Climate zone 4a insulation questions
Climate 4a in southern IN
New construction ranch home now being framed.
2 areas of concern after reading articles here and linked sites.
Scissor truss cathedral GR and MBR with asphalt shingles
I think nonvented sounds better and builder has seen both vented and nonvented in our area. Builder hates open cell for some reason and recs CC for nonvented. He will not put rigid foam under shingles because he has no experience doing it. I’m looking at 2 large cathedrals that need insulated, $$$! Can I use closed cell foam of a minimum thickness and then use open cell to save money?
He also wants to use a synthetic roof underlayment due to the amount of time (months) between decking and shingles. From reading this is off label and possibly bad. Should I make him exchange underlayment when shingles go on?
2x6x10′ walls
Builder normally puts foam on top of osb to stop thermal bridging but our architect only left space for 1/2″ foam. Builder recommends cellulose interior with this 1/2″ foam ext. Is this plan going to leave me with wet OSB and ultimately where does the vapor barrier go? Would I be better off not putting ext foam, doing as builder says or not put ext foam and use a Mooney wall? Or in 4a I’m just worrying about a problem that won’t ever happen?
Thanks,
Brian
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Replies
I feel duty-bound to remind readers that insulation decisions need to be made at the design stage, well before framing begins. Here is a link to a relevant article: Plan Ahead For Insulation.
Brian,
Q. "Can I use closed-cell foam of a minimum thickness and then use open-cell to save money?"
A. Yes. To determine the minimum R-value of the closed-cell spray foam layer, use the rules outlined in this article: Combining Exterior Rigid Foam With Fluffy Insulation.
Q. "He also wants to use a synthetic roof underlayment due to the amount of time (months) between decking and shingles. From reading this is off label and possibly bad."
A. You can go online to look up the installation instructions for the synthetic roofing underlayment that you are considering using. Note that (a) this is only a problem for unvented roofs -- manufacturers always permit the use of their products on vented roofs, and (b) there is a trend towards changing the installation instructions for this type of product -- more and more manufacturers are beginning to allow the use of their synthetic roofing underlayments even on unvented roofs.
Q. "Builder normally puts [rigid] foam on top of OSB [wall sheathing] to stop thermal bridging but our architect only left space for 1/2 inch foam. Builder recommends cellulose interior with this 1/2 inch exterior foam. Is this plan going to leave me with wet OSB?"
A. No. You'll be fine. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "Ultimately where does the vapor barrier go?"
A. You don't need a vapor barrier. It isn't required by code, and building scientists don't recommend one. However, you should certainly include a durable air barrier. For more information, see Do I Need a Vapor Retarder?
Q. "Would I be better off not putting on the exterior rigid foam?"
A. The rigid foam is useful, but 1/2 inch of rigid foam has an R-value of only R-2 or perhaps R-3, depending on what type of rigid foam you plan to use. It would have been better if your architect planned ahead for a thicker layer of rigid foam.
Q. "Would I be better off using a Mooney wall?"
A. Just one inch of cellulose provides more R-value than 1/2 inch of rigid foam, so a Mooney wall with an additional 1.5 inch thickness (compared to a 2x6 wall without interior strapping) would provide more R-value than a 2x6 wall with 1/2 inch exterior rigid foam. Of course, you will lose interior space if you go this route.
1/2 inch of rigid foam will slightly reduce thermal bridging, but it will certainly not stop it. When people use that terminology, I am concerned that they are thinking about it by analogy to something that really isn't analogous. For example, you may be using the analogy of a bridge over a gorge. If the "bridge" goes only 90% of the way across the gorge, then it is no bridge at all. No traffic can cross the gorge using it. Thermal bridging doesn't work like that. A thermal bridge that goes 90% of the way across your insulation layers still provides a pathway for heat to get through the insulation.
Now at the decision stage for insulation. The local "pro" insulator recommends OC on everything. He states that for our climate it is not only a good option but the best. because it all allows drying in either direction based on temp and vapor gradients.
Current plan:
Walls:
above grade walls 1/2" foam on exterior with 2x6 filled with OC
Attics:
Spray OC on all wall caps and areas of air leak concern and then use loose cellulose with soffit and ridge vents
Cathedrals Scissor trusses:
place a baffle, http://adoproducts.com/as_durovent , from soffit to ridge vent for a "vented attic" and spray OC below to seal the space as if it was a non-vented attic. Pro says it is over kill to vent above the OC but that will make me more comfortable and it will be considerably cheaper than CC foam. (I and my wife really want recessed lighting in the spaces and this will allow that in the most efficient way possible.)
Basement:
Poured walls with 2" exterior rigid foam.
Pro says frame out 1.5" off wall and use OC to a thickness of 3 inches to cover that and the sill boxes. This makes me nervous since it is below grade and on concrete. He says not to worry because it will dry to the inside and more importantly it will stop all air flow thus stopping any condensation on the "cold concrete" that would happen with a fiber type insulation . He says with the rock, rigid foam and the thick waterproof coating on the exterior there will be no water getting into the concrete and CC is over kill.
Thoughts?
Brian
Here is an excerpt from one of Martin's articles (for the rest go to https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing) Just to clarify, this caution apply to the roof/attic area.
“Open-cell foam is risky in all climate zones”
The authors wrote, “Every best-performing unvented attic roof is constructed with a closed[-cell] SPF and with a low indoor moisture supply. The opposite is true for the most risky roof. In all cases, except for climate zone 4, a high duct leakage has a positive effect on the moisture content (MC) of the OSB, most likely due to the dehumidifying effect of the HVAC cooling coils, which, by a higher rate of air leakage, will have a higher influence on the vapor content of the attic air during the operating cooling mode.”
As an example, the worst-performing OSB in climate zone 4 is the OSB on a north-facing roof insulated with open-cell foam, over an attic with a duct system that is fairly well sealed (4% duct leakage), an attic floor which is fairly leaky (2.0 cfm50/CFA), and a house with high indoor moisture production. After the software was run for one simulated year, the net result of these factors was OSB with a moisture content of 54%.
At his presentation in Florida, Simon Pallin summed up the researchers’ findings this way: “Open-cell foam is risky in all climate zones.”
The authors wrote, “The MC in the OSB sheathing varies mostly due to whether the spray polyurethane foam (SPF) is vapor closed or open. Having an open SPF is actually a risk in all the investigated U.S. climate zones, 1 to 7, depending on the chosen values of the other varying parameters. Naturally, the outdoor climate will influence the MC of the OSB, but also the indoor moisture production rate has a significant impact. … A high air leakage rate from the air distribution duct system has a positive impact on the MC of the OSB sheathings due to the dehumidifying effect of the HVAC unit, though it has a negative influence in terms of the HVAC energy demand.”
Read more: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing#ixzz4dmu5OLRF
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If there is a vent channel above the open cell would this not fix all problems of OC in the attic? Spraying OC on the top of dry wall with a vented attic should be OK so this would be a variation of this technique.
Brian
The half inch rigid foam on the exterior would be code legal, but provides no dew point control in a zone 4 climate, and very little thermal break. Bumping to 3/4" rigid polyiso would buy you a meaningful thermal break and some dew point control. If staying at 1/2", unfaced EPS would provide reasonable drying toward the exterior.
With a vented cathedralized ceiling it's fine to go with ~13" of open cell foam to make the R49 code minimum. To ensure there is no summertime moisture cycling,half-perm "vapor barrier latex" on the ceiling is cheap insurance, but probably not needed (depends on the venting details.) If you skip the vent channel you'll DEFINITELY need the half-perm paint.
Open cell foam or wood studs against the foundation wall is risky, unless the concrete is mixed with a high capillary-resistance material to prevent ground moisture from wicking into concrete from the footing. The exterior 2" foam does nothing to mitigate that problem. It's cheaper and easier to skip the exterior foundation foam, install an EPDM sill gasket between the top of the foundation & the foundation sill, and install continuous 3/4-1" foam (any type) with foil or plastic facers against the interior side of the foundation as a capillary & thermal break for the finish wall studs and insulation. If there isn't going to be any under-slab insulation, install an inch of EPS (not polyiso) under the bottom plate of the non-structural interior studwall too.
Brian,
If you are contemplating the use of open-cell spray foam under roof sheathing, I definitely recommend that you include a ventilated air gap between the top of the open-cell spray foam and the underside of the roof sheathing. You can do this by installing sturdy site-built ventilation baffles.
For more information, see Site-Built Ventilation Baffles for Roofs.
-- Martin Holladay