Replies

1. GBA Editor
   Martin Holladay | Jul 21, 2018 06:24am | #1

   Dylan,
   You are using vocabulary that I'm unfamiliar with. I'm not sure what you mean by a "lean-to." Is that the same as a shed roof? If so, why does your roof have two sides? If the roof has two sides, it is usually a gable roof or a saltbox roof.

   After I wrote the above paragraph, I found the illustration reproduced below. Are you talking about a roof like the one shown in the illustration as a "skillion and lean-to roof"?

   If this garage has a horizontal insulated ceiling with a vented attic above, here are the rules for the insulation above the top plates of the perimeter walls:

   1. The insulation needs to be at full depth. For an R-49 ceiling, you need R-49 of insulation above the top plates of the perimeter walls.

   2. If the insulation is air-permeable (that is, fiberglass, mineral wool, or cellulose), then you need a ventilation gap between the top of the insulation and the underside of the roof sheathing. Because insulation can shift of blow around, it is almost always necessary to include ventilation baffles near the soffits to keep this ventilation channel open.

   For more information, see "All About Attic Venting."

   .

   
2. 
   walta100 | Jul 21, 2018 02:02pm | #2

   My guess is his roof look something like this photo I did not see it in your very nice chart.
   The unpainted part of this barn, are 2 lean to steads one added on each side.

   
3. 
   dylhill28 | Jul 23, 2018 10:54am | #3

   I apologize for the wrong terminology - I can understand now how I made it hard to visualize what I described. The garage has an open gable roof...nothing fancy. But on one side of the ridge, the slope is slightly less steep and comes out much further from the perimeter wall and is supported by 8' tall 4x4 posts that are about 8 feet away from the perimeter wall.

   From what it sounds like, ventilation baffles would still need to be installed on this side of the gable roof, so that air movement would not shift around the insulation. Would this still be necessary even with fiberglass batts? I understand the air currents shifting around blow-in insulation, but would there be any concern with batts stapled to the ceiling joists?

4. GBA Editor
   Martin Holladay | Jul 23, 2018 11:04am | #4

   Dylan,
   Q. "Ventilation baffles would still need to be installed on this side of the gable roof, so that air movement would not shift around the insulation. Would this still be necessary even with fiberglass batts?"

   A. Yes, absolutely. If you are installing fiberglass batts -- either between your rafters, or on the attic floor (but touching the roof sheathing near the eaves) -- you absolutely need ventilation baffles to prevent the fiberglass from touching the roof sheathing. The purpose of the ventilation baffles is to assure that there is a clear, unobstructed air space between the top of the insulation and the underside of the roof sheathing.

   For more information, see these three articles:

   "All About Attic Venting"

   "How to Insulate an Attic Floor"

   "How to Build an Insulated Cathedral Ceiling"

5. 
   dylhill28 | Jul 23, 2018 06:18pm | #5

   Hi Martin,
   Thanks for your patience. Maybe I will try to get a picture when I get back tomorrow, because I am thinking there is enough clearance between where the top of the insulation will be and the roof sheathing since the roof is higher up on that side (at the point right above the wall perimeter) since it extends about 8 feet past the wall perimeter.

6. 
   dylhill28 | Jul 25, 2018 12:16pm | #6

   I took a few pictures, but the files seem to be too large to upload.

   There is 20 inches of clearance between the top plate of the perimeter wall and the roof sheathing (on the side where the roof pitch is not as steep), as opposed to 6 inches on the other side of the roof.

   So if we install R-30 fiberglass insulation, which is 9.5" thick, we should have 10.5" of clearance, which I think would be sufficient space for ventilation to occur and for the insulation not to be disturbed. Would you agree?

7. GBA Editor
   Martin Holladay | Jul 25, 2018 01:22pm | #7

   Dylan,
   Yes, I agree. A wind-washing dam (described in "Site-Built Ventilation Baffles for Roofs") is still useful at the perimeter of your attic.