Should I put my 2″ DOW insulation board on the outside or inside of my semi-conditioned crawl space?
We at Habitat for Humanity are new to building with crawl spaces in Albuquerque. We are doing it out of necessity to use the dirt elsewhere. I am looking for your thoughts on placing 2” DOW Blue Board, over water proofing, on the exterior of the cmu stems. I would like to continue the 2” blue board up the exterior osb of the framing to prevent thermal bridging. I can build 2X4 walls with cellulous insulation because of the added R-11 blue board. I like the idea of having one thermal line on the exterior wall all the way down to the footing. Some want me to jog the insulation to the inside of the stem. The floor joists are hung inside the cmu stems on a sill plate. I will vent the crawl space with a HRV. The price should not be considered in your comments. We get the Dow Board and other products at no cost.
Thanks for recycling at ReStore and supporting Habitat across the country and around the world.
Doug Keaty
Greater Albuquerque
Habitat for Humanity
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Replies
Others here will differ, but I would never install exterior foam board over OSB, since it is far too vulnerable to moisture damage, including rot and mold (in fact, I won't use OSB anywhere because it is an inferior product to plywood).
Installing a continuous layer of external XPS risks creating a hidden entry for termites. Metal cap flashing is necessary over foundation foam board, whether outside or inside the crawlspace, and outside foam requires protection from physical damage and UV.
You can meet the IECC R-value recommendations with 2x6 advanced framing and dense-pack cellulose, which will create a far more breatheable wall assembly than a foam-covered one. I would avoid the wasted energy of a conditioned crawl space and insulate under the floor joists with foil-faced polyiso board insulation. Make sure you install a ground vapor barrier in the crawl and radon vent through the roof.
Doug,
Robert and I sometimes disagree, and your proposal seems to be an example of such a case. I'm a fan of the detail you propose: continuous XPS foam from the footing to the top of the wall.
1. I live north of the termite line, so I'm unfamiliar with termite issues. If termites preclude your use of exterior foundation foam, then do as Robert suggests.
2. The XPS will greatly improve the thermal performance of your above-grade wall; you'll have less thermal bridging than you would through a 2x6 wall filled with cellulose.
3. You're the first one I've heard of to suggest ventilating a crawl space with an HRV. My reaction is, hunh? Use the HRV upstairs, to provide ventilation for the house. Just seal the crawl space, using all of the recommended sealed-crawl-space details.
Doug,
I design homes in ABQ and at one time did several designs for H4H. You can use the Dow board but make sure you have a protective shield when backfill and landscape; you could use http://www.energyedgeform.com/ for that or have a metal contractor fabricate a protective metal flashing.
A conditioned crawl space is better than a vented crawl space but make sure you follow instructions from http://www.buildingscience.com/ and do a search on Conditioned Crawl Space Details or you could Google too.
To ventilate a crawl space all you need is to supply 1cfm of air per 50 sqft of crawl space. Also install a Class I vapor retarder. See IRC Section 408.3.
3 coat-stucco + #15 Construction Paper (JumboTex 60 min. is better) + taped 2” Rigid foam + 2x4 + cellulose + sheetrock is the best wall assembly for our climate.
I don’t understand “The floor joists are hung inside the cmu stems on a sill plate.” Why not on top of the Sill Plate?
Contact Steve Hale at http://www.buildgreennm.com/ and try to get your project certified to get the tax credits. I’m sure with those specs you can get a HERS 60 or better.
Sorry... I forgot to add the OSB or Plwd to the wall assambly.
Thanks for the information so far.
Armando, you and I met with Diego the day before you left Abq.
I will submit some follow-up questions in a day or two.
Thank you all so much.
DK
The thing to remember is, hot always moves to cold!
And usually the idea of insulation is to keep the heat in, or the heat out.
The heat loss or gain for most homes is by conduction.
The heat in the room, heats the ceilings and walls, then the joists, framing and rafters then the roof,exterior wall or the other way round with heat from the sun.
If we look at this from the point of keeping the heat in, then the insulation needs to be on the inside of the room, breaking the movement of heat from the room into the wood. While dry wood is a good insulation, damp wood is not. The timber in your home is the weak link, helping the heat to disappear.
Two inches of closely butted Blue Board placed below the joists, and on the room side of the frame, will make a very big difference. The key thing is to avoid any gaps or holes, as your expensive heat will be sucked through these holes by the passing wind.
To make an even better job up to Passive House standard, completely fill the spaces between the joists and framing with Blue Board......... with six inches of Blue Board between you and the cold..... your heating will stay off most of the year.
If you insulate, with two inches of Blue Board over your floor with another layer below the joists, again making sure there are no holes or gaps, the joists will stay warm and dry.... there will be no risk of wood rot, damp or mould and your feet will stay warm. Once this is done water vapour will enter your crawl space and make for the cold concrete surfaces to condense.....this will not cause a problem.