Venting attic: gable vent, yea or nay?
Background: My home is a 1950’s brick Cape Cod with a traditional asphalt shingle roof over 1×8 planks. 2 inch vent channels are installed under the roof deck in the rafter bays and connect the soffit and ridge vents. Insulation consists of one layer of R15 Roxul over the vent channels between the rafters and another layer of R23 Roxul in the space I furred out over the original rafters. Insulation runs from the soffit to the ridge. The original gable vents were closed off and sealed when the new insulation was installed.
The HVAC duct work is housed in the small triangular space created above the collar ties and the insulated roof. While running some electrical I noticed the it is quite a but warmer up there! This is with AC running and the temp at 72 at floor level and no drywall yet installed.
Location: Tidewater area of VA. VA/NC coastal border. Hot, humid summers and mild winters with some occasional stretches of days with freezing temps.
Question: Should open up the gable vents to allow that space to vent or keep them sealed as I originally planned? Please provide a rational for your answer.
Thanks in advance,
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If you insulated your sloped roof properly, the attic above your collar ties should be within the conditioned space of your home. If the attic is noticeably hotter during the summer that the rest of the house, I suspect that (a) there is an imperfect air seal at your thermal boundary (the boundary that follows the insulation), (b) there are problems with the quality of the insulation installation (perhaps insulation gaps), or (c) both a and b.
Has your home ever had a blower door test?
If my understanding is correct -- and you insulated your sloping roof -- you certainly don't want to have any gable vents, since your attic should be inside you home's conditioned envelope. You never want to invite outdoor air inside your conditioned envelope -- unless you are opening a window in mild weather, or pulling air into your home through a mechanical ventilation system.
"The HVAC duct work is housed in the small triangular space created above the collar ties and the insulated roof. "
Are you saying that the insulation is at the collar ties, and the HVAC was installed outside the insulation in that vented micro-attic above the insulation? Or is the HVAC all inside the insulation & pressure boundary of the house, but at the very highest part of the attic?
With the roof insulated the gable ends (including the former vents) should all be air sealed and insulated too. Opening up a gable vent into conditioned space is the same as leaving a window open. But it's worse than a window open at some intermediate floor level, since it maximizes stack effect depressurization & air infiltration flows.
If you can stand listening to somebody who talks as fast as Corbett Lunsford, this 50+ minute primer on the merit-order priorities for retrofit improving the building envelope is pretty good:
If you don't have 50 minutes, the bottom line is that in your heating dominated /mixed climate you need the attic to be as air tight as possible, not punching vent holes in it.
It's normal for air in a room to stratify, with the warmest (= lightest) air congregating to the highest point. So if the HVAC is in above the bar in the A, it's going to be considerably warmer than near the floor, since it's both higher in elevation and more concentrated due to the lower volume. In the cathedralized ceiling of my fully insulated family room there is often a 25 F or higher temperature difference between the floor and apex directly under the ridge.
The insulation runs soffit to ridge and is above the HVAC duct work which is in the conditioned space (no micro attic). I did the absolute best I could while insulating but I'm sure that the air seal is imperfect. I do have several rolls of certainteed membrain smart air barrier that I could install under the the insulation to improve the air seal.