Spray Foaming Under Roof Sheathing

 
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Old 04-21-2013, 12:56 PM   #1
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Spray Foaming Under Roof Sheathing


Ive heard a lot of good things about spray foaming underneath the roof sheathing. I need to do something with my house but could use a few answers from someone experienced. Our house was built in 1904 and the square footage of the attic floor is about 1100. Then the peak of the roof is about 12' tall. To me it seems like a lot of attic to heat and cool. Id like to know if its still the best route or is there something else that would be better? The ceiling joists up there have no insulation in them now.
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Old 04-21-2013, 01:12 PM   #2
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Re: Spray Foaming Under Roof Sheathing


The only answer one can really gives you is it depends...
What are your goals, needs of the house, budget, etc...What is the shape of the roof, the sheathing, what type of wiring do you have - these are just some of the questions we consider before recommending one over the other
As for conditioning costs - in most cases they go down with a hot roof (especially if the ducts are run up there) but once again it depends on your situation
I would recommend you start here & ten check out the related articles on attic sealing; http://blog.sls-construction.com/201...-is-a-hot-roof

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Old 04-21-2013, 03:02 PM   #3
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Re: Spray Foaming Under Roof Sheathing


By heating the attic you eliminate the need to vent it. We did this one house that we did and Chris had to fight with the city to get approval, but in the end it worked out. He will probably see this and add more.
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Old 04-21-2013, 04:01 PM   #4
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Re: Spray Foaming Under Roof Sheathing


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Originally Posted by FramingPro View Post
By heating the attic you eliminate the need to vent it. We did this one house that we did and Chris had to fight with the city to get approval, but in the end it worked out. He will probably see this and add more.
Doing a hot roof in Ontario always seems like a battle royal for approval.
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Old 04-21-2013, 04:29 PM   #5
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Re: Spray Foaming Under Roof Sheathing


How deep are your rafters? What climate zone are you in? What is on the exterior of your roof?
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Old 04-21-2013, 06:04 PM   #6
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Re: Spray Foaming Under Roof Sheathing


2x6 rafters. 1 layer of shingles are on the roof. Live in Northeast Iowa
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Old 04-21-2013, 06:29 PM   #7
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Re: Spray Foaming Under Roof Sheathing


2x6 isn't much. I would flash them with closed cell. 3" is usually good. Depends on your climate zone and total r value. Then I would cross strap the rafter for additional depth and dense pack it with cellulose. The cross strapping will give you sufficient thermal break. To get an R40 you will need to build your roof down 6". That's a bit much. Maybe closed cell on the whole roof and then foam board the interior face of the rafter with 2" eps. That would get you about R40.
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Old 04-22-2013, 10:47 AM   #8
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Re: Spray Foaming Under Roof Sheathing


Anytime we do that in Ontario we have to install 1X3 on the side of the rafters tight to the sheeting and install thin material to keep a 2 1/2" air space to keep the sheeting cooler. Then we install a ridge vent. Its the only way to keep the shingles from cooking off the roof. If you do not have the air space it can and will reduce the life of the shingles by as much as 50%. It is a lot of work to do it but if you want to use the space you have too.

Also if we do it in Ontario we need to get about R 45 in the roof. You would have to build up the rafters by adding at least a second 2 X 6 on the bottom and add plywood plates to each side. At least one every 8' of rafter length. One at the top and one at the bottom.

Its a lot of work and it is cheaper to add insulation on the top of the ceiling joists.
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Old 04-22-2013, 12:23 PM   #9
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Re: Spray Foaming Under Roof Sheathing


The difference in roof temp between a hot roof/unvented and cold roof/vented has been proven to be irrelevant. With 2x6 rafters your in a tough spot regardless of the path you choose to take.
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Old 04-22-2013, 01:52 PM   #10
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Re: Spray Foaming Under Roof Sheathing


Quote:
Originally Posted by Greg from K/W View Post
Anytime we do that in Ontario we have to install 1X3 on the side of the rafters tight to the sheeting and install thin material to keep a 2 1/2" air space to keep the sheeting cooler. Then we install a ridge vent. Its the only way to keep the shingles from cooking off the roof. If you do not have the air space it can and will reduce the life of the shingles by as much as 50%. It is a lot of work to do it but if you want to use the space you have too.

Also if we do it in Ontario we need to get about R 45 in the roof. You would have to build up the rafters by adding at least a second 2 X 6 on the bottom and add plywood plates to each side. At least one every 8' of rafter length. One at the top and one at the bottom.

Its a lot of work and it is cheaper to add insulation on the top of the ceiling joists.
shingles cooking in Ontario, while those of us in the south do it all the time without any cooking issues - thank goodness for our cool summers

By the way, I know this is some quirky code issue up there, but it is easier to just slap some strapping on top of the sheathing & add another layer of sheathing to get your ventilation in.

The other part is the required minimums based on dew point calculations which would be at least 6" if not more closed cell foam, so going with a 2x6 you would be best with at least two offset layers of foam above the sheathing, taped seams, strapping & sheathing (to get your ventilation) & CC foam sprayed into the cavity
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Old 04-22-2013, 02:40 PM   #11
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Re: Spray Foaming Under Roof Sheathing


Actually Greg, I've got the engineering report completed and stamped by Stow Engineers (probably the most prestigious engineering company in Canada, and used by municipalities all the time in court cases) allowing foam directly to the underside of sheeting with no air space. This is the documentation I provided to the city of Toronto during our 'discussion' of my chosen method.

Spraying the underside is now considered a cathedral ceiling application and our code calls for R 31, on the flat it is R 50

As for life expectancy of shingles cut in half, it's a myth.
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Old 04-22-2013, 04:25 PM   #12
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Re: Spray Foaming Under Roof Sheathing


I was not aware of that change as far as no spacing needed. Let alone the shingle not being reduced in life span. Can you upload the drawings you have for us? Well maybe not may be too big. Its interesting that they ok'ed your drawings its been a long time that they have not allowed an air space. The building dept in our area has perpetuated that myth as well.
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Old 04-22-2013, 04:29 PM   #13
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Re: Spray Foaming Under Roof Sheathing


Quote:
Originally Posted by Greg from K/W View Post
I was not aware of that change as far as no spacing needed. Let alone the shingle not being reduced in life span. Can you upload the drawings you have for us? Well maybe not may be too big. Its interesting that they ok'ed your drawings its been a long time that they have not allowed an air space. The building dept in our area has perpetuated that myth as well.
The problem is the ice damming that screws shingles. I can think of 3 buildings I've worked on that had shingles and hot roofs.
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Old 04-22-2013, 04:39 PM   #14
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Re: Spray Foaming Under Roof Sheathing


Thats true. It has always been told that lack of air flow under the sheeting can make the shingles wear prematurely as well. Never once have I been told it was ok to not leave that space. Even popular shows say this. If its wrong then it will take time to correct this fabrication.
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Old 04-22-2013, 05:04 PM   #15
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Re: Spray Foaming Under Roof Sheathing


I have seen foam sprayed directly to the underside of the roof sheathing but there was also fiberglass added to it. I don't know for sure but I recall reading somewhere, where it's cost prohibitive to go beyond a certain thickness. I don't know if this was closed cell or open cell.

I don't get the concept of no air movement under the shingles or what needs to get done if you had to replace sheathing due to a leak problem and why condition unoccupied space. How about flame spead or smoke developement? I would love to spay it in my crawl space but I understand that it can't be left exposed.

I know there has to be someone here that knows a lot about this stuff.
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Old 04-22-2013, 06:05 PM   #16
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Re: Spray Foaming Under Roof Sheathing


GCT - if sprayed in your crawl it has to be Closed Cell due to the possibility of water

Codes dictate how much air-impermeable must be applied above or below the roof sheathing before you can use fiberglass or cellulose - based on testing some open cells can be used

As for fire spread, you either have to spray on or attach a fire retarder unless it has been tested to meet the flame & some specs required

As for sheathing - you replace it & respray that area, with that it should be less than normal with closed cell as its ability to migrate is minimized as compared to open cell (a sponge) & vented

A lot of this is covered in the link in my first post
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Old 04-22-2013, 06:24 PM   #17
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Re: Spray Foaming Under Roof Sheathing


So why are roofing under-layment manufacturers touting their product as

a "breath-able" membrane, similar to tyvek?

Something is going on with "hot roofs".
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Old 04-22-2013, 06:39 PM   #18
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Re: Spray Foaming Under Roof Sheathing


Not to sound to much like a smart a55, but do you only use tar paper, etc... when you have a hot roof? The reason is any water that gets in, they want it to get back out / pass through & seriously you guys up north need to learn how to roof as the only argument I hear is not if but when the roof leaks.

If you are worried about ice dams creating issues, do a proper hot roof & use I&W like the codes state as smaller ones will still form, but not the monstrosities you are used to seeing
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Old 04-22-2013, 07:08 PM   #19
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Re: Spray Foaming Under Roof Sheathing


The IRC allows for "hot roofs".
Quote:
R806.5 Unvented attic and unvented enclosed rafter assemblies.
Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope. 2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly. 3. Where wood shingles or shakes are used, a minimum 1/4-inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing. 4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class III vapor retarder coating or covering in direct contact with the underside of the insulation. 5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing. 5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing. 5.2. Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control. 5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation. 5.4. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.


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Old 04-22-2013, 07:19 PM   #20
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Re: Spray Foaming Under Roof Sheathing


Oh great, there goes Andy with his facts....

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