To avoid these ads, REGISTER NOW!

garage Roof/ Rafter Insulation... To Vent or Not to Vent ?

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
ok, so i am working on insulating my detached 1 car garage. it is 20 wide by 30 long. single garage door on front 20 end that is 8x7. man door on front left wall toward house. garage was originally a 20x20, then old owner added a 20x10 addition on the rear for a workshop, and a man door from front to rear. small 30" door. i recently removed the man door and widened the opening so i can transverse easily front to back, and move items around.

walls are insulated with r13 and drywalled in.

want to insulate the rafters, as i store stuff in the overhead and want to keep it open and easily accessed. brand new roof put on earlier this year, 50yr arch shingles and such. walls are ONLY 8' tall, roof peak is 15' tall from ground level. no lift, no plans for lift. new roof has a ridge vent, and NO soffits, more of a cape style garage, there is about an 1/2" gap around the top of plate between top plate and roof sheeting. rafters are 2x6 with approx 24" OC.

question is should i

1-install a vent channel, like this one ADO Products Durovent 22-in x 48-in from ridge to gap to allow air to flow freely up the deck to vent. then install some r15 or r19 faced fiberglass over the vent. then some strapping to secure the insulation and hold it in place, just in case. $300 insulation, $150 vent channels, $25 strapping, $50 tools & other

2-seal up ridge vent and gap, and install some r19 faced fiberglass in the cavities. then some strapping to secure the insulation and hold it in place, just in case. $500 insulation, $25 strapping, $50 tools & other

looking at either way it estimates about $500-600 in parts, with me doing the labor. would be using a pneumatic stapler to save me time and hassle.

had a local insulation come out and quote me what they want to do it and, they recommend sealing up the vents & gap, r19 unfaced fiberglass all over, strap in to be safe, Quoted $900 to do this, BUT i will have to clean ALL things out of the rafters, move thing on floor out of the way, possibly to driveway while they work. I plan to work around the stuff. would take a day to move it all out and back in. a lot of work !!

Also I plan on conditioning the space with a AC/ heat Pump.
 

Attachments

  • Garage Rafter Insulating.jpg
    Garage Rafter Insulating.jpg
    6.9 KB · Views: 266
Last edited:
To avoid these ads, REGISTER NOW!

James-W

Well-known member
Joined
Feb 3, 2013
Messages
12,432
Location
Southeastern Wisconsin
The idea of a ridge vent is to let the hot air escape to the outside. If the hot air escapes to the outside, then you need fresh air coming in to replace the hot air that escaped. Since you have an exchange of air on a continuing basis, that negates the effectiveness of insulation.

I would say you need a ceiling and to insulate the ceiling rather than the rafters. That would be my take on the situation.
 

quick60

Well-known member
Joined
Nov 5, 2010
Messages
214
Location
Virginia
From what I understand is that if you insulate the rafters it either has to have an air gap that can allow moisture to escape or use closed cell spray foam insulation and spay the entire underside of the roof deck and rafters. Keep us updated on what you decide, I have to do the same thing in my detached garage.

Check out Finehomebuilding.com they have a ton of stuff on insulation.
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
yeah he said either vent it or seal it all up

vent moisture and allow air to flow freely from gap tto ridge or seal it up, gap & ridge to keep moisture and air out
 

Al Bundy

Well-known member
Joined
Aug 1, 2011
Messages
2,026
Location
Upstate NY
I wouldn't seal it up unless you want to replace your 50 year shingles in 5 years. Put in a ceiling and insulate it. Air flow is your friend.
 

Cyberbear

Well-known member
Joined
Nov 23, 2013
Messages
1,524
Location
California
I'm not familiar with your weather patterns, but a ridge vent in most cases is a good thing and can always be closed off when retaining warm air is wanted, especially during high snow build up where hot air can help melt that snow.
 

theoldwizard1

Well-known member
Joined
Feb 22, 2011
Messages
43,106
Location
SE MI
I wouldn't seal it up unless you want to replace your 50 year shingles in 5 years. Put in a ceiling and insulate it. Air flow is your friend.

The real reason for having air space between the roof deck and the insulation is to keep th roof deck cold in winter (i.e. get the warm air that get through the insulation a path to escape). If the roof deck and shingles stay cold, the snow will not melt as fast and you will have less chance of ice dams.
 

75gmck25

Well-known member
Joined
Jul 21, 2014
Messages
1,318
Location
Alexandria, VA
I've also read the articles on FineHomebuilding.com and other sites about sealing up the rafters with foam and making the attic conditioned space.

The idea is appealing for having insulated attic space, but I have two concerns. (1) Spray foam is an irreversible solution, since its nearly impossible to get off if it turns out to be the wrong choice. (2) Most spray foam articles state that the foam must have a thermal barrier over it if you want to use the attic for storage. This is an example of one article. http://www.greenbuildingadvisor.com...mal-barriers-and-ignition-barriers-spray-foam .

Bruce
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
seems like option 1 is the one to go with from the feedback so far.

putting in a ceiling is not really an option due to height of walls and storage needs and usage. really need the overhead space !!
 

quick60

Well-known member
Joined
Nov 5, 2010
Messages
214
Location
Virginia
Is it truss or rafter and joist? If rafter and joist why cant you still use it for storage? Dont go with max R value just put wall insulation in and interior finish board . You are not living in it but it will be enough to make it comfortable in most of the time and you could still store stuff up there.
 

Al Bundy

Well-known member
Joined
Aug 1, 2011
Messages
2,026
Location
Upstate NY
The real reason for having air space between the roof deck and the insulation is to keep th roof deck cold in winter (i.e. get the warm air that get through the insulation a path to escape). If the roof deck and shingles stay cold, the snow will not melt as fast and you will have less chance of ice dams.

True statement, but not the whole story. Your roof deck will cook like an oven in the summer with no air flow under it. The roof on one of my houses has poor air flow that turned a new roof into needing to be replaced in 5 years. Nothing to do with ice or snow. If I took a picture and asked you to guess how old the roof was you would miss by a mile.
 

Jlbc212

Well-known member
Joined
Dec 7, 2013
Messages
1,530
Location
Northeast MA
Option 1 - as others have stated ventilation directly under the roof deck will prolong the life of the roof shingles.
 

DC73

Well-known member
Joined
Dec 27, 2014
Messages
1,627
Location
Lubbock TX
seems like option 1 is the one to go with from the feedback so far.

Closed cell spray foam with no ventilation is the best choice if the price fits your budget. But your option 1 is likely the next best choice. The problem with insulation against the bottom of the roof deck is that it allows condensation to build up on the bottom of the deck and leads to roof rot. Closed cell spray foam prevents the condensation from reaching the deck. Your vent channel helps prevent the condensation and provides it a path to escape. Building scientists are coming around to a new way of thinking for some climate zones. They now believe you can put insulation like fiberglass, mineral wool, etc in direct contact with the roof deck AS LONG AS you cut in a continuous ridge vent and then seal the ridge vent with a vapor permeable membrane. This vapor membrane prevents air movement but allows moist vapor to escape thus preventing the roof rot. However, this method is currently only proven for climate zones 1, 2, & 3. You can read more on BuildingScience.com and GreenBuildingAdvisor.com


DC
 

guyerst

Well-known member
Joined
May 10, 2010
Messages
49
Location
Zeeland, MI
I have pretty much the same situation, and haven't decided yet what to do. Right now I'm leaning towards cutting 2" foam sheets to fit between the rafters, leaving about a half inch between the foam and roof deck. My thinking is this will allow the air flow to keep the roof cool. Still trying to find someone that has actually done this so I can confirm it's worth the cost/effort...
 

Colin Len

Well-known member
Joined
Jan 30, 2013
Messages
1,233
Location
Long Beach CA
True statement, but not the whole story. Your roof deck will cook like an oven in the summer with no air flow under it. The roof on one of my houses has poor air flow that turned a new roof into needing to be replaced in 5 years. Nothing to do with ice or snow. If I took a picture and asked you to guess how old the roof was you would miss by a mile.
I have been planning on closed cell spray foam for my garage roof insulation (project that will happen a few years down the road) so upon reading this thread earlier today was worried and thus did a little more research. Through my reading I found that there's a lot of debate here. A study done in Florida showed a 5-10% decrease in shingle life for unvented roofs - IMO a totally acceptable trade off for being able to seal the structure with foam not not need venting. Although I don't believe this study was foam specifically and instead just unvented roofs. The study showed that the amount of sun the roof saw and the color of the shingles were more significantly tied to shingle life than was venting.

http://www.greenbuildingadvisor.com.../insulated-rooflines-and-shingle-temperatures

Lots of debate and lots of anecdotal evidence so I'm not sure what's true or what is most important for my situation. Whenever I get around to my project looks like I'll have some research to do.
 

DC73

Well-known member
Joined
Dec 27, 2014
Messages
1,627
Location
Lubbock TX
Lots of debate and lots of anecdotal evidence so I'm not sure what's true or what is most important for my situation. Whenever I get around to my project looks like I'll have some research to do.

Do some research on BuildingScience.com when the time comes. They now recommend unvented roofs with spray foam. You can read some of their older articles and see the evolution of the thought process.

Unvented roofs with spray foam are becoming more and more common. One telling factor is that early on, roofing manufacturers were hesitant to warranty unvented roofs. They've now come around and as long as the unvented roof is constructed properly, there are no issues with warranty.

There are a lot of excuses as to why not to use spray foam under the roof deck but, if done correctly, the only real knock against spray foam is the cost.

DC
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
Looking at Fiberglass insulation, seems that I can get either R11, 13, or 15 since the cavity is 22.5x5.5 and the vent i am using will occupy 1.5 inches, leaves me 4 inches of space. i do not want to cram in R19 which is 6.5 inches into a 4 inch space and crush the vents. thinking R15 as it has a higher R value, and will not crush the vents.

R11 $350
R13 $375
R15 $440
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
Finally broke down and ordered my R-15 Insulation Last night, no one really carries any insulation in stock anymore, have to order it in.

also ordered my Rafter Vents

everything is scheduled to arrive by Oct 6

need to run out to Harbor Freight and pick up the 20g pneumatic stapler and some staples.

was able to pick up some vinyl strapping from local store going to use this as security once i install the insulation in the rafters and staple in place. just in case.
 

Al Bundy

Well-known member
Joined
Aug 1, 2011
Messages
2,026
Location
Upstate NY
They now believe you can put insulation like fiberglass, mineral wool, etc in direct contact with the roof deck AS LONG AS you cut in a continuous ridge vent and then seal the ridge vent with a vapor permeable membrane. This vapor membrane prevents air movement but allows moist vapor to escape thus preventing the roof rot.
DC

Two things here that make no sense.

1. How can a ridge vent possibly allow moisture to escape from under an insulated roof deck? If the deck is covered, the moisture is going nowhere.

2. By definition vapor is a substance suspended in the air, in this case moisture. If a barrier is air tight, how does it allow the suspended substance to pass through? Sounds like snake oil.
 
To avoid these ads, REGISTER NOW!

OldSoldier

Well-known member
Joined
Sep 14, 2011
Messages
131
Location
DFW
I have pretty much the same situation, and haven't decided yet what to do. Right now I'm leaning towards cutting 2" foam sheets to fit between the rafters, leaving about a half inch between the foam and roof deck. My thinking is this will allow the air flow to keep the roof cool. Still trying to find someone that has actually done this so I can confirm it's worth the cost/effort...

I understand the OP has already gone his direction with the fiberglass (which should work fine) but I did a similar thing on my garage. I had 2x6 rafters and wanted to maximize my insulation so I bought used 4" thick polyiso sheets (insulationdepot.com) and cut them to fit each rafter bay (minus about 3/4" all around) and spray can foamed them in place. I used 1" spacers directly under the roof sheathing to give me the airflow channel from soffit to ridge vent. I used scrap pieces of 1" thick XPS foam board glued to the sheathing for that. It was a lot of work but performs good. I can cool down my two story 26x52 with less than 3 tons in Texas in 100 degree heat in about 4 hours. My wife bought me a FLIR One for my phone for my birthday and it shows me how well it is working.
 

GLTHFJ60

Well-known member
Joined
Oct 31, 2013
Messages
821
Location
Durham, NC
This picture shows an application with a ceiling in the attic, but isn't the purpose of a rafter vent to allow air flow from the soffitt to the ridge vent under the rafter insulation? This type of product allows the roof deck to have ventilation while allowing you to insulate the rafters with spray foam or fiberglass or whatever:

Attic-Rafter-Vent-Composite-web.jpg
 

Al Bundy

Well-known member
Joined
Aug 1, 2011
Messages
2,026
Location
Upstate NY
This picture shows an application with a ceiling in the attic, but isn't the purpose of a rafter vent to allow air flow from the soffitt to the ridge vent under the rafter insulation? This type of product allows the roof deck to have ventilation while allowing you to insulate the rafters with spray foam or fiberglass or whatever:

Attic-Rafter-Vent-Composite-web.jpg

You are correct. The question was spray foam directly on the bottom of the roof deck, not between the rafters. The illustration you posted is ideal IMO.
 

DC73

Well-known member
Joined
Dec 27, 2014
Messages
1,627
Location
Lubbock TX
Two things here that make no sense.

1. How can a ridge vent possibly allow moisture to escape from under an insulated roof deck? If the deck is covered, the moisture is going nowhere.

2. By definition vapor is a substance suspended in the air, in this case moisture. If a barrier is air tight, how does it allow the suspended substance to pass through? Sounds like snake oil.

Why are vapor barriers required in some climates? Because without them vapor would move inside the walls and cause mold, mildew and rot. Drywall is vapor permeable (allows water vapor to pass) but also blocks air movement. OSB and other wood products are also vapor permeable. Most paints are vapor permeable.

In your example, drywall can be air tight but they still require vapor barriers behind it in some climates. Why? Physics.

Spend some time reading on BuildingScience.com

DC
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
Well since i had some of the Rafter Venting on hand, picked up from local Lowe's, remainder on order with insulation. i went on and ventured in trying to install and come up with a way to do so easily and efficiently.

I picked up the 20g Wide Crown Pneumatic stapler from Harbor Freight. had a variety of T50 staples on hand in the garage so i can see what size will work best.

started with some 1/4 long ones, and they just seem to drive right through the foam vents. even at 60 psi regulated air !! installed the first row with these, most going thru. decided to try some 1/2 long and they seem to work much better, as they drive in but not through. maybe because more material to drive into the wood.
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
real easy to install, after the first few rows...

now waiting for the other case to come in so i can finish before insulation arrives.
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
got my shipment in today of vents, starting putting them in, got about 75% complete 1 more night should be complete with the rafter vents !!

Lowes called said my Insulation should be in on Oct 10 !! UGH, always special order, not in stock
 

kinigitt

Well-known member
Joined
Sep 21, 2016
Messages
73
Location
Kahnawake, Quebec
True statement, but not the whole story. Your roof deck will cook like an oven in the summer with no air flow under it. The roof on one of my houses has poor air flow that turned a new roof into needing to be replaced in 5 years. Nothing to do with ice or snow. If I took a picture and asked you to guess how old the roof was you would miss by a mile.

What about having an unvented, insulated attic with a steel roof? I'm trying to figure out how I'll insulate the garage I'm planning. I'd need R38 up there. Ceiling would be vaulted (gambrel trusses) with drywall.
 

pappaf2

New member
Joined
Dec 6, 2007
Messages
2
Location
Bay Area, Norcal
BearsFan315 did you end up drywalling the ceiling after you put the insulation in? I am in a similar situation that you have, I have storage in the rafters and would rather keep them open and insulate against the roof deck. I had always assumed I'd need to be drywalling over that insulation which seemed daunting.
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
No drywall... used kraft faced, and put that to the inside.

no need for drywall, if i was going to cover it it would be with thin plywood, metal, or something similar.
 
OP
B

BearsFan315

Well-known member
Joined
Jun 12, 2014
Messages
689
Location
Portsmouth, VA
Problem is IF i walled/ covered the rafters and insulated above it the ceilings are BARELY 9' tall, i would feel boxed it. at least this way it feels and looks bigger. also i love the open rafter look, and i can store junk up there.
 
To avoid these ads, REGISTER NOW!
Top Bottom