need replaced in the next year or two. I am thinking maybe leaving the batts in place as they are and then when i reroof add a nailable rigid insulation over the roof deck then reshingle. What do you think? Thanks for your input. P.S. the roof is unvented Below is a pic for reference of the upstairs bedrooms showing the angle of the roofline etc.

You might have a mold issue up there. Since the 2x have gaps, in over a century I'm sure some water leaked thru.

Posted By robinnc on 27 Oct 2013 10:38 PM
You might have a mold issue up there. Since the 2x have gaps, in over a century I'm sure some water leaked thru.

Havent found any mold issues. The picture above of the bedroom is pre demo (bad drywall job so pulled it all down). The upstairs is totally gutted now, no sheetrock and i have already pulled most of the batts out between the rafters looking for moisture issues from condensation but so far nothing.

You can put rigid foam on the inside or the outside, but you will lose less space on the outside. Keep it breathable (like staggered layers of taped but unfaced EPS) if you do the inside.

Doing a foam-over with nailbase iso is the right way to go. You'll know when you strip the shingles whether you have any moldy/punky sections of roof deck. Shaded or north facing roof pitches would be the most susceptible, but with only 2x4 rafters and likely R13 (or less) the roof deck runs warmer (=drier) than if it were insulated to current code-min. Whether there is a real hazard or not depends primarily on your local climate & climate zone, as well as your shingle color/type and shading factors. Find yourself on this map for climate zone reference:

http://www.greenbuildingadvisor.com/sites/default/files/images/H-T%20Zones%20with%20Cities%20ABC+.jpg

The amount of exterior foam needed for dew-point control also depends on climate and total-R. If insulating to the current code min with most of it as fiber on the interior side you can refer to Table 806.5 in the IRC:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

But it's the RATIO that matters most for dew point control, and with only R13 in the rafter bays anything over R12 would be sufficient for most of the US, though more would still be long-term cost effective on fuel savings in colder climates (zone 5 or higher.)

A flash'n'batt approach on the roof deck with 1" of closed cell foam would both air-seal the roof deck and protect it from wintertime moisture accumulation, but may not be necessary- it depends again on climate zone & shingle temp, etc. (If zone 6+ it's worth considering.) With a plank roof deck you'd be fine just filling the cavities with half-pound (open cell) foam or cellulose, with R15 rock wool batts being the third-best option, but replacing the existing batts also works, as long as they do not have foil facers (=potential moisture trap.) If you use the rock wool or replace the fiberglass, in zone 6 or higher it's worth putting Intello Plus or Certainteed MemBrain smart vapor retarders on the interior side before putting up the gypsum as insurance.

If you don't foam the interior side, it's still worth detailing the roof deck as an air barrier with caulk if possible. If the seems are too wide and there are a splits & knotholes etc, using a sheet of permeable housewrap that wraps continuously over the sides & interior edge of the rafters, works.

This is more about gaskets, but similar arguments apply to tape. Ie, be careful where you use caulk for air sealing. Same for building wrap with nail or staple holes in it.

When builders first learn about air sealing, they often depend heavily on caulk. After inspecting a home for leaks during a blower-door test, however, they learn that caulk has a few downsides.

My climate zone is 4A but the 4A/5A border is about 15 miles to my north. My zip is 67575 for reference. What about putting a 1.5 inch average closed cell spray foam in the cavity then using the intello plus or membrane and blowing in loose fill like articat to finish off the cavity? Then drywall over it? Thanks

Posted By DENALI on 28 Oct 2013 05:30 PM
My climate zone is 4A but the 4A/5A border is about 15 miles to my north. My zip is 67575 for reference. What about putting a 1.5 inch average closed cell spray foam in the cavity then using the intello plus or membrane and blowing in loose fill like articat to finish off the cavity? Then drywall over it? Thanks

If you put 1.5" of closed cell in the cavity there is no point to the Intello Plus.  The 1.5" of foam is more vapor-retardent than the Intello when dry, and has the same  or greater R-value as the 2" of AttiCat.  If you're going that route, a 1" foam is actually better, since it it's enough R to keep the average mid-winter temp of foam/fiber interface above the ~40F dew point of the interior air, and gives the roof deck better drying capacity.  But you don't really even need that. Your mean outdoor temp in January is about 32F, but the average temp over the 3 coldest months is about 38F.

In your climate just open cell foam or dense packed cellulose would be just fine, and would cost about as much as the 1.5" of closed cell.  Open cell would be more air-tight, cellulose would be more protective due to it's moisture buffering capacity.  You could also dense pack the cellulose to make it nearly as air-tight as open cell, but dense packing would likely cost more than half pound or 0.7lb (Demilec) open cell foam.   If you're a DIY batt kind of guy, cotton batts have a similar R and moisture buffering as cellulose.  If you like playing with insulation blowers, you can use landscaping fabric stretched tightly and stapled ~4-6" o.c. on the rafters as blowing mesh, then see what you can get out of the single-stage rental blower for the box store- you can probably hit 3lbs density or so with cellulose, which is pretty good.  After blowing you can roll it flat to the studs with a rolling pin or section  of 3" PVC or something before putting up the gypsum.

Cellulose has a summertime performance edge over fiberglass in attics & cathedral ceilings due to it's higher opacity to infra-red.  With a hot roof deck the "as used" R-value of the first inch to two inches of fiberglass won't quite meet spec due to the penetration of radiated heat through the translucent fiber at high delta-Ts.  (The ASTM C 518 test used for labeling is 75F center temp with a 30F delta-T, which isn't half as severe as a 140F roof deck with a 75F air-conditioned interior.)

When you re-shingle it's worth putting up ~R20-ish nailbase iso above the roof deck (R15 min, to meet IRC 2012 code) bringing the center cavity R to over R30, with a good thermal break over the rafters. There's is a rationale for more if you're heating with propane, but not a lot more.