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debiasio 08-29-2012 01:37 PM

attic ventilation
 
2 Attachment(s)
Hello -

I live in a 1940s cape home in MA (see photos) and we suffer each summer on the 2nd floor, which doesn't seem to cool down on its own on cool nights. The outside air can be 65-70, but the rooms stay 75-80. I'm suspecting that hot air is trapped in the attic and is to blame. We can run window AC units but that gets to be a drag on our electric bill.

The only venting present are two 12x9" gable louvers on each end of the house. There is no ridge nor soffit venting.

I've read that ridge vents are great, but aren't as useful with the presence of gable vents. Although I don't understand this because wouldn't hot air rise and escape through the ridge vent and pull the outside air through the gable vents? I also don't want to cut an opening in my roof if I don't need to.

Or, I could forget the ridge vent and just add a powered gable fan and plenum to drive hot air out one and gable suck cooler air in through the other? Yes its extra electricity, but I'd only run it for a few hrs at night.

Any other thoughts would be appreciated. Thanks.

AGWhitehouse 08-29-2012 01:46 PM

The ridge vent won't really work with the gable vents because there is not enough vertical distance between them. Code has a measure of 3 vertical feet between intake and exhaust before a different set of ventilation parameters are enacted. This difference in height is what makes natural thermal convection become active.

With your kind of house it would be difficult to traditionally vent the roof/attic without extensive interior work that would require close to a full gut. A powered gable vent fan on a humistat/thermostat does work, to a certain extent, and is the cheapest solution. The best solution, though no-where near cheap, is to replace your roof and provide a venting plane there. You could also insulate at the roof level while your at it and reduce the temperature swings of the upper story. You would essentially add another layer of sheating on your roof and then new shingles. This would provide and air space for heat-laden air to move. here is a generic photo of a vented roof sheathing http://www.alcespostandbeam.com/SIPs...ent_02_med.jpg

They also make insulated vented roof panels. they look like this: http://www.flickr.com/photos/hunterpanels/5654955641/ these offer the added benefit of additional insulation while provided the appropriate ventilation.

debiasio 08-29-2012 03:51 PM

Thanks for the reply and the links. I had never seen panels like that before. Looks expensive to implement that type of ventilation, however.

bcdemon 08-29-2012 05:15 PM

It doesn't appear you have vented soffits, am I correct?
If not, you could go with the power gable vent and I would install it on the side of your house that is in the sunlight in the afternoon/evening so It can blow hot air out and suck in cooler air from the shaded side of your house.

debiasio 08-29-2012 05:38 PM

attic ventilation
 
That is correct. The soffits aren't vented. I was thinking the same thing you mentioned, using a powered gable fan to provide active ventilation.

AGWhitehouse 08-30-2012 09:27 AM

Quote:

Originally Posted by debiasio (Post 999285)
That is correct. The soffits aren't vented. I was thinking the same thing you mentioned, using a powered gable fan to provide active ventilation.

This is the quick/cheap method, though it's not going to solve your heat gain issues in the upper rooms. It may help slightly, but won't solve. You have alot of roof surface that is in direct contact with the rooms in question and the heat transmittance through that assembly cannot be mitigated with a fan in that tiny attic space.


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