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new roof insulation ventilation dilemma

Posted by annemarie29 (My Page) on
Thu, Mar 5, 09 at 10:27

I have dutch colonial with a gambrel roof. About to have a roof installed and trying to get my remodel up to energy star standards. It's a cold brick house. After tear off (shinles plus four layers) planned to sheath with plywood (no sheathing now) and install asphalt shingles. However, I'm stuck with how to ventilate and insulate. Building a wall lengthwise in attic to allow removing ceiling joists and raise ceiling in mbr. Can't really use remaining space because of struts and lots of moisture here so want to do natural ventilation in that portion. Any suggestions on how to get some R value from sheathing especially above raised ceiling and still ventilate remainder of attic. Has anyone done anthing like this?


Follow-Up Postings:

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RE: new roof insulation ventilation dilemma

So you're planning a half ventilated attic, half not?

Why not make the entire attic unventilated, then foam the underside of the sheathing? See Building Science RR 301 at link. Some details will vary depending on your climate.

Here is a link that might be useful: Building Science


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RE: new roof insulation ventilation dilemma

Thanks for responding. That is one option I am considering. However, I am told that foam is really expensive, so not sure using it on the unusable portion of the attic would be worth it. Also, it precludes using an attic fan, possibly moisture issues, shingle manu warranty problems, etc. I am getting the impression there are two schools of thought on this, buttoning up tight, vs. ventilated attic space, both make good arguments. I would like to hear from people that have faced this, done either option, and what the experience has been. Right now I am leaning toward foam on the raised ceiling side only, and having the living space be the envelope. Although having a cold attic does seem like it would make house colder, on the other hand I have heard that foam under the entire deck will give you a cooler house, and this house tends to be cold, even in summer. I will read the link, but is nice to hear personal experiences as well.


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RE: new roof insulation ventilation dilemma

We wanted to renovate the 1/2 story of our bungalow to make a master suite. We also badly needed a new roof and it had 4 layers of old shingles on it. So, we gutted the upstairs and the first stage was a new roof. Off came four layers of shingles. The oldest were shakes, so there were boards they were nailed to, but no sheathing. We wanted a cold deck roof (because DH suffered a bad fall shoveling the da*n thing a few years back - crushing his heel!). The contractor installed sheathing that was foam backed OSB; it wasn't sandwich board - it was OSB board on one side & foam on the other (probably because we had those old nailer boards across the joists). Anyway, the new 40 year shingles went on over the foam backed OSB. That was about 4 years ago. It was really expensive. I swear we have the most expensive roof in town on our little house! Fast forward to last year... still working on the house. Now, it's time to finish the upstairs and we have to insulate between the roof joists in order to get ready to hang the drywall. The roof doesn't have vents, so we can't do fiberglass; the general contractor recommened spray foam, so that's what we did last spring. That was another $6-7,000. The spray foam added another R38 to the two inches of foam under the sheathing, so now we have something like an R52 roof. Let me tell you though, the upstairs does not get hot in the summer like it used to, we have a true cold deck roof - nary an icicle and no ice dams - and it stays cozy in the winter with very little help from a gas fireplace we added to the upstairs as the only heat source. The upshot is we are very happy with the outcome as to energy efficiency, indoor climate control, and roof/snow management. Was it expensive? YES. Hope this helps.


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RE: new roof insulation ventilation dilemma

Where the attic space is constricted, the ideal approach is to use foam and an unvented attic. Otherwise there is often not enough space to allow proper ventilation of the roof, especially with contemporary energy efficient insulation levels

The details of an unvented attic vary by climate. Be sure to pay attention to Building Science's climate specific recommendations.

The only advantage to a vented attic is that is cheaper to insulate; and moreso in a retrofit, which will require that you close up all the existing venting. If you go that route don't use a power ventilator. There's lots of evidence that they cause more problems than they cure.

I'm presuming that in removing ceiling joists and providing substitute roof support, you're getting a structural engineer or architect to do the design.


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RE: new roof insulation ventilation dilemma

Both responses are helpful, thanks. I am working with someone on loads, that's the easy part. Unfortunately, everyone I talk to has a different school of thought on the ventilation/insulation issue. I also have the old cedar shingles nailed directly to boards with no sheathing. I asked the roofer about something besides plywood (SIP?) for increased R value when sheathing, but he didn't offer anything. My understanding, which changes daily, is that if I button up completely I will be required to have some form of mechanical ventilation, egERV. I am concerned about radon, as have already had borderline readings. So first question for worthy, what is difference between ERV and PRV? Also, for kimkitchy how did you deal with air exchange/condensation and do you live in a moist climate? Ours is basically rain forest when we get a normal summer. We also get snow, and with my current idea the ventilated side of attic would be sun facing warm side and foamed closed side would be north facing cold side. Is that good or bad for ice dams. My head hurts.


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RE: new roof insulation ventilation dilemma

I've never heard of a PRV. You probably mean HRV. For the differences, see the link. HRVs exchange heat; ERVs also handle moisture transfer. See Building Science discussion of HRV/ERVs here.

Both vented and unvented attics address the problem of ice dams--vented by keeping the underside of the sheathing the same temp as the outside and unvented by keeping interior heat off the underside of the sheathing. Unvented attics have a better chance of being more effective in this regard due to the practical difficulties of keeping the heat out of vented attics.

Unfortunately, everyone I talk to has a different school of thought on the ventilation/insulation issue.

Instead of talking to people who are almost invariably unaware of the building science research and literature on the subject, read the materials from Building Science and other building scientists on the subject.

kimkitchy's good experience isn't surprising. But in terms of energy savings payback--in other words, how long before the energy savings equal the cost of installation vs. conventional insulation--it also demonstrates that conventional vented attics aren't out of the running in retrofits.

If you choose to do a half and half, so to speak, on your home, the side of the partition on the unvented portion will have to be sealed tight and insulated. If you decide on unvented, be sure to check with your local building inspection authority as to what fireblocking they may require under the foam.

Radon is a separate issue that can be addressed whether you bring your home up to higher insulation standards or not.

Here is a link that might be useful: Energy and Heat Recovery Ventilators


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RE: new roof insulation ventilation dilemma

Worthy is giving such knowledgeable advice and great resources, not sure I have anything much to add from a practical experience perspective. But, I'll answer your direct question to me. :-) We do not live in a moist climate. I live in quite a dry climate, but with long, very snowy winters and very cold. We don't even need or have air conditioning. Maybe we should have an EVR to add humidity to the interior for winter, but we don't. We don't have much of a problem with the interior of windows sweating (we have storms and triple honeycomb blinds on most windows & perhaps that helps with the condensation). There is not an issue with vapor between the drywall and the roof because the foam is the barrier, so there is no mold issue there. Essentially warm air doesn't meet cold air, so there is no condensed vapor to get trapped and grow mold. Ours is closed cell foam. We did put in a high efficiency Carrier furnace to help with the heat loss recovery issue. It all seems to work well for us, but Worthy is right that the payback period is potentially looong with the unventilated foam route. HTH. Cheers, -Kim


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