Hi Everybody,
I am renovating a garage to be a heated space, and am stuck on how to ventilate the hip roof. I live in Northern Minnesota, brrrrrr, and am planning to blow in at least a foot of insulation into the roof. I have drilled holes in the bottom of the roof bays, to let air in from the soffits (will vent those too) and now I need to install (styrofoam?) air channelers into the bays. Does this sound like I am barking up the right tree, and what do I use to let warm air OUT of the roof? I have never worked on a hip roof before, and so would appreciate any thoughts..Thanks!
Replies
I would investigate specific detailing techniques for your area, and consider sealing it up for no flow. ventilated roofs are so 20th-century!
;-)
Forrest
The suggestion of looking into an unvented roof is a good one. It is especially appropriate in a cathedral ceiling.
It is possible to vent the hips. Simply leave the sheathing back the same distance that you would at the ridge and install the vent the same way.
Don't you actually mean to allow air excahnge between the living space and the attic space thereby conditioning the attic space and also to install the thermal barrier at the roof deck? Or do you actually mean to make the attic space a dead air space?
>>>>>>>>>>>>>>>>>>>>>>>>ventilated roofs are so 20th-century!Unvented is the new vented.http://logancustomcopper.com
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When I build a house with a hip roof I put shingle over ridge vent http://www.airvent.com/homeowner/products/ridgeVents-shingleVent.shtml on whatever portion of the ridge is horizontal, and add a few "box vents" to back side of the roof - or maybe even one on each side too. http://www.airvent.com/homeowner/products/staticVents-roofLouvers.shtml This isn't a great solution, but more like the most workable venting solution I have come up with.
I live in a southern climate though and gotta wonder how these would work, or really just about any traditional style roof vent would work with cover of ice and snow for months on end...
Your soffit solution sounds good.
BTW - here is something else to think about. The purpose of attic ventilation is to allow heat any moisture that enters the attic a route of escape. Regarding the moisture, hopefully, the entire exterior of the structure is weather tight so moisture won't be entering that way. Some humidity may enter via the soffit vents, but I guess the thought is that the benefits of the soffit & ridge vents far outweigh the disadvantage of humid air entering the soffits during certain times of the year. That leaves moisture from the interior. Normal living activities like breathing :-), cooking, and bathing all produce indoor humidity. This indoor moisture will migrate into your attic via air leakage. So, if you do your absolute best to stop all air leakage, then you stop the transfer of moisture from the living space to the attic. Leakage around pipes, ducts and wires as well as around any attic entry door are all prime candidates.
Another thought - you said you were "planning to blow in at least a foot of insulation into the roof". Isn't that rather minimal for your climate? That would be about R-30 wouldn't it? Wouldn't you want a minimum of R-38 or more? Also, you didn't say what type of insulation you were planning on using. For a climate like yours fiberglass insulation has 2 distinct disadvantages: 1) it looses some percentage of it's insulating qualities as it becomes very cold, and 2) it's not really an air barrier, and that air will carry moisture. Cellulose might be a better choice for you.
Further, what about vapor barriers in the ceiling? Are these used in your part of the country? I'm guessing yes. Is garage ceiling already installed? If not, you might want to consider installing a vapor barrier, or at least investigate the possibility. BTW - we do not use ceiling vapor barriers in my part of the country, but we have a very different climate.
Edited 12/24/2006 8:46 am ET by Matt
This roof is in an existing garage, already has shingles but no insulation. We will put a vapor barrier on the ceiling, then sheetrock. We are blowing in wool insulation above, are not using batts. It will be more than a foot deep, likely 18" or so. I know that warm air and moisture will escape into the unheated space above the insulation and below the roof, and so am looking for ways to let that moist warm air escape through the roof itself rather than be trapped below.
I think I need to create a "cold" roof; if I just insulate it with no air exchange then I worry about moisture, condensation, etc. Thanks for your advice so far!
Add some top hat vents since you have the soffit vents. You do have less need for venting is you seal and tape the plastic at the cieling well. That will make untrue your thought that you know "that warm air and moisture will escape into the unheated space above the insulation and below the roof,"
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>>We are blowing in wool insulation above << What is wool insulation?
Sheep in your attic.
The last time I used blow in insulation, where you rent the big machine and put packed insulation into a big hopper and it gets torn apart into smaller pieces, the packages (I had a lot of time to read them while waiting for the hopper to clear) said that the insulation was made of wool. It's white, like sheep.
So, I am hearing don't vent the roof at all, and just vapor barrier the ceiling below so moisture doesn't escape into the roof cavity. Makes me nervous, but maybe less so than tearing the ridge course of shingles off and installing cobra venting up there? The other issue is that very few of the bays are empty all the way up to the roof; most have blocking in them that would impede the passage of the cold air through them. So, I'd have to drill holes in the blocking as well. Thanks for all of your comments up to now!
White could be modern rock wool, Chopped fibreglass, or even chopped cotton is sometimes used that way. Cellulose is more grey in colour. All are installed the same, more or less.
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Wanda,
I would vent your roof--even if it is so twentieth century. Here is a hip roof I am presently working on. The shingles are less than 10 years old. The decking has rotted from the inside out because there is no ventilation. I inspected the attic prior to giving a price on this job and the entire underside of the deck was soaked--even though it hadn't rained for days. The house was built around 1960 and the problems started after eave vents were covered by a Sears contractor.
Thanks Mudslinger, I am going to vent. It can't hurt, I have already drilled many many holes in blocking, and fear the kind of damage in your photos. I am thinking of going with those whirly thingys you see spinning on roofs, look like silver mushrooms. My, don't I sound knowledgable! Then I will make sure air can get up the soffits, into the roof cavity, up the bays, and out the whirly thingys. Sound good?
I agree with piffin whirly bird vents.I have started to rethink ridge vent.if you look at the inside of most vents their is filter like fabric (just like a furnace filter)that we think will plug up.Also to properly install ridge vent on hip roof it would need to be installed on hip ridges and the manufacture requires it be set in roof cement.I would not mix top venting ,that is can style vents with ridge vent.I only use one kind of vent and that is GAF's can stle vent with louvers down the side the mushroom stlye almost always let bird in or make a great spot for them to nest.If you don't mind the look of a whirly Bird use them I seen 30 year old models still working and they don't use any electricty
They are called turbine vents - if you are talking about what I think you are.
Regarding the blocking higher up in the rafter bays, unless you are attaching the drywall to the bottom of the roof rafters the blocking is not a really big deal. The air will rise and exit out the top vents in spite of the blocking. If it is a hip roof, many of the rafter bays don't connect to the ridge anyway - right?
Sounds like you are far enough "up nort" that your job will not be inspected? I work in the Minneapolis area and know that R-44 (approx 18 inches of cellulose) is considered the minimum. I would remove the ridge singles and cut the sheathing back to install a continuous ridge vent (you can find a product called Cobra at HD). As for the vapor barrier, in my experience it depends on the local inspector whether or not you need to install one (though not necessarily should install one). The last job i finished, the inspector didn't want to see poly anywhere on the house. Sounds like you are on the right track.
If the hip roof of your garage has a ridge of some length, the continuous ridge vent should work. If your garage does not have a ridge of some lenght, or is a square and has no ridge length, you will need to doo vent hat type venting as a minimum. If you want to introduce some vents with more architectectural design features and want to do more work, you can try to introduce a small louvered dormer feature.
If you use Icynene or other polyurethane liquid spray-foam, you might not need to vent at all. Check with your local building inspector or insulation contractor.
TP
Thanks all - I am going to vent (paranoia wins out) and going with the turbine. There is no ridge venting at all and since I am not going to pull lots of shingles off in the snow to install ridge venting, turbine it is. It's a smallish roof (garage is 24 X 24 or so) so I am only going to install one turbine vent unless there is a marching band that goes by with a banner telling me to install more than one. Thanks again for your help.