chadman51
New Member
Posts:20
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| 31 Dec 2010 06:18 PM |
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I am renovating my house. Gutted the entire house. Put a metal roof on. I am getting ready to use Icynene in the walls and on the roof deck. Does it make sense to put something to vent the roof before we spray it with foam?
The metal roof has a vented ridge. Should we put something that would give about a 1/4" gap between the roof sheathing and the foam for venting purpose? Just to let the roof breath a little?
Thanks in advance
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Matt G
New Member
Posts:93
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| 01 Jan 2011 09:29 AM |
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What does the metal roof mfg say? What does Icynene say with regard to it's product and metal roofs? Assuming some inspections are going to take place what does your local code enforcment official say? He could trump all.
If it were to be ventilated, I'd think that 1/4" may not be adequate. Any specific reason you picked 1/4"? |
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FBBP
Veteran Member
Posts:1215
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| 01 Jan 2011 11:40 AM |
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How would a spray foam roof differ from SIP when it comes to venting? |
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chadman51
New Member
Posts:20
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| 01 Jan 2011 10:33 PM |
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I was saying 1/4" because i was going use luan to do it. The foam guy said that they do it but really coudn't say if it was worth it or not. I didn't want to take the time if I really didn't need to.
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insulateright
New Member
Posts:17
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| 01 Jan 2011 11:13 PM |
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what area are you in. If you are in a climate that requires heating then you will definitely need an air space with Icynene. 2-2/12 inches is normaly recommended. Icynene is not as good as the manufacturer claims. Be careful.
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Matt G
New Member
Posts:93
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| 02 Jan 2011 10:47 AM |
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Posted By chadman51 on 01 Jan 2011 10:33 PM
I was saying 1/4" because i was going use luan to do it. The foam guy said that they do it but really coudn't say if it was worth it or not. I didn't want to take the time if I really didn't need to.
Chad: When I said "What does Icynene say with regard to it's product and metal roofs?" I meant the Icynene mfg not a local installer or salesmen. OTOH, it sounds like the word from the locals is to ventilate. What the issue is is will the underside of the roof sheathing become a condensing surface thereby creating a potential moisture problem? If a problem does occur who will be responsible to resolve it? Metal roof mfg? Icynene mfg/installer? You? I think we know the answer to that. A lot of this depends on what your local climate is. Roof assemblies act very differently in different climates - say hot dry, hot humid, mixed humid, cold dry, etc. What state, etc, do you live in? Also, I'm not sure how ventilating it can hurt. Sorry I don't have an answer for you. Maybe an expert will come along... I'm just a guy who builds homes and can't afford any big mistakes. BTW - another little Q: what is the roof sheathing? Skip Sheathing? Plywood/OSB? 1x? laid tightly? Just trying to get a better 'look' at the situation, if for no other reason to have everything fully defined once an expert does come along. |
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chadman51
New Member
Posts:20
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| 02 Jan 2011 07:29 PM |
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Thanks Matt
I have metal roof with ice and water shield and 5/8" plywood.
I live in Delaware. We have very hot humid summers and very cold winters.
I will try and call the manufacture as well. (good advice) |
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chadman51
New Member
Posts:20
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| 02 Jan 2011 08:15 PM |
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 Here is a quick sketch of what i was thinking. Hopefully it will be better to do nothing. |
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Matt G
New Member
Posts:93
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| 02 Jan 2011 09:30 PM |
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Another idea would be for you to look around buildingscience.com for a detail using open cell foam under the roof deck of a metal roof. I think I have seen details there for metal roofing.
IF you decide to vent... totally different application, but this pic shows insulation baffels I did on my own house using 1/4" OSB and some scrap 2x4s that I ripped to 2x2s. The 1/4" OSB because it was the cheapest type of sheating material I could find plus I felt it would give the best ventilation. Some other cantidates might be 7/16 OSB and Thermoply type sheathing. On the job site we just use those staple up foam insulation baffels although the Energy Star rater we use won't accept those for ES homes - they require cardboard baffels because they can be fitted more securely down to the wall top plate.
BTW - growing up, our family had a house at Bethany Beach DE. |
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greencleaning
New Member
Posts:50
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| 02 Jan 2011 09:33 PM |
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some foam products are strong enough to actually add strength to the building structure; some building products use a wood-foam laminate to produce insulated building panels, for example for use in un-vented cathedral ceiling/roof structures. |
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Here's to you, your health and the health of your family. <a href="http://www.cleaningcleaner.com">Eco friendly cleaning</a>
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arkie6
Veteran Member
Posts:1453
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| 02 Jan 2011 10:55 PM |
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This is probably a bit late given the fact that the metal roof is already installed. If I were installing a metal roof in my climate over any type of wood substrate, I would ventilate it. In the diagram above, I would ventilate immediately below the metal and above the plywood decking. I would apply 30# roofing felt or similar product over the plywood. Then I would run 1x4 furring strips horizontally across the roof approximately every 24". But before I installed these wood strips, I would take a circular saw and make 3/8" deep cuts approximately every 6" perpendicular to the length of the boards. These boards would be installed with the cuts downward to allow drainage and prevent ponding of any moisture condensation on the top side. Then install your metal roofing screwing through the furring strips into the plywood decking. If you are going to spray foam the roof, then I would apply it directly to the plywood decking from below. I don't see any point in ventilating between the spray foam and the plywood roof deck. |
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chadman51
New Member
Posts:20
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| 02 Jan 2011 11:03 PM |
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 I used these clips. They allow for a 1/4" gap off of the sheathing. |
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arkie6
Veteran Member
Posts:1453
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| 02 Jan 2011 11:08 PM |
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Ahh. Standing seam with hidden fasteners that allow expansion and contraction of the metal roofing. Good choice. You've got some ventilation between the metal and plywood decking. I don't see any point in ventilating below the plywood decking. |
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arkie6
Veteran Member
Posts:1453
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| 02 Jan 2011 11:21 PM |
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How much of that Icynene open cell foam do you intend to apply to the roof deck? The spec sheets says R3.7 per inch. I would assume that you would need at least 10+ inches to meet the current energy code requirement of R38 in your area. |
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chadman51
New Member
Posts:20
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| 02 Jan 2011 11:24 PM |
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The house has 2x6 rafters. He said they would be covering them. |
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arkie6
Veteran Member
Posts:1453
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| 02 Jan 2011 11:41 PM |
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"Covering" the 2x6 rafters leaves a lot of leeway. 6" deep spray foam would cover the 5.5" deep rafters but would fall far short of even the minimum energy code requirement (R22 vs. R38 minimum). Is R22 ceiling insulation OK with you?
Any particular reason you chose spray foam insulation on the roof deck vs. less expensive blown in ceiling insulation?
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Matt G
New Member
Posts:93
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chadman51
New Member
Posts:20
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| 03 Jan 2011 08:13 AM |
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Posted By arkie6 on 02 Jan 2011 11:41 PM
Any particular reason you chose spray foam insulation on the roof deck vs. less expensive blown in ceiling insulation?
We have lots of duct work in the attic. Thanks |
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Dana1
Senior Member
Posts:6991
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| 03 Jan 2011 10:55 AM |
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Posted By chadman51 on 02 Jan 2011 11:03 PM
I used these clips. They allow for a 1/4" gap off of the sheathing.
What arkie6 said- with the gap between the metal and the roof sheathing it is already inherently vented, and further venting it from below wouldn't be necessary. The purpose of a ventilated roof is to allow the decking to dry even when there's rain or snow water on the exterior. With the ventilation gap on the exterior of the deck it dries even better than if vented on the interior side of the wood. Even with highly permeable open cell foam applied below, the moisture drives from the interior can't get ahead of the outward-drying capacity, as long as there isn't some highly impermeable material such as Ice & Water Shield or EPDM membrane on the exterior side of the roof deck. (Roofing felt is permeable enough.) Only if there IS such a material on the deck would further venting from below be warranted. |
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chadman51
New Member
Posts:20
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| 03 Jan 2011 06:44 PM |
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thanks Dana. I do have ice and water shelid on the entire roof. Is that a problem?
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