I am a newbie to this forum but have subscribed to Fine Homebuilding for years. I live in Salt Lake City UT (approx 4500 ft elevation with cold dry winters/hot dry summers). I am currently in the process of building a well insulated new home. My insulation contractor has recommended spraying the underside of the cathedral ceilings with open cell foam directly onto the underside of the roof sheathing. My contractor says he has always left a one inch airspace between the insulation and the sheathing, drilling holes in the blocking and placing a ridge vent to allow condensation to evaporate. Insulation pro claims that is old school, that with high R-value foam and decreased air penetration that the dew point is far removed from the underside of the roof and by minimizing the vapor barrier on the ceiling(warm side) that the structure actually dries from the inside and is a better system. He also cites the new code requirements that no longer mandate the airspace. The building science website seems to argue that both can work depending on the climate.
Does anyone have any experience in this area to offer a third opinion?
Thanks,
Jack
Replies
Your insulation pro is correct, 1" air space is not required so long as you use Elk or Certainteed roofing shingles.
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"You cannot work hard enough to make up for a sloppy estimate."
With enough sparay faom, no venting is needed re dealing with condensation, which is what codes adressed.
Some shingle manufacturese still fail to warrant product on hot roof (which is the term for having insulation with no venting immediately on the back side of the sheathing) because the shingles do stay a bit hotter and accelerate cooking off the volatile oils, tho some studies show the aging is negligible, at most about a 2% reduction in lifespan which is a few months to a year.
The other factor is that in a good tight house like this, you NEEED to consider a heatr rocovery and ventilation system for the whole interior airspace to avoid environmental conditions that are hard on your health or that encourage mold growth
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The first two posts are absolutely correct. Any shingle will be fine. The Elk & Certainteed just keep the already lame warrantee in place with the "hot roof" insulation method.
The code says the insulation must be "air impermiable". That defines to a certain air perm rating. The only open cell foam I am aware of that meets that code is Icynene. So, make sure it's Icynene they use and not a cheap imitation.
Stu
Read this it just came out. Good info on your Q
http://www.jlconline.com/cgi-bin/jlconline.storefront/45eaceff0018964427177f00000105d2/UserTemplate/82?s=45eaceff0018964427177f00000105d2&c=99bed6c24948e67882b7559f41b7f06d&p=1
What a great link! Thanks reinvent! It just answered 6 very basic questions about the whole debate! I'm getting into the whole closed cell thing with Zerodraft, and i'll pose the link to by building officials! Cheers!
d
Its a horse thing!
I have run into the same situation. My take from all I have read on BT for the last 3 weeks is that closed cell in teh entire above-grade home in Michigan is the best choice. I have insulation contractors trying to talk me into open-cell foam and I cannto figure out why.
Maybe it is easier for them to work with, maybe there is higher profit margin on open-cell, I don't know. Maybe they get less pushback from inspectors for vented attic space or other considerations (perceived insulation value with such a thin application of material with closed-cell), etc.
Or maybe it is the best choice. My gut says go with closed cell based on all I have read in articles and on this forum. My gut has never insulated a home.
Thoughts?
My only concern with the open cell foam would be the "minimizing the vapor barrier" part of your post.
I'm not familiar enough with your climate to tell you right or wrong regarding the vapor barrier, as you're in a high elevation, somewhat arid environment and I'm in New England.
But as far as having foam against the bottom of the roof's sheather, your insulation guy is correct.
Personally, I prefer closed cell foam with its higher R-value per inch and better air sealing and vapor retarding proerties.
You may want to take a gander at buildingscience.com and see if you can get some info there.
Mongo
Thanks to all of you for taking the time to post in response to my question. I feel much more comfortable accepting Dan the insulation guy's recommendations after hearing your opinions. I'll have to work on my general contractor now.
Reinvent; Thanks for the link. I've read through it. It seems to apply directly. I'll show it to my general and see what he thinks. He has argued that he is ultimately responsible for the construction and has always vented.
Mongo; Thanks, I agree about closed cell foam. I asked for the bid specifying closed cell but Dan recommends open cell because he says that closed cell is more expensive and I would be wasting my money. I have 12" in the joist space so maybe he is right.
Piffin; Thanks. I'm planing a standing seam metal roof for low maintainice and to let the snow slide off rather than shingles. I agree with you about adequate ventilation in a tight house and asked for a HRV in the HVAC bids. Interestingly in my favorite bid right now the HVAC guy argued against an HRV and designed in just fresh air being drawn into the house into the cool air return duct back to the air handler with exhaust of air from the house(no heat exchange). He argues that the heat saved is not worth the money spent on an HRV. Interesting how many opinions there are out there!
MASprayfoam; Thanks. Didn't realize there is much of a difference between open cell products. Most just seem to specify "1/2 lb foam"
in the bid. I'll pin him down for a product name.
Sorry this is so long. Thanks againt to everyone.
Jack
Your general likes to 'vent', maybe he should see a therapist.lol. He falls under the category of old dog new tricks. Keep us posted on what happens and show us some pics.
Gasser, has your HVAC guy ever run the calcs on heat loss due to ventilation without HRV? Show him. For example, for a house of 30,000 cu.ft. volume (floor area x wall height x #floors), at 0.35 ACH, air density 0.076 lb/cuft, heat capacity 0.24 BTU/lb/degF, inside at 70 F, outside at 20 F, that comes to 9600 BTU/hr, rounded up. That is a sizable piece of the total heat loss for a house that meets energy code, more for one that supposedly is Energy Star. An HRV at 75% would cut that "controlled air infiltration" to a more reasonable 2400 BTU/hr. The savings, 7200 BTU/hr, 172,800 BTU/day, or 50.6 kwh/day. At electric rates of, say, $0.10/kwh (YAMMV), that is about $5/day, every day while the air is cold outside. Scale down according to fuel costs in your area.
"Thanks, I agree about closed cell foam. I asked for the bid specifying closed cell but Dan recommends open cell because he says that closed cell is more expensive and I would be wasting my money. I have 12" in the joist space so maybe he is right."
Did you ever ask Dan why it was more expensive? Their is a reason! One that he should be able to distinguish bewteen.
Open cell against a roof deck should only be done if using a vapor retarder interior to that as open cell is vapor peremable.
Closed cell is more expensive because it offer a high R value and it negates one having to add a vapor retarder as its a vapor retarder. Study up on the permability of open cell vs closed cell and the R-Value that each offers.
Edited 3/9/2007 5:15 pm by fall50
The blanket statement is not always true concerning the VB over soft, or low density, foam like Icynene. In cold climates it may be required depending on heating degree days, indoor environment and so on. Not as cut and dry as you lead people to believe.BTW, the ICC and every code based on that states the insulation against the roof deck must be an air barrier. Icynene is the only low density foam I know that qualifies!!! Do not be a test subject for the Johnny come lately foam!!!Stu
MaSprayfoam,
Fair enough. Would a better way to put it be in most but not all cold climates open cell foam would require a VB. Hence considering closed cell foam in most cases may be benefical as one would have to condider the extra cost and labor to add such when closed cells offers it all in one.
And in predominately cooling climates open cell, may, but not always be the more logical choice.
I saw the OP was in UTAH to me the he falls into a cold climate hence closed cell.
MASpray, question lets say open cell is sprayed in a cold climate to a roof deck and the need for a VB exists. Would drywall serve as appropiate VB or would you need sheet rock+ a vapor retarding paint? Or even perhaps some XPS then sheetrock
I hear you. This is exactly why I spray both. Different opinions. In the cold climate roof you mention where a VB is desired one could either paint the foam with a VB paint or sheetrock and VB paint that. The VB in direct contact with the insulation is beat so I would choose to paint the foam inself.
Closed cell is more expensive because it takes more raw material to produce the foam.1/2 lb foam (open cell, soft foam)takes a half a pound of material to produce a cubic foot, 2 lb (closed cell, hard foam) takes 4 times the material for the same cubic foot, while the
R-value is not double. That's why it cost more to install, not to including the install time diference.Both foam products have there uses but I still like wall spray and dense pack cellulose when I can make it work.Garett