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Community and Q&A

Insulation advice

Coopsdaddy | Posted in Energy Efficiency and Durability on

Sorry to repost this, but we are overwhelmed.

Just got some prelims back and he’s want to know about insulation details so they can be noted.
Zone 3,12/12 pitch standing seam roof with 5/8 plywood an clips,2 story with a option I bonus room on the third,it has a half story truss,24 on center,we were thinking about spray foamin the inside roof line but know if that the best option.
Walls are 2×6 on 24 centers,we planed on putting 1 inch exterior foam board under hardie board and over tyvek and osb but don’t know if that’s te best option.
I tend to over think things butni like doing things right,in this case I just don’t want to spend excess money on things that’s would take forever to pay back or cause my home to rot.
We will finish the third floor so it will have sheetrock. we need to have the roof vent(ridge vent),if so what’s the best option(site built baffles going from soffit to ridge)?
2.what do we insulate the interior roof line with,spray foam or batts and then sheetrock? we still use 1 inch exterior foam flat down over plywood or membrane? foam exterior board 3/4 or 1 inch needed under hardie board?will we benifit from it in zone 3?
5.would we benifit from a rain screen with or without foam?
6.are batts the best option for exterior walls or batts and the foam combo?
Man my head is spinning?

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  1. Dana1 | | #1

    In zone 3 with as little as R5 foam above the roof deck you don't need venting under the roof deck, as long as the interior side is finished with latex paint, or you have a "smart" vapor retarder to limit. If you air seal it meticulously and install batts without any gaps, compressions or voids, they will perform as well as open cell foam. This means caulking every rafter & ridge element to the roof sheathing, completely filling the rafter depth with the batts tucked into the edges & corners, then tugged out slightly proud of the rafter edges so that it's a compression fit to the gypsum board as it goes up, and sealing all electrical & plumbing penetrations of the framing & sheathing with can foam or the appropriate caulks.

    If you use spray foam on the interior the air sealing in the cavities is almost a given. You'd get comparable performance to batts at any depth using open cell foam. If you did the same center-cavity R-value with closed cell foam it would be lower performance, since the thermally bridging path through the rafter is much shorter, and lower R. More than 2-3" of closed cell foam also becomes fairly vapor tight, limiting the drying path for the roof deck. In general it's better / cheaper / greener to use open cell foam than closed cell and spend some of the cost savings on going higher than the absolute R5 code-minimum with the above-the-deck foam.

    For the above-deck foam, polyisocyanurate would slightly outperform XPS inch-for inch in your climate, and it's a much greener product due to it's much lower impact blowing agent. XPS will lose it's environment damaging blowing agent within the lifecycle of a standing seam roof, setting in at about R4.2 per inch over the very long term, despite it's labeled LTTR of R5/inch. EPS and polyiso don't have this issue. It takes 1.25" of EPS to hit the code-min R5, whereas the fully-depleted R-value of 1" of polyiso is about R5.5.

    Exterior foam on the walls has a couple of benefits- it keeps all of the structural sheathing & framing closer to the same humidity levels as the interior, providing some vapor retardency against the summertime humidity in zone 3A. An inch of foam also roughly doubles the R-value of the 25% of the wall area that is thermally bridged by framing, cutting the heat transfer through that low-R fraction in half.

    A rainscreen helps preserve the finish of the siding by doulbing it's drying capacity. If you used unfaced EPS for the insulating sheathing a rainscreen also provides a drying path toward the exterior for the wall assembly. If you used foil-faced foam with nothing between the foam and the air-gap it delivers another R1.5-R2 of average performance to the assembly, due to the radiant-barrier effects.

    For cavity fill in the walls, it's the same story: If you're obsessive about air sealing the sheathing and fitting the batts, they going to be as-good or better than foam solutions, but if you're sloppy on air sealing there is a performance hit. Here again, R for R a full fill of open cell foam is cheaper and higher performance than a partial fill of closed cell, and a full-fill of closed cell is very vapor-tight, limiting the drying capacity of the assembly.

  2. Expert Member

    With 5/8" plywood on your roof you don't need H clips.

  3. dburgoyne | | #3

    I am building a small house in a 3B zone and applied polyiso insulation over exterior wall and roof sheathing, followed by the water resistive membrane and a rainscreen (1x4 battens attached to studs). My exterior finish is stucco, and my stucco installers were unfamiliar attaching paper and lath to rainscreen battens, but it is working out. Hardie Board should install easily over rainscreen battens. Remember the vent at the bottom of rainscreen wall, and soffit vent. Above roof venting is possible as well, depending on your roofing type (mine is concrete tile).

  4. Coopsdaddy | | #4

    I read this article,open cell foam and a damp roof by Martin holiday,could you school me some more on the differance in the closed cell vs open in my application,thanks for all the help.

  5. GBA Editor
    Martin Holladay | | #5

    There are lots of resources to help you here on the GBA site.

    Here is a link to an article that explains all of the different ways that you can insulate a sloped roof assembly: How to Build an Insulated Cathedral Ceiling.

    Here is a link to an article that tells you what you need to know about rainscreens: All About Rainscreens.

    Here is a link to a GBA Encyclopedia article that explains the differences between open-cell spray foam and closed-cell spray foam: Spray Foam Insulation: Open and Closed Cell.

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