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grover
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# Posted: 23 Mar 2014 02:34pm
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I think if I don't change my mind we are going to go with the spray foam on the back side of the roof and that will make it a conditioned attic with no venting.
Just to back up and provide some info...my trusses are scissor type with 12/12 outside pitch and 8/12 inside pitch. This makes for a small attic area.
My question is about additional insulation. I was thinking about adding some additional insulation, either batts or blown in, just before I finish the ceiling. Would this create problems in the attic or just create some additional R value.
Thanks
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bldginsp
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# Posted: 23 Mar 2014 03:02pm - Edited by: bldginsp
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The spray foam, in theory, creates the vapor barrier between the conditioned space and surfaces that are exposed to the cold (roof plywood). So additional insulation inside that envelope does not need ventilation to deal with moisture SO LONG AS the foam is thick enough that the inside surface of the foam won't get cold enough to condense moisture. If you did ventilate above the blow in and below the foam, you would lose all the insulating of the foam, since the venting would let in the cold. I think you're fine with the additional blow in or fiberglass if the foam is at least 3 inches thick. Where you located?
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grover
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# Posted: 23 Mar 2014 04:13pm
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Southern Indiana, I can never remember which zone that is.
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bldginsp
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# Posted: 23 Mar 2014 05:16pm
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I'm guessing you don't get real extreme cold there very often. The colder it gets the greater the possibility of the foam getting cold enough to condense water. Whether you put the additional insulation or not, the problem is really the same- whether or not the bottom surface of the foam will get cold enough to condense moisture in the attic that came from the house. The additional insulation won't stop moisture passing through.
An additional precaution you could take would be to put a plastic moisture barrier on the bottom of the trusses before you put on whatever finish material.
In general I question how effective spray foam is in such situations, even though the industry consensus is that it is effective since it is such a good moisture barrier, unlike other insulations. I'm just old fashioned- I like to see a well ventilated attic. You are creating a totally enclosed space over conditioned space and relying on moisture barriers to prevent a problem. Like I said before, if the foam is thick enough, should work. But if the foam is thick enough not to get cold enough to condense moisture, do you need additional insulation? Can't hurt, I guess.
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MtnDon
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# Posted: 23 Mar 2014 08:18pm - Edited by: MtnDon
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Probably zone 4 http://energycode.pnl.gov/EnergyCodeReqs/?state=Indiana
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I do not have any specific resource to point to on this, but to me splitting the insulation seems like a bad idea. I seriously doubt that the R-value of the two layers added would be the same as if all the insulation was in one plane. How much difference I don't know. Partly I base this on some observations in our home. The house was built with a two car attached garage, somewhat deeper than a lot. The wall between the garage and the habitable space was insulated R-11 (1985). A few years later we built a second wall to create a utility / storage room between the garage and the habitable space. The new wall was insulated as well; R11. The utility room is not heated or cooled. In winter it is always almost as cold as the garage. I've never taken temperature readings to compare garage to utility room. But it is much colder than the habitable space and as stated closer to garage temperature than house temperature. In summer it also warmer in the utility room than in the house, but not quite as bad as the garage.
Oh. the garage is also insulated although the overhead door has fairly low R-value, maybe R6. ??? But no heat as a rule, no cooling.
My point being that the space between the two layes (walls) of insulation is always closer to exterior temperature than the interior, heated/cooled space.
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IRC rules do NOT permit a vapor retarder in the attic floor when the attic is built as a conditioned space.
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I believe that all the insulation you want / need should be, must be (?), installed in one layer, even if consisting of layers of different composition. I just can't point to a resource that states so. So I could just as readily be completely wrong.
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Sustainusfarm
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# Posted: 23 Mar 2014 11:34pm
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I was told by the contractor to use 1" pink foam board on the rafters befor I put up drywall to stop the thermal bridging of the roof rafters to the roofing material! Works great! Warm drywall too!
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TheCabinCalls
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# Posted: 24 Mar 2014 11:06am
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We spray foamed the 2.5 inches of foam and filled the rest with pink fiberglass (R30). It is a common way of doing it. The reason you don't fill the entire area with spray foam is that it would cost too much. If you can afford it then have it done that way.
So far we haven't had nay issues in 3 years. Nice and cozy!
Some retro-fitters have put the foam on top of the roof deck, but then need to add to fascia material.
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SonnyB
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# Posted: 23 Apr 2014 10:41am
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I spray foamed my home that I live in full time in Ga and it does a great job here. So far this year my highest bill, total elec., has been $136 and that's heating and cooling over 3000 sq ft. I am going to spray foam my cabin also just to help keeping it warm in winter, only heat with wood stove, and cool in summer with window units.
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