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Old 04-10-2015, 09:09 AM   #1
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Cool my garage attic


I've been storing stuff up in my garage attic but here in Texas, especially when the afternoon sun beats down on the west-facing half of the roof... well, I would prefer it to be cooler up there. Garage is a 20x24' new construction, with asphalt shingles on OSB, vented gable roof. I put R-13 between the floor joists and have some loose decking boards over that.

Would it help to put some insulation on the rafters? I'm looking for a good balance between cost, performance and ease of a one-man install job?

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Old 04-10-2015, 07:00 PM   #2
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how about a power vent.....
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Old 04-11-2015, 03:24 AM   #3
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+1

More venting will probably be more effective and won't impact shingle life either.
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Old 04-14-2015, 02:26 PM   #4
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Power vent might be the best way to cool but I want something passive. Though if some kind of insulation on the rafters wouldn't do any good, I'd like to hear that, too.
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Old 04-14-2015, 03:09 PM   #5
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Insulation in the rafters won't do much good and would over heat the roof potential causing premature shingle failures . More ventilation . A larger gable vent, perhaps a ridge vent and then some way for cool air to get in the lower part of the garage. You need to exhaust the hot air and let cooler site near the floor
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Old 04-14-2015, 05:54 PM   #6
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Solar powered attic fan is probably your best bet here.
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Old 04-15-2015, 08:18 AM   #7
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My thought was to nail mylar of some kind on the rafters, with the idea that the trapped heat would migrate up to the ridge vent instead of coming into the storage area. Not going to be worth the trouble?

The roof has a ridge vent but no gable vent.
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Old 04-15-2015, 10:10 AM   #8
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Soffits?
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Old 04-17-2015, 03:59 PM   #9
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Power vents are expensive to run, solar ones are also, if insulated already, per code minimum; http://ecobrooklyn.com/solar-powered-attic-fans/

Payback of solar powered with varied insulation; http://epics.ecn.purdue.edu/hfh/_tea...larresults.htm

Only 5-10 % less service life of shingles; http://seattlewindowsanddoors.com/20...-temperatures/

RB will work great for your application on the rafter bottoms, not over the insulation- #4; http://www.mytexasinspector.com/pdf/...heet%20DOE.pdf

Stops about 28% of the radiant heat flux; http://www.rfoil.com/pdf/8-Effect-of...erformance.pdf

OR, a pricier RB (foil side to interior) that is vapor permeable; http://www2.dupont.com/Tyvek_Weather...AWSBK14127.pdf

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Old 04-25-2015, 12:28 PM   #10
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Thanks Gary. Third link was pretty close to my situation:

Example 1
I live in Orlando, Florida in an 1800 square foot ranch style house. I have R-11 insulation in my attic, and the air-conditioning ducts are in the attic. A contractor has quoted a price for a radiant barrier installed on the bottoms of my rafters and on the gable ends for $400. Would this be a good investment? Answer: Yes
this would be a good investment.
So it does seem like a RB attached to the garage rafters will keep the attic cooler. Should I try to install it all the way down to the top plate, or leave a foot or two open at the bottom?
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Old 04-26-2015, 10:30 AM   #11
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Do you have rafters or engineered trusses?

Radiant barrier can be of some effectiveness but you still need to vent the roof at the ridge if you are going to take full advantage of that radiant barrier. There will be considerable convective currents behind the radiant barrier and there will be appreciable draw near the eaves. In order to get that to work, ideally you would vent at the ridge and have a nice completed (i.e intake to work with exhaust) system.

You would leave it open at the bottom.

Sounds like you have the ridge vent so you are good there. Just make sure the soffits are good and open and unobstructed.
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Old 04-26-2015, 11:01 AM   #12
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Quote:
Originally Posted by Gary in WA View Post
Power vents are expensive to run, solar ones are also, if insulated already, per code minimum; http://ecobrooklyn.com/solar-powered-attic-fans/
Gary,

I agree with everything in the article as it pertains to the potential issues with PAVs (depressurization and sucking conditioned air out).

I wholeheartedly agree that passive ventilation is superior, safer, and should be the default.

I don't see how a solar powered fan, in this case, cost money to run. The attic and ductwork in the attics should always be well sealed. Saying to not use "X" because it may highlight deficiencies in the other building systems is like saying we shouldn't build a 2 story house because it is more likely to collapse (as compared to structuring it correctly).

The RB will probably be the better fit if the attic is set up for it and will work.

I am guessing the home is on a heat pump (i.e. not requiring the heating BTUs of a colder climate) so the concerns about venting the combustion should me moot. The homeowner should still try to seal up the top plates and ductwork for general efficiency purposes and performance.

Dropping on a solar power PAV, while short circuiting the venting, would still help to strip out some heat in those times with the sun is directly heating up that attic.

If it will work with the RB option, that is probably better too.
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Old 04-26-2015, 03:36 PM   #13
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Figure your energy savings with a solar powered fan against the initial outlay for the product, after the ducting (or not) is air sealed/insulated/vapor barrier; http://www.greenbuildingadvisor.com/...r-do-they-hurt

Really need venting at top/bottom as others brought out. OR go with a sealed attic...

RB is open at top and bottom to exhaust attic heat. 20 x 24 garage with heat pump.....

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Old 04-26-2015, 07:37 PM   #14
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This is OP. To summarize, the gable roof garage in question has a ridge vent. I do not want to add any more venting. Garage attic floor has R-13 batts. But is still too hot up there on summer afternoons.

Based on responses so far it looks like I can reduce attic temps by installing reflective sheets on the rafters. This will be enough to go from unbearable temperatures in the attic to bearable, like maybe from 95 to 80?

Installation should start a foot or two above the top plate and go up to a couple of feet from the ridge. All sound correct?
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Old 04-27-2015, 06:40 AM   #15
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Quote:
Originally Posted by Gary in WA View Post
Figure your energy savings with a solar powered fan against the initial outlay for the product, after the ducting (or not) is air sealed/insulated/vapor barrier; http://www.greenbuildingadvisor.com/...r-do-they-hurt

Really need venting at top/bottom as others brought out. OR go with a sealed attic...

RB is open at top and bottom to exhaust attic heat. 20 x 24 garage with heat pump.....

Gary
Solid advice. The Solar powered fans are not cheap.

I do agree with you by the way. I had a previous client that was convinced that an attic fan kept her storage area cooler. I tried to explain it to her but she could not grasp the fact that she was effective air conditioning the attic via the fan. Go figure.

Quote:
Originally Posted by puttster View Post
This is OP. To summarize, the gable roof garage in question has a ridge vent. I do not want to add any more venting. Garage attic floor has R-13 batts. But is still too hot up there on summer afternoons.

Based on responses so far it looks like I can reduce attic temps by installing reflective sheets on the rafters. This will be enough to go from unbearable temperatures in the attic to bearable, like maybe from 95 to 80?

Installation should start a foot or two above the top plate and go up to a couple of feet from the ridge. All sound correct?
I still don't hear a mention of soffits? We need to confirm that the soffits are intact and adequate.

That is the only way that RB is going to work.

As Gary stated...it would be open at both ends. I stated that it needed to be open at the bottom because I thought it was settled that it was open at the top.
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