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how can I install fiberglass batts on underside of garage roof w/out blocking vent airflow?

I'm insulating and drywalling my garage, so I can set up a climate-comfortable workshop out there.

Nice size garage with a full hip roof, exposed joists and a solar-powered vent fan near the ridge.

 

Insulating and sheetrocking the walls is no problem, but I want to insulate the underside of the roof and maintain the open ceiling joists. How can I install fiberglass batts between the rafters without impeding airflow from the sofit vents to the roof vents?

 

Some of the videos I've watched make it sound like air will still be able to flow up from the sofits behind the fiberglass and out the vents. Doesn't make sense to me, if the batts are sized 3.5 inches to fill the side width of the 2x4 rafters,

where's the room? Are insulating baffles made to direct the airflow around the end of the batt and up the outside of the insulation to the roof?

 

 

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Posted 2013-06-25T22:40:51+0000  by sam23 sam23
 

Sam,

 

The simplest way would be to have the entire underside of the roof spray foamed. The ventilation problem would then be mute, as the whole area will be conditioned air and the roof sufficiently insulated that condensation could not occur. However, I doubt that this would be the cheapest. All existing vents would then be closed off. Building codes might not allow this foam to be left exposed.

 

If you go with fiberglass batts on a hip roof,only those rafters that meet the ridge board will have access to ventilation. One way or the other, 3 1/2 inches of fiberglass isn't much, especially in a really hot or cold climate.

 

I am assuming you want that overhead space for condtioned space storage. If not, you would be far better off by installing a normal ceiling with more adequate insulation. A normal ceiling would completely solve the ventilation problems, assuming that your existing vents are adequate for the area enclosed. A quality pull down staircase is not hard to install before the ceiling goes up. There are some very efficient , weather stripped pull downs on the market.

 

As a bonus, a standard ceiling makes heating and cooling the area much easier and cost efficient. Heat is going to run right up to the top of the roof if left open. Your head will be warm and your feet cold!

 

Just some points to ponder.

Posted 2013-06-25T23:13:28+0000  by ordjen
 
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