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Attic insulation and moisture management

I am in Chicago (Climate zone 5) and have a deep energy retrofit on my hands. The 111 year old building has a “low-sloped roof” and we recently finished insulating the attic. We packed 10 ½ inches of rock wool batts between the roof joists and mounted 4 inch thick XPS sheets to the bottom of the roof joists. The joints between the XPS sheets were sealed with a thick bead of closed cell SPF. This should render the system pretty airtight.

I have to assume, however, that some moisture will still migrate into the insulation assembly and roof structure. To let that moisture dry out, I plan to ventilate my roof deck by mounting two by two’s parallel to the roof slope onto the existing deck. A layer of ½ plywood over the two by two’s will be my new roof deck. The ventilation intake and exhaust will be along the low and high point edge of the low-slope roof.

I have read the GBA Blog posts “Insulating Low-Slope Residential Roofs” and “How to Build an Insulated Cathedral Ceiling”, which were very helpful, but have two questions which keep nagging me:

1) Am I kidding myself by thinking that ventilation will keep my cold roof deck and roofing structure sufficiently dry?

2) I don’t quite trust the venting rule of 1 square foot of net free ventilation area for every 300 square feet of roof area. Shouldn't there be a minimum airflow requirements that must be met?

Asked by Marcus de la fleur
Posted Mon, 12/02/2013 - 23:04

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2 Answers

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Marcus,
You'll hear a lot of conflicting advice on different ways to insulate low-slope residential roofs. You'll get a lot of bad advice, and much of the advice you hear will be contradictory.

I have compiled my own knowledge and advice in the article you mention, Insulating Low-Slope Residential Roofs.

I'm not a fan of attempts to vent low-slope roofs. My experience as a roofer taught me that vents in low-slope roofs are more likely to introduce moisture (wind-blown snow and rain) than reduce moisture. So I think that low-slope roofs should be unvented.

Joe Lstiburek disagrees with me. He says that you can vent a low-slope roof, as long as (a) your attic is deep enough to allow an air gap of at least 6 inches between the top of your insulation and the bottom of your roof sheathing, and (b) you can let air enter the ventilation gap around the perimeter of the roof, and you build a ventilated doghouse or cupola in the center of the roof. All of this is explained in my article.

As you guessed, you aren't going to get much air flow following the "soffit-to-ridge" approach.

Your description of "roof joists" confuses me. Are these rafters or ceiling joists? Do you have both or just roof rafters? Is there an attic between your ceiling joists and your roof rafters, or not?

If you are conscientious about your air sealing details at your ceiling plane, my guess is that your plan will work. You just need to know that you are entering uncharted territory, because you aren't following standard advice.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/03/2013 - 06:41

2.
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Martin, to answer your question about the roof details:
- I insulated between the old growth 2 by 10 roof rafters.
- I also have old growth 2 by 6 ceiling joists (except for the back 1/3 of the building, where I had to remove the ceiling joists to have enough space for the XPS insulation attached to the bottom of the roof rafters.

For an image of the insulation assembly see:
http://delafleur.com/blog/?p=4198
http://delafleur.com/blog/?p=4203
http://delafleur.com/blog/?p=4205

For the air sealing of the XPS boards see:
http://delafleur.com/blog/?p=4264

Answered by Marcus de la fleur
Posted Tue, 12/03/2013 - 23:58

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