New home wall assembly
I am currently designing a new single-family home in Northern Virginia (Zone 4, Mixed-Humid Climate) and am looking for some feedback to my tentative plans for wall and roof assemblies. I am not trying to build a passive house, just a more durable house. My primarily concern is controlling/managing air and vapor movement.
Walls: 2×6, 24″ oc w/ open-cell spray foam insulation in the cavity, ZipSystem R3 sheathing on the exterior and 5/8″ drywall foamed to the studs.
Ceiling/Roof: Truss framing w/ dense pack cellulose (R50+), Zip System 5/8″ sheathing, a ventilated attic with foam panel air baffles at the eaves and a vented ridge. A 2nd zone mechanical system (gas) will be in a conditioned mechanical room.
Basement: I don’t have a detail here yet. I heard Martin on the FHB Podcast suggest 2″ XPS rigid foam on the interior wall but I’m not sure if this is necessary in my climate zone?
I appreciate any feedback you guys have!
Thanks,
Mike W.
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Replies
Michael,
1. You don't have to foam the drywall to the studs to make your drywall layer airtight. For more information, see Airtight Drywall. Moreover, you already have several other potentially airtight layers -- the taped Zip sheathing and the open-cell spray foam -- so you could probably relax a little with the drywall layer.
2. I'm not sure what you mean by "foam panel" ventilation baffles. More information here: Site-Built Ventilation Baffles for Roofs.
3. Where do you plan to put your "conditioned mechanical room"? I hope it is on the first or second floor -- not in the attic.
4. I think that you misunderstood my recommendations for basement insulation. Green builders usually try to avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see Choosing Rigid Foam.
5. According to the 2012 IRC and IECC, basement walls in Climate Zone 4 need to be insulated to at least R-10. I recommend that you use either polyisocyanurate or EPS foam for this task. For more information, see How to Insulate a Basement Wall.
-- Martin Holladay
Martin,
Thanks for the quick response.
1. The rational behind foaming the drywall to the framing had more to do with improving adhesion and reducing the likelihood of future screw pops. Any added air sealing benefit would be a bonus. I'm curious whether you think the Zip R3 (or maybe even R6) sheathing is worth the added cost since I'm achieving an R-19 with the open-cell spray foam? Is thermal-bridging a big deal in a Climate Zone 4?
2. Cut-in-place rigid foam to keep a 2" air space between the underside of the roof sheathing and the cellulose insulation. I think we're talking about the same thing.
3. We plan to have two mechanical systems...one in the basement to service the basement and first floor and one in the attic to service the 2nd floor. I thought it might make more sense to create a conditioned space in the attic just for the HVAC equipment rather than cathedralize the entire attic. Am I wrong?
4/5. Got it, thanks!
- Michael
Michael,
If you are designing a new home, you want to make sure that all of the ductwork is in your conditioned space. You certainly don't want any ductwork in an unconditioned attic. Luckily, you can avoid this mistake, since you are at the design stage.
Needless to say, trying to build an insulated mechanical room in an unconditioned attic doesn't solve the problems. Your ductwork still ends up in the unconditioned attic. Moreover, your HVAC installers (and maintenance personnel) will have access problems. This type of mechanical room is usually too small. If it is airtight and insulated, it can get too hot during the winter. If it includes atmospherically vented appliances, the appliances will be starved for combustion air.
So don't do it.
You need to plan for a second-floor mechanical room. Or else you need to plan for an unvented conditioned attic.
-- Martin Holladay
Martin,
I've done a little research on creating a conditioned attic and have some concerns with potential issues with vapor migrating through my open-cell spray foam and causing damage to the roof sheathing. Would you recommend continuous baffles between the insulation and sheathing (with soffits and ridge vent) to help dry the sheathing or using closed-cell insulation instead?
Thanks,
Mike
Mike,
Either approach can work. You can create ventilation channels between the exterior side of the open-cell spray foam and the underside of the roof sheathing, or you can simply use an adequate thickness of closed-cell spray foam.
The ventilation channel approach works with gable roofs that don't have valleys, hips, dormers, or skylights. It doesn't work if the ventilation channels are interrupted -- the channels need to be continuous, from soffit to ridge.
-- Martin Holladay
Open cell foam under the roof deck works fine in zone 4A as long as you use half-perm paint, or a smart vapor retarder on the interior side.
An inch of closed cell foam is more than sufficient for protecting the roof deck, but you would still want to use a class-II vapor retarder to mitigate the risk of "moisture pumping", in summer, where during the day the sun drives moisture out of the o.c. foam into the room, re-adsorbing it overnight.
Read this short report of WUFI testing different foam stackups across a range of climate zones:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Skip forward to Table 3. At 1" the vapor retardency of closed cell foam is sufficient for normal indoor humidity levels, even with the rest of the insulation being air-permeable fiber. With a smart vapor retarder or half-perm paint added you will have no moisture accumulation problems in the roof deck.