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mj1angier
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# Posted: 16 Jul 2019 08:19am - Edited by: mj1angier
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Planing out the roof insulation process for our sheds.
The roof is metal over Techsheild osb ( it has a foil face on inside) on 2x6 rafters. At the wall end, it is just boxed in- no air vent in the soffits. It has a ridge vent and gable vents. The plan is to use 2" polyiso foam between 2x6 with air gap to the Tech shield. Put thick black plastic to inside for vapor barrier and the T&G over plastic.
My question is do I need to put vents in the soffits in each bay between rafters?
Photo of soffit area from inside:
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Wilbour
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# Posted: 16 Jul 2019 09:34am - Edited by: Wilbour
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I may not be fully understanding your situation but the concept behind venting from sofit to either ridge vent or some sort of roof cap is to encourage a natural flow of air using the principle of warm air rising. As the air within the pocket created by the 2x6 and sheathing warms up it wants to rise. You need an exit for this as well as an entrance for replacement air, which would be cooler.
If you are using 2x6s for rafters and placing 2" insulation between them, how are you installing the 2" insulation?
I would find a way to keep the insulation flush with the insides of the rafters and create an air pocket between the insulation and the roof sheathing. Then vent the soffits so air can flow from the bottom of the roof to the top and out.
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mj1angier
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# Posted: 16 Jul 2019 11:44am
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Quoting: Wilbour I would find a way to keep the insulation flush with the insides of the rafters and create an air pocket between the insulation and the roof sheathing. Then vent the soffits so air can flow from the bottom of the roof to the top and out.
That is my plan. The foam board polyiso will be flush to the inside of the 2x6 leaving air gap between roof deck and insulation.It will be a friction fit.
But without it having soffit vents it does no good in my mind. Not sure why they built it like that. I think a 2-3" round screened vent in each rafter bay would be enough. Cut hole with drill and hole saw bit, caulk around vent edge, pop into place.
The TechShield says directly on it not to let insulation touch it, lol
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Brettny
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# Posted: 16 Jul 2019 12:25pm
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Your ridge vent will have a spec on lower end venting. Follow it.
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NorthRick
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# Posted: 16 Jul 2019 03:23pm
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Quoting: mj1angier My question is do I need to put vents in the soffits in each bay between rafters?
Yes, you do. Your plan for using a hole saw and the pop-in vents will work, you just need to make sure you are allowing for enough venting.
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ICC
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# Posted: 16 Jul 2019 04:02pm
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Venting... the minimum venting is square foot area of attic space divided by 150 = total square footage of vent area.
Half the vent area is for the ridge venting, the other half is assigned to soffit vents. Gable end wall vents are not the best types and are of no use in a cathedral ceiling situation.
Vents are screened and that reduces the effective vent area. The vent area calculated as above is the effective vent area. Venting grills will be labeled with their effective area.
That is the theory on venting. Often actual air flow does not follow the theory. There are ways to make better roofs, ones that don't need vents. See... https://www.greenbuildingadvisor.com/article/all-about-attic-venting
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mj1angier
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# Posted: 16 Jul 2019 06:39pm
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See this is the kind of thing that messes me up:
The conclusions of Rudd and Lstiburek echo those of Parker. Rudd and Lstiburek wrote, “In the hot humid climate, the best solution to eliminate the potential for moisture condensation in attics may be to keep the moisture out of the attic altogether by sealing the attic to the outdoors.â€
So I am in North Carolina foothills- pretty much the poster child for hot and humid...
This is what ridge cap looks like and what inside looks like at it
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snobdds
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# Posted: 19 Jul 2019 12:29pm
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https://hammerandhand.com/best-practices/manual/6-roofs/6-2-vented-and-unvented-roof- assemblies/
I have a non vented roof. I plan on using 2.5 Inch polystyrene panels cut to size to fit in the rafter bays. I am installing them directly under the roof sheeting with no air space. I will then use cans of closed cell spray foam to seal the sides of the sheet foam to the rafters.
As long as there is no conditioned air in contact with either a hot or cold roof, there will be no moisture issues.
There may be different ways to do it, but my insulation guy has specifically recommend this as an alternative to spray foam.
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