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I saw r-tech today with one side foil faced. I have always installed foam direct to sheathing then WRB with either drain wrap or drainage plain then siding. The foil faced Ive used in the past has always been foil both sides or no foil at all.

Im wondering what thoughts are on which way the foil face should go in a mostly heating days climate. My thoughts are to reflect the heat loss back into the house and not back into the siding. The manufacture only has directions for brick over cmu.

Thoughts??

 

[Tinstaafl]

You're right, the foil is intended to reflect radiant heat. If you want to keep heat in, the foil would go to the inside.

Want to go nuts over insulation and vapor barriers? Give this a read. :thumbsup:

 

[Tom Struble]

the foil needs to face an airspace to work as a radiant barrier,otherwise it don't work as one

 

[[email protected]]

Its funny . Ive participated in quite a few energy and green building workshops etc. Everyone has a different way of WRB and moisture control based on scientific research of buildings. I still do not understand where they come from sometimes to be so at odds with each other and the conditions that are found in the field.

First thing is your have to keep moisture in all forms out.

Then you have to let condensate escape. I have been contemplating if using a draining house wrap like green guard would help behind the foam and then still having the true WRB on top. This would allow any moisture moving from the inside out to escape. Im just not sure if it would be an advantage or if you would loose the benefit of the foam insulation.

Most issues i believe are because of lack of air movement inside the house, people not using there exhaust fans enough. Ive been over to numerous friends homes and seen them cooking and such without the kitchen fan on at all.

Where does the vapor go if you dont suck it out. It moves into the walls thus you have to let it out or it will cause problems.

 

[[email protected]]

the foil needs to face an airspace to work as a radiant barrier,otherwise it don't work as one

So it would need an airspace on the inside to work to retain heat in the building. This would have to be sealed or it would make the insulation worthless. This air space would then have condensation on the face of the foil most likely right? Unless you drain it making it worthless. UGG.

Then the only thing the foil is really good for is warmer climates to radiate heat away from the building in into a drainage plain?

 

[Tom Struble]

Chris how i understand it is that heat travels by convection,conduction and radiation,they are all important but not equally important,if the radiant is in contact with anything it simply becomes a conductor of heat

it needs an airspace to ''bounce'' back heat radiation,from what i understand this is maybe best done at the building surface instead of in the wall system

Sean from SLSTeck knows about this way better then me:sad:hopefully he can put in his 50 cents:clap:

 

[TNTRenovate]

You're right, the foil is intended to reflect radiant heat. If you want to keep heat in, the foil would go to the inside.

Want to go nuts over insulation and vapor barriers? Give this a read. :thumbsup:

Love that article! Used it several times concerning VP's. Don't think Mike Holmes would agree with it...so we may want to rethink it as a go to source of information. :laughing::laughing::laughing:

 

[GRB]

Love that article! Used it several times concerning VP's. Don't think Mike Holmes would agree with it...so we may want to rethink it as a go to source of information. :laughing::laughing::laughing:

I prefer to get my info on building science & performance from someone who understands it's NOT okay to wear overalls without a shirt.

For any of you that want an even more in depth peek into building science, I'd encourage you to check out the seminars Joe Lstiburek does around the country.

I attended one last year & it was well worth the investment.

 

[Tom Struble]

the other determining factor on where the foam under wood siding may come from these guys

http://www.wrcla.org/cedarspecs/installing/rigidfoamsheathing.asp

wood siding directly against foam may cause moisture build up,they want to see building paper in between

 

[SLSTech]

Tom has it right - there are 3 forms of heat transfer & to stop the radiation version you need an airspace - the general rule of thumb says 1" though you should be ok with 3/4" typically used with strapping for rain screed details used up in your area.

Failure to have the airspace allows for the heat to simply keep conducting through. For example your typical wall assembly - because the inside is hotter than outside, the heat starts flowing through the wall cavity via conduction & convection (air leaks, FG, etc...) until it gets outside. Once it hits the outside it can no longer travel via conduction but via radiation or convection (wind, etc...) If you have the radiant barrier pointed properly, you have now stopped it from conducting (minus the strapping area) and radiating (mostly) which only leaves losses due to convection

Personally, the more you can reduce the losses to convection & conduction - the better you will be up there. I would save the money on the coating & go with a thicker layer of foam on the outside (or better yet two thinner layers offset) to help eliminate thermal bridging &/or better insulation in the wall cavities.

 

[VinylHanger]

Years ago when I saw the foil faced for the first time we started to install it with the foil out. It of course was a hot summer day. We were both blinded and burned to a crisp within a couple of hours.

 

[Randy Bush]

When I install fan fold under siding, 1/4 0r 3/8 with foil on it always goes to the inside.

 

[Tom Struble]

Its funny . Ive participated in quite a few energy and green building workshops etc. Everyone has a different way of WRB and moisture control based on scientific research of buildings. I still do not understand where they come from sometimes to be so at odds with each other and the conditions that are found in the field.

First thing is your have to keep moisture in all forms out.

Then you have to let condensate escape. I have been contemplating if using a draining house wrap like green guard would help behind the foam and then still having the true WRB on top. This would allow any moisture moving from the inside out to escape. Im just not sure if it would be an advantage or if you would loose the benefit of the foam insulation.

Most issues i believe are because of lack of air movement inside the house, people not using there exhaust fans enough. Ive been over to numerous friends homes and seen them cooking and such without the kitchen fan on at all.

Where does the vapor go if you dont suck it out. It moves into the walls thus you have to let it out or it will cause problems.

my opinion? is that its probably best drainage wise is to consider the foam as sheathing and furr out the windows and doors,put the drainage plane at the face of the foam instead of behind it

or at the very least install a wrb ''skirt'' that can lap the foam,again bringing drainage to the front

 

[MasonS]

Foil would go out, but its insulation value is suspect. An ideal radiant barrier has air space on BOTH sides, e.g. tacking it to the rafters under a hot roof. It just blocks the radiant heat and then doesn't conduct it anywhere.

My biased opinion is that XPS is a better choice for exterior sheathing. It is more water tolerant and more permeable, although the R value is slightly less. But if the Iso gets wet, the R value plunges. Iso can absorb water and be damaged, so if there is any penetration in the foil the iso could be compromised. Foil faced iso has a very low perm rating, because of the foil of course.

There is a reason why iso is not recommended for below grade work. My (possibly wrong of course!) opinion is that the more water tolerant XPS is a safer choice in the long run.

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stupid me forgot that r-tech is eps. xps has a higher r-value and is still more permeable than foil faced eps. bias still in favor of xps.