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Old 01-24-2017, 02:49 PM   #1
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Insulating attic with flat roof under sloped roof


Been trying to figure out how to do this best for a while now and thought you all might have some suggestions.

I have a house that has a flat roof that was later covered by a sloped roof. Because of this, I have no access between the joists. I'm pretty sure the insulation between the ceiling and the flat roof is severely lacking so I'd like to add more. I believe the obvious solution is to just add fiberglass/cellulose over top of the flat roof. However, I've become accustomed to the easy of moving around in the attic because of the flat roof and use it for a lot of storage. I had the idea of screwing down 2" rigid foam board onto the flat roof, but wasn't sure about fire codes. Any other ideas?
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Old 01-24-2017, 07:13 PM   #2
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Re: Insulating attic with flat roof under sloped roof


Is the sloped roof vented?

Rigid foam could work here and the code has evolved over time. Are there any utilities in the attic?

Will you be accessing it for storage on the regular?
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Old 01-25-2017, 08:04 AM   #3
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Re: Insulating attic with flat roof under sloped roof


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Originally Posted by Windows on Wash View Post
Is the sloped roof vented?
Yup.

Quote:
Originally Posted by Windows on Wash View Post
Rigid foam could work here and the code has evolved over time. Are there any utilities in the attic?
No, only thing in there is a dryer vent and a furnace flue that come up through the flat roof and make a turn and go out through a gable.

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Will you be accessing it for storage on the regular?
Not daily or anything, but usually a few times a year.
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Old 01-25-2017, 04:42 PM   #4
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Re: Insulating attic with flat roof under sloped roof


So you have an impermeable full surface vapor barrier (the old glue-down material) with minimal insulation and no/any? venting -----and you want to put more insulation above this?

Gary
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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
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Old 01-25-2017, 06:43 PM   #5
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Re: Insulating attic with flat roof under sloped roof


I think OP said the sloped roof is vented. And yes clearly in his post he is saying he is trying to find a way to add insulation between the old and new roof..

Why not answer his question with a recommendation instead of just a condescending question back to him Gary? If you weren't intending to be condescending, then apologies but it sure looks like it to me.

OP clearly says he isn't sure how to tackle this which is why they came here.. Might be more helpful to tell him vs asking a question back that is basically just asking them to repeat what they already said in their first post...

Your question seems to imply that they should not do that.. So what should they do?
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Old 01-26-2017, 10:12 AM   #6
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Re: Insulating attic with flat roof under sloped roof


With a vented design, you should be just fine and the rigid foam will work just fine there.

I would check with local code enforcement on the thermal/ignition barrier requirements as different municipalities are using different versions and you want to make sure that you are all squared away and legal.

Depending on the climate and temps that you are dealing with, a foil faced foam might help with heat rejection and actually supply the thermal/ignition barrier requirements depending on code.

Be aware that as dust covers the foam, the radiant barrier effect is dramatically lessened and as Gary pointed out to me one time, you can actually create some ice damming issues as the eaves with foil faced foam if you aren't careful.
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Old 01-26-2017, 11:46 AM   #7
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Re: Insulating attic with flat roof under sloped roof


Thanks for the mention, WoW.

I added a sloped roof over a flat before. Be careful because the flat roof waterproof layer is also vapor proof. This will stop all moisture from below and unless you opened up the flat roof perimeters for more air flow under it, OTHERWISE you may have condensation there (if limited ventilation), depending on the ratio of R-value on each side of VB per climate; https://buildingscience.com/document...rchterm=attic+

If you don't have enough FB on top; https://buildingscience.com/document...3-over-roofing


Putting enough insulation on top to prevent condensation is tricky, due to the compressed state of the old insulation and amount of air flow to figure now that the soffit vents are supplying both roofs for ventilation. https://buildingscience.com/document...bout-diffusion

(1.) OR did you block the existing (in flat roof) lower fascia vents and add new ones between rafters?
(2.) Is there still an outlet/exhaust stack/vent in center of flat roof from before? With good air path under the flat roof you will not promote condensation BUT will deflate the R-value of any insulation put on top. It wouldn't be a "dead air" space that you could work with.
(3.) Did you cut any holes in the roof or it was damaged- breaking the air barrier?
(4.) Did you add baffles at the new soffit vents?
(5.) Are you open to the (optimum) option of cutting holes in every cavity for vapor escape and possibly air-sealing the ceiling/wall drywall joint, any plumbing/wiring holes) above all walls through rectangular access holes/slots over each wall below?
Sorry ryansdiydad, I was trying to help someone else get on track... won't happen again.

Gary
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