i am building an icf house with an estimated R-50 value on the walls. The rafters/ceiling will be 2×12. I will be doing exposed rafter tails with bird blocking between the tails.we will be putting faced batts up each rafter run.
there will be about a 3′ space in the collar ties from gable to gable with gable vents at each end. Or possibly ridge vent.
i guess the other option is veting the bird blocking, which th HO doesn’t want to do.
any suggestions would be appreciated.
RTC
Edited 1/20/2006 9:11 pm ET by RTC
Replies
"we will be putting faced batts up each rafter run."
Why?
A high eff house like that deseves a better roof/insualtion system.
because the rafters are the cieling.
RTC
There is a vent product that goes on top of the roof sheathing that is vented into the attic though a channel cut in the sheathing. I do not know what roof products it works with.
wyatt
Edited 1/20/2006 11:34 pm ET by wyatt
You are in Houston Texas. Don't vent. Spray foam on the bottom of the roof decking or use 4x8 sheets of polyiso foam on top of the roof. I think that's what Bill was referring to.
should i vent the collarties through the gable ? wouldn't do a lot.
RTC
Edited 1/22/2006 12:47 am ET by RTC
Do a search on unvented attics to learn about the benefits in our climate and methods for implementing unvented attics. There are many threads here and on other sites about this subject. You can also look through related issues at http://www.buildingscience.com. Look on that site for recommendations for hot humid climates.
Hi, Me again, from your other thread about the 60 foot ridge
I see problems here too. The main one is that you are letting the HOs tell you what to do when they obviously have no clue, just like with the structural issues.
Quickie primer on insulation and venting for ya-
For venting to work, air has got to flow in at the bottom before it can flow out at the top. A ridge or gable end vent will do almost nothing ( and can do more harm than good) when there is no provision for makup air coming in at the soffit - or the bird blocks in your case.
Another problem you would have that negates any potential help from a gable end vent is that you have that large room in the center with cathedral cieling that would stop any through flow from one gable to the other.
Notwithstanding all the other issues this house seems to have, the best insulation solution would be using Corbond or other spray foam applied in place. That is one that clients love, once they get over having paid for it.
BTW, most ICF walls, while they claim to be R-50, are actually only R-28 to R-36 based on the elements they contain. The manufacturers claim higher performance based on thermal mass and assumed 'equivalent' performance. Your climate may actually let you acheive this, but know that it is based on assumptions and not performance testing.
But since you are aiming for a high end product with this choice, and since most heatloss/gains are in the roof, that should also be equally high end, which is another reason for choosing the spray urethene foam. It is undeniably the best you can have.
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Not to start a fight but....
R-28 on an IFC is the high side.
It's a good wall system strong, high thermal mass and well sealed. But you can't get R-50 out of less than 6" of foam.
Realy good foam has an R value of 6 per inch. Most ICF's are not that dense, and have less than 6" of foam. Concrete R value is approx .2 per inch (that's from memory)
You can do the math yourself.
Garett
ps - spray foam is the complement to your ICF walls
But that's a real, whole wall R-number, unlike you get with say, a framed wall and batts.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Andy and Piffin Your both right.
I really have a problem with the IFC manufacturer that started the whole R-50 advertising. They just don't truthfully represent the product.
A test was done that related that an IFC wall tested preformed equally with a r-50 Fiberglass insulated stud wall. (whole wall test) Given that at low temperatures FG insulation loses over 30 percent of its R-value it is not hard to understand the results of this test. This has been misinterpreted and transformed by salesman and installers to an R-50 "rating" it is not.
The customer is sold a product that is not as advertised.
I do like the product, but it would be nice to see "truth in advertising" extend to ICF sales. Check out the regulations that are applicable to insulation sales : ttp://www.ftc.gov/bcp/rulemaking/rvalue/16cfr460.htm
and the R-value myth, http://www.energywisemortgage.com/r-value_myth.htm
this case is for loos fill FG but others show similar results for batts.
Not fighting either,
Garett
Actually, I'd like to see the truth in advertising laws applied to fg...
One part of how I earn my keep these days is doing home energy consultations. Not even the installers understand how FG insulation works and doesn't. It's frustrating to crawl through the attic of someone's new house and see missing insulation, compressed insulation, leaking recessed lights, you name it. "But the builder said I have the code-required R-value." Makes me wonder what the insulation in the walls that I can't see looks like.
I even saw a bad Icynene job the other day. That is, I don't think you're supposed to be able to view the back side of the drywall in a foamed attic. Where were the adults?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
I won't fight - but here is how I got my figures, other than memory that they are advertised that way - up to R-28. I've also heard manufacturers claim R-50 and I was pointing out to the OP that he will fall short of that - anyways, I figure R5.2 for the foam, which is 2.2" thick on each side. That gives 22.88 R-value for just the foam.
Crete is R-one for a foot unless it is air entrained or has beads, but there is no need or desire for either in ICFsSo I have a wall in raw form that is maybe a bit over R-23.now I'm going to finish the interior and exterior with something. That something will add to the R-value, generally no more than R-one or two, but can be more, depending on material and detailing. Still I agree, R-28 is likely the max.
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