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Wall and roof moisture concerns; electric HVAC and domestic hot water choices

Hi all,

I would like folks to offer any advice/criticism on the following wall and roof assemblies from a moisture control standpoint as well as any other concerns they might have. In addition I have a few specific questions on HVAC options. I know that many of my concerns have been addressed in various Q&A's but it is the specific circumstances that I am concerned about here.

Currently, I am building a 1000 square foot one story 2 bedroom/1 bath single family home on a 4 3/4" concrete slab (this will be the finished floor) poured on top of 6mil poly over 2" XPS. The frost wall is of masonry block construction with 2" XPS applied (held against the wall by backfill resting on the footers that have drains; the XPS extends to top-of-slab elevation) to transition to 1.5" XPS on the exterior of the house to the top plate. We are at the stage now of just platform framing walls.

The exterior walls have been specified to be(interior to exterior): 5/8" GWB, 24" C/C 2x6 with 1/2" OSB filled with closed-cell polyurethane foam water blown,Tyvek,1.5" XPS continuous with the 2'", exterior siding.

The roof is called out as: 5/8'" GWB, 11 7/8" I-joists 24" C/C with 1/2" CDX plywood deck filled with closed-cell polyurethane foam water blown, EPDM membrane roofing (the roof line are two opposing sheds meeting in the midline; 1.5/12 and 2.5/12 roofs)

This is the first "tight" small house I have ever built. I'm deathly afraid of the potential for moisture trapping inside the wall. What are the long-term ramifications for this house using the construction described? The other concern I have is for the OSB's integrity in the plane that it lies- between moisture trapping materials. One detail I am planning to do is take a 6" strip of I&W shield and apply 4.5" to the bottom of the OSB which is flush to the bottom plate and extend 1.5" down the frost wall and then have the Tyvec flash over and taped to the I&W shield.

The other issue yet to be decided is HVAC/domestic hot water choices for this house (there will be an HRV). After reading this informative article on the drawbacks of in-floor hydronic heating
http://www.oregon.gov/ENERGY/CONS/Pages/res/tax/Radiant.aspx I decided not to go with radiant, in addition to the fact that natural gas would be costly for me to pull. The 2" XPS under the slab was placed mostly for "foot comfort" although the jury will have to decide (In my house, I have 2 slabs on grade- one with foam and one without. The one without is definitely cooler) Energy calculations (with its inherent assumptions) done by my architect for this house show that envelope heat loss will be about 11,000 BTUh and infiltrative heat loss will be about the same for a total of 22K Btuh.

Personally I dislike the interior aesthetics of both electric baseboard and minisplits. My architect favors minisplits (3 different units are being proposed), my electrician favors "good-looking" (really?) baseboards that they have now with central A/C and my HVAC installer is suggesting a combination central heat pump/air conditioner (I live in Ithaca, NY Zone 5B) that incorporates an electric heating element for those really cold days. My HVAC person prefers this over minisplits because of "their multiple plastic parts" that seem to need replacing after 3 years. Your suggestions would be greatly appreciated here.

Also your choice for domestic hot water would be welcomed.

I think that does it. Any other information you need will gladly be provided.Thank you.


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Asked by Lowell Garner
Posted Aug 20, 2014 5:11 AM ET
Edited Aug 20, 2014 5:19 AM ET


7 Answers

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I'm not sure what you mean by “24" C/C,” but I'm guessing that you meant to write “24" o.c.” (on center).
Is that right?

You have described the insulation that you will install between framing members as “closed-cell polyurethane foam water blown.” But most closed-cell spray foam is not “water-blown.”

Most water-blown spray foam is open-cell foam, not closed-cell foam. This detail matters, so I want to be sure I understand what kind of spray foam you intend to use.

Answered by Martin Holladay
Posted Aug 20, 2014 7:28 AM ET


You wrote, "My electrician favors 'good-looking' (really?) baseboards that they have now with central A/C."

What does this mean? Electric resistance baseboard units? Hydronic baseboard units? Or a type of forced-air register that is designed to look like a baseboard?

Answered by Martin Holladay
Posted Aug 20, 2014 7:31 AM ET


I think you are likely to be happier with ductless minisplits than with the other suggested HVAC systems. There are lots of articles on ductless minisplits on the GBA site; if you enter "ductless minisplit" in the GBA search box, you should get dozens of articles.

For more information on water heaters, I suggest that you read these two articles:

All About Water Heaters

The Water Heater Payoff

Answered by Martin Holladay
Posted Aug 20, 2014 7:38 AM ET


You have a permit for a set of detailed plans. You are well into building. You are now asking advice about what you already have detailed in your plans and half built????

IMO, the home looks very nice.

IMO the planning is nuts. The design details are horrendous. The BTU numbers for that house should be much less. My old home that is twice the size uses 22k. Yours should use half that or less much of the cold season.

You are overusing foam. You are sandwiching foam. You are using OSB which is a crappy cheap product that loves to fail if kept wet in a foam sandwhich.

You are not using rainscreening.

Your roof is using 1/2" sheathing it should be 5/8".

You are not using a vented roof. That is OK but your roof has to be perfect. Your roof is expensive.

Your insulation is expensive.

No mention of windows or South facing.

EPS as Dana is convincing me slowly is a better choice of rigid foam. 2" of foam is not much under a slab these days. I do 2" under a basement slab and would do more if I ever do a slab on grade especially here in Upstate NY.

I prefer less foam and more mini splits or natural gas or pellet stoves or a combination of them.

You prefer foam and more foam and OSB.

Good luck with that.

Your home would be nice done via drawn up by Bruce Brownell Adirondack Alternative Energy.

Who designed the home?

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 20, 2014 9:38 AM ET


Thank you to you both for your questions and feedback. For sure it sounds like minisplits.

Martin, to answer your questions... Yes, 24" on center studs. By electric baseboard I was referring to the resistance type. And I am talking about water blown closed-cell foam. However I now realize that the use of polyurethane requires HFC (always I think?). We were thinking of this product ECOBAY™ CC CAN. Just curious about icynene (not going to use)- the fact that it is castor oil-based, can it act as a biologic substrate for any "pathogens"?
AJ, totally agree that there is a ton of foam here. According to the new energy codes (at least going by what I have been told) this is what the envelope requires. I can't say that I have done the calculations myself, but my architect is meticulous with numbers. I also agree that OSB is a crappy product when wet compared to Huber's material. However it meets all structural ratings(especially with foam) and I don't intentionally build roofs that leak- to my knowledge they have been perfect. I am concerned about water behind the foam on the wall and I suppose it depends in part where the dew point might actually be located in this wall- in fact that was part of my question here. I did forget to mention that the rigid foam will be installed with Z-flashing/furring. Again I am told that code does require some sort of continuous insulation assembly so that is why there is footer-to-roof foamboard. So this should act as a rain screen. As for the foam sandwich with the OSB between, I am worried- again that was my question. How much of REALISTIC worry should I really have here? To answer your window query, I have attached another photo to show you the south-facing windows present. As for the alternative architect you mention, as you point out, I have plans and a permit and have already begun.

Thank you both again.


Answered by Lowell Garner
Posted Aug 21, 2014 6:17 AM ET
Edited Aug 21, 2014 6:22 AM ET.


If you plan to install a continuous layer of rigid foam on the exterior side of your sheathing, then you shouldn't install closed-cell spray foam between your studs, since this "foam sandwich" approach prevents the OSB from drying. You would be much better off if you specified dense-packed cellulose in your stud bays.

Answered by Martin Holladay
Posted Aug 21, 2014 6:38 AM ET


Martin- he specified water-blown closed cell foam, and it exists. Icynene's water blown 2lb foam is much more vapor permeable than the HFC blown goods. It's a pricey option (you get better bang/buck applying that to the exterior), but it is NOT a moisture trap.

MDR-R-210 is 2.2lb density, about R4.9/inch and about 1-perm @ 2,4", woudl be about a half-perm in a full cavity fill, which is a bit tight, but not any tighter than a kraft-faced batt:


MD-R-200 runs 2lbs density, R5.1/inch and about 1.3 perms @ 3" , and would be about 1-perm in a full cavity fill, which is just fine.


With 24" o.c. spacing the framing fraction is going to come in at about 20%, so when the cavity fill comprising the other 80% is 4x that of the framing wood, the wood is conducting as much heat as the cavity fill, and going much beyond that has severely diminishing returns, since it doesn't thermal break the 20% that is wood. In NY/NE they usually use species, which runs about R1.2/inch, so the framing fraction will be R6.6. With R28 cavity fill more than half the heat loss is going through the wood.

You'd get better overall thermal performance with R20-R23 fiber cavity fill and adding another R6 to the exterior, and it would give you FAR better drying capacity for the sheathing.

With 1.5" of XPS on the exterior you only have about 0.7 perms of drying toward the exterior. It would be sufficient dew-point control for R20 cavity fill, but not R28 cavity fill, but you'd be counting on the 1-perm cavity fill (assuming MD-R-200) to limit winter loading. If instead you went with 3" of unfaced type-II EPS (R12.6)on the exterior and R20 cavity fill you'd have about 1-perm of drying capacity to the exterior, huge dew point margin, and 2-5 perms of drying toward the interior. It's a cheaper and more reliable assembly, and slightly higher performance to boot.

And since EPS is blown with pentane instead of HFC134a, it delivers less than 1% of the climate damage. (7x CO2 instead of 1400x CO2 global warming potential.)

Answered by Dana Dorsett
Posted Aug 21, 2014 6:43 PM ET

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