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Old 08-28-2018, 02:55 PM   #1
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Attic foam board?


well, here is what I've been thinking of, not sure if this works.

I have a traditional attic, with old/little insulation above ceilings in Dallas, high heat, high humidity.

I'd like to setup something like a 1" RMAX thermasheath-XP (foil on both sides) under the rafters and seal the attic so that it vents from the suffits, up through the air gap to the existing vents. I'd put a top flat/horizontal piece some 12-18 inches under the ridge line. /-\ completely growing the home envelope of conditioned space.

The only board with dual facing foil I've found is the thermasheath-XP and it's expensive. the alternative is to use two regular 1/2" RMAX thermasheath boards back to back with the foam sides facing each other. This is about 1/2 the price of XP.


1) would this meet code / 15 minute flammable delay on the service only attic?

2) does it make sense from both a radiation, and convection, and conduction perspective? I think it does.


I'd further this by placing active/solar fans to exhaust (battery/main backed) so it runs as needed by temps day or night to keep the smaller air gap temp as close to ambient.


Well that's the plan... thoughts?
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Old 08-28-2018, 03:59 PM   #2
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Re: Attic foam board?


so something like this approach but thinner..

https://www.drenergysavernewi.com/at...uperattic.html
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Old 08-28-2018, 04:16 PM   #3
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Re: Attic foam board?


Greetings,Thinner is basically the problem. You can shift too un-ventilated in the attic but the insulation between or under those rafters needs to take over the total requirement and that space would need to share the ac from the house.

But let's back up for a moment. The job of protecting the living space from the attic heat is the responsibility of the insulation in the ceiling, in tx I would suggest well above the code minimum, r-30 or r-38 depending upon location what your local authority has adopted. But bury that attic in r-49, a common requirement in cold country, and any heat up there will not affect the living below.

Note, I'm from the north so some of your methods down there are foreign to me. One is to install sheathing with a foil surface facing the attic. Now, you can't replace your sheathing but you could cover the bottom of the roof between the rafters with just a radiant material. Actually, a thin layer of foil faced foam would do the trick without getting you in trouble with condensation issues. Actually doubt you would face any condensation issues .

Are you looking to utilize that space where 2' of insulation would be an issue?

Bud
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Old 08-28-2018, 04:34 PM   #4
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Re: Attic foam board?


right, the picture I see in the link I added looks like a 4+ " board which would be above R30. so 2 x 2" boards back to back then? or locate a distributor that sells thermasheath XP or a cheaper equivalent.
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Old 08-28-2018, 05:40 PM   #5
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Re: Attic foam board?


I'm not familiar with thermasheath XP but I'm not the one who needs to approve it, that authority resides with your local code office. As for leaving it exposed, again the local authority needs to approve it, not the data sheet provided on the internet. As far as I know only one foil faced rigid insulation board has gone through the rigorous process to meet one of the higher standard and even that product still needs local approval, Dow Thermax.

The insulating values for the thermasheath add up in a strange manner. 1" = r-6 where 4" = 4-27.4. Seems like the whole is greater than the sum of its parts.

Your local authority is going to be drawing his information from well accepted books and very little from any internet research, thus you need to bounce your plans off of him or her to be sure they meet the requirements.

Here's some reading on insulating a vaulted ceiling: https://www.greenbuildingadvisor.com...hedral-ceiling

Bud
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Old 08-28-2018, 09:34 PM   #6
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Re: Attic foam board?


Thanks for the links, it does seem some places seem more concerned with ventilation code and saw in California they were very specific to bring ventilation from 1/300 to 1/150...

I'll have to talk to my local city about this just to be on the up and up.
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Old 08-29-2018, 05:58 AM   #7
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Re: Attic foam board?


Quote:
Originally Posted by mycorrado View Post
Thanks for the links, it does seem some places seem more concerned with ventilation code and saw in California they were very specific to bring ventilation from 1/300 to 1/150...

I'll have to talk to my local city about this just to be on the up and up.
I love how code people and others get a hold of some numbers and suddenly they become the gold standard. Here's some research from a well respected energy professional who points out that neither 1/300 nor 1/150 originated from the testing and science we expect of numbers like that. Basically it started as a WAG (wild ass guess) and now is the law. Code people have to deal with providing a simple requirement that many can understand and follow but unfortunately the one size fits all doesn't help everyone.

Enjoy
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