Gable Vents? block them off?

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  #1  
Old 05-06-14, 10:36 AM
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Gable Vents? block them off?

I'm going to start getting ready to insulate my home better, and before I get too involved, I'd like to get your input. I have soffit vents, two gable vents, and 5 ridge vents on the house.

Should I close off my gable vents? is this fighting with my soffit vents?

Thanks in advance.

Adam
 
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Old 05-06-14, 12:31 PM
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Hi Adam,
There are reasons to close off the old gable vents, but reducing the airflow through the soffits isn't one of them, despite how often we see that excuse. Air moves due to a pressure difference across a path or vent in our case. Leaving the old gable vents in place actually increases the pressure difference across the soffits, thus increases the total air flow.

Too much air flow can be bad when it brings in rain or snow, then removing the gable vents is required. But when they have been in place for years you have some idea as to prevailing winds and a good idea as to if that has been a problem in the past.

Leaving gable vents in place also increases the pressure difference across the ceiling plane, so a well air sealed ceiling is very desirable. Since air sealing that ceiling plane is a high priority to start with, we assume it is or will be done anyway.

Natural venting for an attic is a very desirable feature and a little extra air flow can be good. But there are other factors which should be considered, ducts in the attic, plenty of insulation above the ceiling, and well distributed venting. Have you calculated your total NFA vent areas? (Net Free Area).

Bud
 
  #3  
Old 05-06-14, 01:21 PM
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Thanks for the quick reply Bud, I don't think I've had issues with rain, or snow, so that's a good thing. I'm going to do my best to air seal the upper floor of my cape. I'm going to have a difficult time with sealing the top plates, but I'll seal any light fixtures, etc.

I plan on installing approx. R-49 on the ceiling of the 2nd floor, and the two crawlspaces on either side.

I have not done any calculations, but my home is approx 1000 Sq. Ft, and I've got 5 active soffit vents and 5 ridge vents on the south side, and the two gable vents running east-west.

Right now, I don't have very well sealed attic doors, but I plan on tackling all of these issues.

Thanks!
 
  #4  
Old 05-06-14, 01:41 PM
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I'm doing a deep energy upgrade to my cape as well, so do understand the difficulties of getting to everything that needs to be air sealed. Get the big ones, plumbing chaseways, chimney (if there is one) and be sure to seal under the kneewalls. I'll attach a pdf that illustrates one approach to kneewalls.
http://www.finehomebuilding.com/pdf/021230088.pdf

Bud
 
  #5  
Old 05-06-14, 01:50 PM
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Thank you Bud....I air sealed my chimneys, and my stack pipes are under the living area of my 2nd floor, but I was going to use 3/4in poly foam and use caulk, or expanding foam to seal as best as I can.

A cape is a very difficult thing to insulate, especially the sloping parts of the ceiling upstairs...

Bud, is the recommended insulation depth R-49 on either side of the knee walls in the recommended? as well as the ceiling on the 2nd floor? I have not gotten a definitive answer on that.

Thanks!
 
  #6  
Old 05-06-14, 02:24 PM
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R-49 sounds right for code in your area for flat ceilings. Vertical walls are less, not because they require less, but it is difficult (not cost effective) to build an R-49 wall. The slopes are another compromise as they often are not deep enough to accommodate all of the insulation we would want to install. Plus, as in my case, the vent space above provides air flow right across what little fiberglass insulation is already in there. Net result is virtually no r-value. I believe the slopes are allowed to drop to r-30, but even that would require some extra depth. I would have to do some searching to confirm.

If you have this entire area stripped to the studs, then it isn't too bad. Baffles, Roxul, and 2" of rigid insulation, then drywall. Or skip the rigid and strap with 2x4 on edge and install 3.5" Roxul going the other way. Roxul is a bit more expensive but once you work with it you will never go back to fg. And it performs better.

Any air channels that would be considered a fire path need sheet metal or some other fire rated block.

Bud
 
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