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Old 11-19-2017, 02:40 PM   #1
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Attic insulating, rigid foam and batts.


Hi all,
I'm renovating a century old house with an extremely low pitch tin roof sloping one direction; about 5" over 14 feet. I hammertacked runs of Raftrmate in between the roof trusses to allow airflow under the roof deck, then batted with Roxul until I got to the bottom of the room ceiling joists; that gave approx 11" insulation on one end and 6" on the other.
I now plan on adding a 1" layer of Owens pink XPS ( Foamular Codebord) on the underside of the ceiling joists, 6 mil vapour barrier, strapping, then drywall.
From all the reading I've done, I'm now confused as to whether this is going to be a condensation issue or not at the point where the foam meets the Roxul.
Any adice on whether this is a good idea or not?
I'll add that the bldg inspector will insist on the 6 mil plastic barrier.
Thanks.
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Old 11-19-2017, 03:36 PM   #2
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Re: Attic insulating, rigid foam and batts.


Your location will tell us how cold it gets there.
Essentially you have a flat roof in terms of ventilation which means zero to very minimal air flow. You also didn't mention any vents for air to enter and exit. Having a space above the insulation does nothing in terms of exhausting moisture. Any ventilation there or plans to add some?

A plastic vapor barrier is often mistaken as doing double duty, moisture barrier and air barrier. In most cases it does serve as a moisture barrier but fails as the air barrier and blocking air flow has been found to be more important. Most moisture is transported by air leakage.

Bud
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Old 11-19-2017, 09:48 PM   #3
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Quote:
Originally Posted by Bud9051 View Post
Your location will tell us how cold it gets there.
Essentially you have a flat roof in terms of ventilation which means zero to very minimal air flow. You also didn't mention any vents for air to enter and exit. Having a space above the insulation does nothing in terms of exhausting moisture. Any ventilation there or plans to add some?

A plastic vapor barrier is often mistaken as doing double duty, moisture barrier and air barrier. In most cases it does serve as a moisture barrier but fails as the air barrier and blocking air flow has been found to be more important. Most moisture is transported by air leakage.

Bud
I live in Ontario, not far from Montreal.
Where you would normally find soffit, is basically closed in it's entire length but it does have two 4x12 grills that allow minimal airflow throughout the roof overhang. The ends of each bay between the roof trusses were blocked off but had gaps around the blocking. I figured it was best to replace them, drill holes in them so air could run through the Raftrmate and keep the roof deck dry. The insulation itself would be in close proximity, if not touching the Raftrmate. To clarify, all this is taking place on the rear half of the house. Once that air travelling through the Raftrmate reached the center of the house, it dumps into the other attic area which has little to no insulation and a similar venting setup as the rear, all closed in with deteriorated blocking, gaps and some airflow. If I remove the light fixture on that side, you can feel heat getting sucked out. I hope that helps.
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Old 11-20-2017, 08:18 AM   #4
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Re: Attic insulating, rigid foam and batts.


Sorry but I get weak knees when I think about the old houses I've worked on, always another problem popping up, soo much to cover.
Pictures will help, inside and outside. I'm having difficulty with the low slope and open attic understanding what you have.
The heat that is being pushed up through that opening when you remove the light fixture carries a lot of moisture which gets deposited on the cold surfaces, tin roof or rafters. Air sealing is a must along with good insulation. You won't reach recommended code minimums and I'm not familiar with your requirements up there, others may fill us in.

Basically, once I understand what you have the recommendation will be to establish as much natural air flow as possible. Wind will be necessary as you effectively have no height. Natural air flow is directly related to that height.

See what you can get for pictures.

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Old 11-20-2017, 11:26 AM   #5
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Ok, here we go. The first pic is the roof deck, trusses and original blocking. The daylight you see is where the roof deck pulled away from the blocking.
The second picture is where I have replaced that blocking and drilled the blocking to allow airflow to come in from the eave and travel through the Raftermate which is stapled to the roof deck; primarily to create a barrier between the deck and insulation. The third is to illustrate how close the insulation.
So, rigid foam will be fastened underneath the Roxul, vapour barrier applied, light fixtures sealed to the vapour barrier, then drywall. There should be no airflow in or through the insulation itself, nor will it touch the roof deck, just the Raftermate.
Unfortunately, I'm out on business for 2 days and don't have exterior pictures with me. I can tell you it's a standard two story 21x28 house with a vented 8" eave, that's where my airflow for the Raftermate comes from.
Thanks
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Old 11-20-2017, 02:26 PM   #6
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Re: Attic insulating, rigid foam and batts.


I'll leave the ventilation question to the experts, but I suspect as other do that you might have issues given the pitch of the roof.

I believe code minimums on Ont. for ceiling with no attic space above is R-31.

Verify yourself here: http://insulation.owenscorning.ca/pr..._Jan_01_17.pdf

With 6" roxul and 1" XPS you will get to about R-29.

With 11" roxul and 1" XPS you will get to R-49.

One suggestion I would have is, instead of XPS, use iso foam. It will give slightly more r-value per inch, and I believe it is cheaper than XPS.

Iso is known to derate badly at cold temps, but you won't have that issue under all the roxul. It's known to be a "greener" material than XPS, if you care about that.

Iso is also usually foil-backed. Meaning if you tape it up properly, you shouldn't need a plastic vapour barrier.
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Old 11-20-2017, 03:50 PM   #7
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Re: Attic insulating, rigid foam and batts.


I don't think you need the vapour barrier with rigid foam but if the inspector wants it . . .

I would wonder if the two grilles plus the holes you drilled in each bay will provide sufficient ventilation. You also need to consider the 'exit plan' for the air in the connected attic. Maybe your inspector can provide advice - some will others won't.

"I live in Ontario, not far from Montreal"
. Has there been a coup? (Only kidding - I understand what you mean - just had to say something.)
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Old 11-20-2017, 09:51 PM   #8
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Re: Attic insulating, rigid foam and batts.


I'll talk to the inspector, but I'm sure his response is going to be along the lines of "it's a whole lot better than before." It's definitely not going to meet code, but if I lower the ceiling more than it is, then I'm encroaching on the top if the windows which are quite high. But, future plans are to put a peaked roof on it which will being it up to code, that's a few years away.
My main concern is to prevent condensation in the attic, I can't say that I fully understand how and why it happens. If I can get more R from iso, I'll go that route. As for the vapour barrier, the inspector says he'll accept anything that meets code for vapour, which includes the taped iso.
Thanks.
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Old 11-20-2017, 10:11 PM   #9
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Re: Attic insulating, rigid foam and batts.


Quote:
Originally Posted by lenaitch View Post
I don't think you need the vapour barrier with rigid foam but if the inspector wants it . . .

I would wonder if the two grilles plus the holes you drilled in each bay will provide sufficient ventilation. You also need to consider the 'exit plan' for the air in the connected attic. Maybe your inspector can provide advice - some will others won't.

"I live in Ontario, not far from Montreal"
. Has there been a coup? (Only kidding - I understand what you mean - just had to say something.)
I can't speak for Dow, but I do know that OC pink requires it as per their installation instructions as I interpret them.
I wonder if the holes I drilled are adequate as well, but I have no other options. The air in the connected attic has lots of gaps and cracks in the soffit area to vent everything out. In 3-4 years once I put a peaked roof on, it shouldn't be an issue, but I still have to consider that everything I do now has an impact in the future, peaked roof or not.
No, no coup yet. It's suprising the difference in climate between Toronto and Ottawa. Surprisingly enough, many people are more familiar with Montreal's location on a map than Ottawa, that's where the Montreal reference comes into play. Strange but true.
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Old 11-21-2017, 12:10 AM   #10
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Re: Attic insulating, rigid foam and batts.


To be clear, natural air flow for ventilation occurs between low vents and high vents and the less than a foot you have will produce NO effective air movement. Thus any moisture that gets in there will not be readily moved out.

Now, natural air pressures pale in comparison to wind pressures so if the house is situated where it regularly receives a reasonable breeze then that will help with your all low ventilation.

If the ventilation fails and results in moisture problems a high exit can be created, think fake chimney. But those air pressures don't care where the air goes as long as it is higher than where it comes in.

As for the permeability of of the O C Pink (Owens Corning) it is very low but not considered a class I vapor retarder.
https://energy.gov/energysaver/vapor...sion-retarders
Under most conditions it would be fine, but with your limited air flow adding a plastic vapor barrier could help.

Bud
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