Blown insulation 2:12 pitch?

Reply

  #1  
Old 11-05-18, 08:52 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
Blown insulation 2:12 pitch?

I'm ready to insulate my garage addition (ceiling and walls) and I'm contemplating whether I should do batts or blown-in insulation in the attic (roof trusses). Cost for the batts (materials only; I would install) is just under $400, and I imagine it would be about $300 to $400 to have someone do blown-in insulation. However, is it possible to use blown insulation in a pitched roof? It's only 2:12, so pretty gradual. And what is the recommended R value for a garage attic space with unconditioned area below? Should I go with R-30, or is that overkill?
 

Last edited by mossman; 11-05-18 at 09:21 AM.
Sponsored Links
  #2  
Old 11-05-18, 09:12 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,523
Received 37 Votes on 34 Posts
Sounds like you are insulating a sloped roof directly below the roof sheathing. If so you need an air space above the insulation with low and high venting.

If this will be conditioned living space you need to check local code requirements as local authorities can choose whatever they wish for guidance.

Also of concern is the activity in the garage portion itself. If the garage doors remain and vehicles could possibly be parked there then air sealing and moisture management become high priorities. That is in relation to the floor/ceiling below.

How deep are your rafters, 2x?.
What is in place for roof venting?

Bud
 
  #3  
Old 11-05-18, 09:37 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
Sounds like you are insulating a sloped roof directly below the roof sheathing. If so you need an air space above the insulation with low and high venting.

If this will be conditioned living space you need to check local code requirements as local authorities can choose whatever they wish for guidance.

Also of concern is the activity in the garage portion itself. If the garage doors remain and vehicles could possibly be parked there then air sealing and moisture management become high priorities. That is in relation to the floor/ceiling below.

How deep are your rafters, 2x?.
What is in place for roof venting?

Bud
They are 2x4 scissor trusses and there's about 24" of empty space between the top and bottom chords at the highest point, tapering off towards the eaves (13'-6" of span in each direction from peak to eave). There are soffit vents on each end/eave and a ridge vent across the entire 12'-6" width. I will be installing baffles at the eaves to prevent the vents from being blocked. The garage space will be unconditioned and a vehicle will be parked in the garage most of the time. Other than that, just my tools and lawn/gardening equipment. However, I will be using a portable propane space heater at times during the winter (maybe once a week for a few hours at most) when working on my car/truck/motorcycle. And since it may be difficult to get blown insulation all the way down to the eaves (there will only be a single attic access near the peak), perhaps it would be best if I used batts for the first 4ft or so at the eaves then blown insulation for the rest. That way I can ensure there is no leakage near the eaves where moisture could get into the attic and freeze. Make sense?
 

Last edited by mossman; 11-05-18 at 10:11 AM.
  #4  
Old 11-05-18, 10:16 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
If there aren't any advantages to using blown-in cellulose and there's a risk of not being able to get insulation everywhere it is needed (especially near the eaves) considering the truss design and the complications it poses (at least 15' distance to each corner from the attic access, shallow and decreasing depth from top chord to bottom, sloped roof, truss webbing in the way), then it seems like batts may be the way to go. I grossly over estimated the cost of using batts during my initial estimate, which is why I started looking for alternatives, but it is looking like batts would be about the same cost as having someone do blown-in cellulose, and likely more effective considering the aforementioned complications. And a vapor barrier (meaning the batts) is probably a good idea even though the area won't be conditioned since I'll be using a propane heater at times during the winter and an air conditioner at times during the summer.
 
  #5  
Old 11-05-18, 11:09 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
Assuming the consensus is to go with batts, the building code does not dictate R-value for exempt/unconditioned space, so I'm free to use whatever I want. However, I planned on going with at least an R-30 for the ceiling, which happens to be the minimum code requirement (walls will be the standard R-13). Any reason I should be with and R-38 as space allows (closer to the ridge), or just stick with R-30 for the entire ceiling?
 
  #6  
Old 11-05-18, 11:25 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,523
Received 37 Votes on 34 Posts
R-30 should be fine, the extra 8 would make little difference.

Note, burning Propane generates a lot of moisture vapor that has to go somewhere and it requires combustion air from that space, just a heads up.

Getting the baffles in place to block the insulation from going into the soffits and maintain an air channel is a challenge, unless the drywall hasn't been installed yet.

Snow melt and rain on vehicles are two major sources of moisture. Tools will be the first to show signs of rust.

Bud
 
  #7  
Old 11-05-18, 11:28 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
R-30 should be fine, the extra 8 would make little difference.
Okay, great. Saves me a little money.

Note, burning Propane generates a lot of moisture vapor that has to go somewhere and it requires combustion air from that space, just a heads up.
Yes, I am aware of that. I would have a window or the garage door cracked. Not only because of moisture, but the exhaust.

Getting the baffles in place to block the insulation from going into the soffits and maintain an air channel is a challenge, unless the drywall hasn't been installed yet.
The rafters are currently open. No drywall anywhere. Installing baffles will be very easy.

Snow melt and rain on vehicles are two major sources of moisture. Tools will be the first to show signs of rust.
I see. Thanks!

Bud
 
  #8  
Old 11-05-18, 02:09 PM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 4,002
Received 2 Votes on 2 Posts
I'm comparing prices and HD has an 88sq ft (eleven 24" x 48" pieces) package of R-30 faced batts, #BF71 for $90 and Lowe's has what appears to be the exact same product with #E56 for $79. Anyone know the difference between BF71 and E56 insulation? Curiously, the HD website has the E56 product in the image, but BF71 in the description.
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: