# Insulating garage with flat roof



## VitoB (Nov 30, 2012)

Hi guys,

My house has an attached garage with a flat roof. The flat root has continuous soffit venting all around the perimeter.

What would be the proper way to insulate it?

I believe I need to keep at least a 1-2" air gap below the plywood for air circulation. The joists are 2x8, right now it has fiberglass batts stuffed in between and they're not doing a very good job of keeping the cold out.

Any suggestions would be appreciated.

Thanks!


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## Windows on Wash (Aug 30, 2011)

Vito,

Where's the home located and can you post a picture of the garage, both interior and exterior.

How are you on ceiling height?


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> Vito,
> 
> Where's the home located and can you post a picture of the garage, both interior and exterior.
> 
> How are you on ceiling height?


This is not my home exactly, but it's the same model:

http://www.diychatroom.com/albums/t...340997fce2fb8233b4692f_7417.jpg?dl=1423584041

Ceilings are 9 feet high in the garage and 8 feet high in the powder room/laundry room behind the garage, they are both underneath the same flat roof, hopefully that makes sense.


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## Windows on Wash (Aug 30, 2011)

As long as you have contiguous ventilation and it is balanced, intake and exhaust, you can insulate any number of ways.

How efficient are you trying to make that garage space?

Some 2 x 6 Roxul batts, cover the framing with some foam board, seal the seams, and cover with a fire rated drywall would give you a pretty good assembly.


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> As long as you have contiguous ventilation and it is balanced, intake and exhaust, you can insulate any number of ways.
> 
> How efficient are you trying to make that garage space?
> 
> Some 2 x 6 Roxul batts, cover the framing with some foam board, seal the seams, and cover with a fire rated drywall would give you a pretty good assembly.


I want it to be as warm as possible, we have crazy cold winters here and I also have living space under that roof with the powder room/laundry room.

I am not crazy about using batts. How thick of a foam board are you thinking of? 1"?

I had an idea of fitting some rigid foam between the joists and spray foaming underneath that maybe 1" to hold them in place and increase the R value. 

Your approach would probably be better though since I am stopping the thermal bridging from the joists by using the foam before drywalling.


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## Windows on Wash (Aug 30, 2011)

What are you thinking of using besides batts? 

You can certainly do what you are talking about which is creating a rigid vent space via cut and sealed foam. You will want a minimum of 2" of vent space but you can fill that last 4.25" with whatever you want. 4.25" of SPF will work well there. 

The reality is that once you stop the air loss, 5.25" (aggregate) of well placed and sealed insulation will make that space warm/cool.


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> What are you thinking of using besides batts?
> 
> You can certainly do what you are talking about which is creating a rigid vent space via cut and sealed foam. You will want a minimum of 2" of vent space but you can fill that last 4.25" with whatever you want. 4.25" of SPF will work well there.
> 
> The reality is that once you stop the air loss, 5.25" (aggregate) of well placed and sealed insulation will make that space warm/cool.


What would be the process with the Roxul batts? Fit them in between the joists and then nail some rigid foam with some cap nails to the joists? 

What thickness of foam should I be using?

My only concern is putting up light fixtures and maybe an outlet for my garage door opener might be more difficult that way.

I also have pipes for my furnace running just below my ceiling drywall right now. That will also present a challenge.


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## Windows on Wash (Aug 30, 2011)

If clearances are tight, you will need to skip the foam across the joists plan.

Put up foam in the interstitial joist space and use 2" sleepers to keep the vent space open properly. Fill the rest of the joist bay with Roxul and cover with fire rated drywall. 

It will work well about keeping the cold and heat out. Make sure the foam is sealed to the joists to keep the air loss to a minimum.


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## 1985gt (Jan 8, 2011)

How does your venting exhaust? 

What kind of roofing material do you have on there?

Why not skip the vented space and make it a warm deck?

If you leave a 2" gap in the joist area you would have to have a vent for each joist space, other wise you may as well fill the cavity and block off the soffit vents.


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## VitoB (Nov 30, 2012)

1985gt said:


> How does your venting exhaust?
> 
> What kind of roofing material do you have on there?
> 
> ...


The top of the roof is tar and gravel. It will need to be redone in the next few years, I plan on having a membrane torched on.

The top of the roof is attached to the master bedroom, I am really not sure from where it is exhausting. Although I do understand what you mean. Air needs to travel by entering and escaping like a regular roof with a pitch.

Where the roof ends facing my yard it also has the soffit vent around. What type of exhaust should these roofs have?


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## VitoB (Nov 30, 2012)

Is there any way to verify if the roof was built with a vent space at the top if I have my ceiling open in certain areas or do I need to check this from outside by removing part of the aluminum soffit?


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## Windows on Wash (Aug 30, 2011)

Unless you can get to the eave, pop a piece of soffit loose.


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## 1985gt (Jan 8, 2011)

VitoB said:


> The top of the roof is tar and gravel. It will need to be redone in the next few years, I plan on having a membrane torched on.
> 
> The top of the roof is attached to the master bedroom, I am really not sure from where it is exhausting. Although I do understand what you mean. Air needs to travel by entering and escaping like a regular roof with a pitch.
> 
> Where the roof ends facing my yard it also has the soffit vent around. What type of exhaust should these roofs have?


There is in our area very very few low slope roofs that are vented. Either the insulation is on the top of the decking or the insulation is on the inside. With traditional low slope roofs there is less of a need for it like with shingles. The heat build up does not affect the roof materials as much and Ice dams (sometimes) do not create much of a problem if the roof is installed correctly. The mansard area (the steep slope where the shingles are) should be vented, unless there is interior drains there. If it were me I would fill the joists with insulation and block off the soffit vents. If you want more insulation, when its time to reroof have them put some Polyiso (ridged insulation) down then a coverboard. Be very very careful who you choose to put a torch in their hand and let them up on your roof. IMO an asphalt and gravel roof is far superior. 



VitoB said:


> Is there any way to verify if the roof was built with a vent space at the top if I have my ceiling open in certain areas or do I need to check this from outside by removing part of the aluminum soffit?



If you have the ceiling open then move the existing insulation and see if you can find any extra space. If not look through the soffit, still will be hard to tell for sure.


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