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Old 04-13-2007, 06:09 PM   #1
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advice for insul./vent. of garage roof?


The scenario is this, the primary purpose of my two car garage (just under 400 sqft) is to shelter one vehicle. The secondary purpose will be a home shop. I'm a cabinetmaker by trade. I want to insulate the space for both acoustic and thermal reasons. The location is in a mild climate zone that mandates neither active heating or cooling. The insulation will provide however, the passive benefits of both.

The garage has a mansard style roof. It's built with 2x4 trusses. I want to insulate it along the rooflines, keeping open the framing to the lower work area. I have already furred out the rafters (or top cords) with 2x2s, so as to create air space between the insulation and the sheathing. There is a central roof vent already in place. I'm thinking that I should sister in some framing that creates a larger air space under the central vent. Question #1. How much space would be wise?

Now the harder part...

My biggest dilemma is that there is almost no overhang between the fascia (with gutters attached) and the top plate of the wall framing...i.e. no real soffit space. I bought a bag of 1 in. diameter soffit vents but it seems an awkward solution to drill holes in that slight 1.5" wide overhang (which seems to be a subfacia piece of lumber), as the holes have to be canted to come through above the top plate. I started this process in one section, but stopped after breaking my spade bit on a nail and having second thoughts about the structural compromises I was making to the framing. Question #2. Is it essential that I provide some type of soffit venting? And or... can someone with more experience and knowledge tell me of another way to deal with this issue?

I hastily bought the insulation, which is now robbing me of valuable floor space, and enough time has passed, so that I'd have trouble returning it. So I've decided I ought to get back on track with my original plan, & for many reasons would like to figure out how to get the insulation where it was intended to be (without creating problems with ventilation). I'm looking for an informed and creative solution, as I want to do the right thing ... and I need to get past this roadblock to my goal.

Can you help me make this happen?

P.S. I'm not interested in insulating the bottom cord of the truss, as I want access to and use of the height up in the truss area.

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Old 04-14-2007, 08:14 AM   #2
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advice for insul./vent. of garage roof?


I don't think you could put in enough 1" soffit vents to be useful. With a 1 1/2" overhang, I think the best you could do is a continous soffit vent. The amount of venting you should have is based on a formular. So many linear feet of ridge vent requires so many sq inches of soffit vent. Do the math before the job to see if this type of venting is even feasible. Other options are gable vents, roof vents or power vents.
I would drop the common space for air flow 2 feet below the ridge beam. More if you want to install a power vent. Then just put the rafter ties below the vent.
Ron

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Old 04-15-2007, 10:47 AM   #3
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advice for insul./vent. of garage roof?


To: Ron 6519

Thanks for replying. Food for thought. In my searching around, I read some info from a site posted by the Canadian govt'. It was some kind of treatise on insulating, and it had some specifics on Mansards and it almost lead me to believe that the steep angle is still more like a wall than a roof? This made me wonder if I could just leave the two feet of air space below that central mushroom vent at the ridge, and just insulate the steep part, leaving the furred out space of the 2x2s. Also since it is just a garage space, and I'm not so worried about the thermal (as much as the acoustic), would it be absurd to leave air flow space open from the inside (above the top plate)?
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Old 04-15-2007, 02:37 PM   #4
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advice for insul./vent. of garage roof?


You just want air flow through the underside of the sheathing. You can install the insulation up the Mansard "wall", just leave a space between the insulation and the roof sheathing for air flow. Otherwise the roof shingles/sheathing , "cook" , reducing their effective life. The open area will help the air flow as long as you have adequate intake to facilitate the air stream.
Sort of like a whole house fan. You need to open doors or windows to have the air flow properly out of the house.
Ron
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