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Has anyone used a roof venting system (intake air) like Smartvent

Posted by mahnrut (My Page) on
Wed, Mar 25, 09 at 18:35

In our family we have a 1940s house with a relatively low slate roof which we need to replace. We have decided on asphalt shingles. The house has no place for soffit vents and currently has one gable vent. Other than this low attic space (you can't stand up in the middle) getting very very hot in the DC area summer there have been no problems with this roof set up - no leaks or condensation that we can see over the past 70 years and although only used to store boxes it would be nice to be able to access it in the summer.

Although with a new roof there are many seemingly viable options for venting the hot air (ridge vents, solar vent fans etc.) these will only work if the intake air is sufficient and without the possibility of soffit vents this is problematic.

I read a bit about the smartvent system where a vent for intake air is put into the bottom part of the roof decking but I have not read any independent studies or reviews of this system. Are there any other systems and does anyone have any information to share on this issue in general. In our area there are hundreds and thousands of houses like this one and most of them I see with retrofitted asphalt roofing have no vents other than the mostly one gable vent the house came with when it had a slate roof. A big market here I would think. Any help much appreciated.

Follow-Up Postings:

RE: Has anyone used a roof venting system (intake air) like Smart

The SmartVent system will allow a small amount of outside air to enter at the eave and if there is a ridge vent a similar amount of air will exit at the ridge. The purpose of this system is to allow excess moisture to escape from the attic. Don't expect this system to lower the temperature of the attic air or to lower the temperature of the asphalt shingles or increase their longevity.

Asphalt shingles get hot because of direct exposure from the sun. If you want your asphalt shingles to last longer select a lighter color or shade the roof. If you wish to decrease the heat gain in your house add more insulation in the attic floor or between the rafters (maintaining the vent space). A radiant barrier under the rafters will also lower the temperature of the attic air.

RE: Has anyone used a roof venting system (intake air) like Smart

Thanks Mightyanvil. I hoped that the smart vent would do a bit more but this information is clear and very helpful. The insulation is pretty good in the attic floor, there is nothing under the roof.

Do you think that it is asking for trouble with a new asphalt roof (which will be applied over the old boards as they seem to be tight enough) plus the existing passive gable vent to do nothing more?. Or would you do all of the things you mention. Thanks.

RE: Has anyone used a roof venting system (intake air) like Smart

Check the open area of your vent and compare it to the floor area of your attic. The typical moisture control ventilation ratio is no different for asphalt shingles than for slate shingles. You usually only need a clear ventilation area of 1:300. If there is a vapor barrier at the ceiling below or a eave & ridge vent system that ratio can be 1:150.

Moisture vapor pressure is like air pressure only much slower. Excessive moisture will leave the attic if the outside air is drier and there is an adequate path. In a humid climate where outside air can be more humid than the attic air, ventilation can hurt more than it helps.

RE: Has anyone used a roof venting system (intake air) like Smart

It is a small house and the attic floor area is 450 sq ft but the one gable vent is pretty large with the bottom part of the triangle being 6'. Assuming 1" of air at each vane (likely more than that but hard to see from the ground as the vanes slope in) I estimate about 2 sq ft of opening so the ratio is met for 1/300. I couldn't understand why "with a vapor barrier at the ceiling below or a eave & ridge vent system" the "ratio can be 1:150". I would have thought that you would have wanted less ventilation per area. Please explain. I appreciate your time again.

RE: Has anyone used a roof venting system (intake air) like Smart

Your assumption is correct; I reversed the numbers.

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