Does an attic need to be ventilated?
This is related to another thread, but is really a different topic.
Here’s the situation, we’re insulating a previously unfinished attic of a 1915 home in order to create more living space. My assumption going into the project was that we would put baffles in all the bays to allow air to flow from the soffit vents, behind the insulation, all the way to the roof vent in order to keep the roof cool.
However, upon further inspection yesterday by the GC, it turns out that there are no soffit vents at all, and no ridge vent. Taking a look up and down my street (all built around the same time), it looks like none of the houses have soffit vents. Unfortunately, we have a slate roof, so while putting in a ridge vent is possible, it’s not at all an easy, or inexpensive thing to do.
So, the GC suggested that we don’t bother trying to ventilate under the roof at all, but use Icynene insulation in all the bays to create an airtight seal under the roof. We talked about bumping out the rafters to allow space for 6-9″ of Icynene, which would create a very solid thermal barrier (except where the rafters are, of course). With that level of insulation, the roof should be able to stay cool. But, this is definitely a controversial topic.
Replies
Forget the vents at this point and spray foam the underside of the roof deck (don't forget the knee walls).
"Forget the vents at this point and spray foam the underside of the roof deck (don't forget the knee walls).Forget the vents at this point and spray foam the underside of the roof deck (don't forget the knee walls)."Forget the kneewalls and do the underside of the roof deck. If you do all of the roof deck that puts the kneewalls in conditioned space.And eliminates problems with short circuiting the insulation with air paths under the floors..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Did not know if the exterior walls were framed above the ceiling. I have seen a couple of cases where there was a 2-3' wall at the roof edge and they foamed the roof deck and not the perimiter knee (actually stub) wall of the building. Of course if there is a knee wall set in from the perimeter of the dwelling then he would want to include that storage space as conditioned space.
There will be 5' kneewalls, which are required in my area. I will insulate down to the eaves, so that we can use the area behind the kneewalls as storage space, and also so that the ductwork that is running in there is in conditioned space.
IMHO you are on the right track.
It is only a controversial topic to those who are uneducated on the subject.
Educate them:
http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
You haven't filled out your profile, so I have no idea what your typical climate might be.
That said, houses built back then weren't especially air tight. While there was no specific ventilation incorporated into the roof structure, it occured naturally, to some degree.
But the idea is not to keep the roof "cool".
The idea behind ventilation is to keep the interior of the roof at the same (or close to) temperature as the exterior, in order to avoid condensation. But this goes back to your local climate.
J. D. Reynolds
Home Improvements
I'm in the NorthEast. What I meant by "keep your roof cool" was to try and make sure that the roof doesn't become warm to melt snow, creating ice dams.
What you're proposing is actually called a Hot Roof and is done successfully all the time.
Like Piffin said, polyurethane foam (closed cell) is better than Icynene (open cell) for your application because it doesn't allow as much moisture transmission.
Is there a big cost differential between closed cell and open cell? Closed cells seems a lot better, so what would be the advantage of open cell unless its cost?
Cost varies a lot, but generally closed cell is anywhere from the same price to twice as much as Icynene for the same R-value. The difference is you get more R-value per inch with closed cell.
Some people claim there are advantages to letting the foam "breathe." I don't buy it, but I have had at least one situation where it really didn't matter if the foam "breathed" or not. Also Icynene expands more, which could be good in some situations. Generally closed cell is better on all accounts though.
Where are you in the NE? We have a good foam contractor here in the Portland area.
spray foam is your answer. Polyurethene is far better than the icy tho, IMO. It is a true VB and offers about twice the R value per inch of foam.
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About time you gave up on that idea.
Ths the same GC who told you vents were required?
Joe H
I answered your question about the importance of roof venting in your other thread. An unvented roof is OK as a last resort where venting is too difficult.
If you must use a hot roof, then I would also suggest you go with urethane (R-5.9/in) instead of Icynene (R-3.6/in).
The rationale (which is still highly controversial among those who are highly educated on the subject, contrary to what reinvent claimed), is the spayed-in-place foam won't allow any moisture to migrate to the underside of the roof surface. But Icynene has a perm rating of 10 at 5" thickness, and is by no means a vapor barrier.
Regardless of what insulation system you decide to use, I would strongly recommend a thermal break on the underside of the rafters, otherwise you will lose 30% of the R-value by the thermal bridging of the rafters (at 9" Icynene, more than that if you use a higher R-value foam).
Edited 1/22/2008 8:26 pm ET by Riversong
Why use urethene at R5.9 when you can get it R7?
or are you just using od figures?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
"Why use urethene at R5.9 when you can get it R7?or are you just using od figures?"
I'm not sure what you mean by "od figures", but ASHRAE lists sprayed urethane as R5.6 - R6.2.
If someone is trying to sell you R-7 urethane, they probably also have a bridge in Brooklyn for sale.
Corbond goes in at 6.8 to 7.2 and settles to 6.8 testedFoam can be applied at R10. it is all a matter of density and how much expansion agent is usedin the mix.http://www.builderonline.com/industry-news-print.asp?sectionID=0&articleID=50812"The R-value rating for soft foam is about 3.7 per inch, while rigid foam has an initial R-value rating of 7 per inch. And because rigid foam cures stiff, it helps construct a stronger wall."
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It does seem that a few inches of closed cell foam directly to the underside of the roof would accomplish the goal of VB and getting an airtight seal. Then the rest could be filled in with spray in cellulose. In terms of bang for the buck, I think I'd get better insulation with a few inches of cc foam, and putting 1" of hard foam over the rafters and behind the drywall, than 6" of cc foam which would leave the rafters as a thermal bridge.
You're on the right track now.
More bang for the bucks and no tearing up the roof and soffits.
Joe H
Had a chance to speak to an insulation contractor this evening to give some prices around these options.
He gave me prices of:
R38 Fiberglass with baffle - $1.40 sqare foot
R38 Cellulose with baffle - $2.50 square foot
Polyurethane (4" deep R24) - $6 square foot
Both of the first two options would require firring out the rafters with a 2X4, on end, and securing them to the rafters with a gusset plate. They would also require putting in a gable vent - he suggested putting in a powered fan with a thermometer and humidity sensor.
Now, I have about 1700 square feet of rafters that need to be insulated. So, the economics come out to be:
Fiberglass: $1,680, plus cost of gable vent
Cellulose: $3,000, plus cost of gable vent
Polyurethane: $7,200
He didn't recommend putting in more than 4" of poly foam. He said that he doesn't believe it is really necessary, that you get a really good barrier with only 4". I know technically it is only R24, but that R24 is a heck of a lot tighter than the R24 you get with other types of insulation.
If you're asking my opinion, none of the above.
If you've done your reading here and elsewhere you know the fiberglass is a waste of money.
#2, you're back to ventilating the attic for no reason.
My choice would be 2" of foam and densepack cells.
Best might be with an additional sheet of foam on the bottom, taped and foamed at the edges & then the DW.
Joe H
<<6-9" of Icynene>>
You might want to price that. That much foam would be $$$. You could spray a couple inches of foam to get the vapor resistance and adhesion, then fill the rest with cells for a lot less money.
Splintergroupie,
Responding to a discussion about Icynene, you wrote, "You could spray a couple inches of foam to get the vapor resistance and adhesion." But Icynene is vapor permeable -- and the thinner the foam, the more permeable it is. If you want vapor resistance, a couple of inches of Icynene certainly won't provide it. It is sponge-like.
Agreed, but i was quoting the OP. Perhpas i should have emphasized the depth not the material, but the other posters have told him already about closed v. open cell.