Wall and roof moisture concerns; electric HVAC and domestic hot water choices
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.
Posted Wed, 08/20/2014 - 05:11
Edited Wed, 08/20/2014 - 05:19
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