I'm building a r50 sip 8/12 pitch roof on a icf walled house.  Can I use reflective foil for my underlayment on my roof or should I just stick with felt or go with something else?  I would like to go metal roof but may have to do asphalt to please the wife.  Just trying to get this thing as efficient as possible.

Posted By oldsamdog on 20 Nov 2011 10:14 PM
I'm building a r50 sip 8/12 pitch roof on a icf walled house.  Can I use reflective foil for my underlayment on my roof or should I just stick with felt or go with something else?  I would like to go metal roof but may have to do asphalt to please the wife.  Just trying to get this thing as efficient as possible.

Do not experiment and use something for which it was not intended.
If you use shigles , you would most definitely void the warranty by using something other than an approved underlayment.

If you go with a metal roof make sure you use a "Hi-Temp" peel n stick underlayment

SIPs do not have a problem with radiant heat penetrating thru the core.

I would like to go metal roof but may have to do asphalt to please the wife.

Okay, I am curious as to why asphalt would be more pleasing than metal.

I prefer metal myself, but our deed restrictions allow only shingle or tile

For a reflective foil to provide any thermal enhancement requires an air-gap, and through R50 foam the benefits of an exterior radiant barrier (fully air-gapped) are more academic than real. The reflectivity and emissivity of the shingle or metal roofing make an order of magnitude more difference, but are also "in the noise" from a cooling season energy use or comfort point of view. On an 8:12 roof the convective cooling to the exterior is also quite significant.

One needs to take care when using foils in any stackup,but even more so on a roof. Foils are extremely vapor-retardent (less than 0.05 perms), which would block the ability of moisture that finds it's way into the skin of the SIP to dry toward the exterior. Even through felt, the drying capacity through asphalt shingles is already fairly impaired, and it can a problem with OSB skinned SIPs. The dimensional changes of OSB with moisture content can degrade air-tightness the seams, and particularly the ridge joint, leading to wintertime moisture accumulation in the OSB at the air-leak-lines, sometimes to the point of rot conditions (this risk is much higher in US climate zones 5 and higher, but it still possible in zones 3 & 4.)

It's an additional expense, but the prefered solution from a long-term resiliancy point of view is to install a ventilated nailer deck for shingles above the outer skin of the SIP, mounted on 2x furring (1x might be OK in some instances), vented at both the bottom and at the ridge. That way the outer skin of the SIP as well as the nailer deck can dry easily and quickly to the vented cavity, preserving both. If the roof lines are complicated with valleys ridges & hips that make top/bottom venting impossible or less effective, segmenting the furring in to 2' chunks with 12-18" of gap cross-ventilates the entire roof, and no ridge venting would be required. In this type of structure you can use a housewrap (highly permeable) on the skin of the SIP for max drying weather barrier, and use standard underlayements (felt, etc) between the nailer deck and shingles or metal roofing. If metal roofing, simply mounting the metal on purlins (no nailer deck) provides a drying path for the SIP, but it's a lot noisier in the rain/hail.

Greetings,

Putting a RB over the sheating and then asphalt shingles would probably lead to condensation problems.
RB/RI require air spaces.

However if you go with one of the shingle type metal roofs they can put a perforated RB down and the the furr'g strips. Just had one done around the corner from me and is beautiful. They use a low profile shingle with RB.

As far as the attic insulation, an advanced RI sys will result in a 2 btu/sf/hr performance. See Marks Mech. Eng. Handbook, 13th ed. I've doneicf buildings, including a furniture store and it really adds to the energy savings. Had the local utility come out and change the meter because the icf/RI sys was using too little energy. Your house with RI will probably never exceed 80 degs on 95+ deg day, NO AC.
Be sure to use an eff ridge/soffit sys. See: cor-a-vent.com

RB is obviously reflective barrier, but what is RI?

Posted By randomhitz on 01 Dec 2011 10:42 PM
RB is obviously reflective barrier, but what is RI?

it is an oxy moron

Greetings,

RB is obviously reflective barrier, but what is RI?
ANS: RI is where a 2 or more reflective airspace is enclosed by sheathing or other, such as wall cavity.
The sys I mentioned above will out perform anything that others have suggested and cost less without any of the inherent problems associated with the other sys.
IT uses a 2 layer sys that is installed in the rafter cavity ( http://www.fifoil.com/Builders/Products/ProductInfo/?ID=3 ) and a single sheet (http://www.fifoil.com/Builders/Products/ProductInfo/?ID=5) attached to the bottom of the rafters with 7/8" primed steel furring strips across the rafters and then drywall or other. It also allows you to install an effective ridge/soff sys without having to do additional carpentry/ buildup. SEE: cor-a-vent.com I've done many of these and I can assure you that on a 95+ deg day with an unshaded, dark roof, you can put your hand on the drywall and it will feel cool to the touch. The room will feel considerably cooler without the ac on too.

Quote>it is an oxy moron
ANS: You should never use the word moron where your name is in the vicinity.
Oh my, that was so easy. Have you got anymore like that?

OLDDOGSAM: You'll have to forgive the banter. The builders of today have a cult mentality and anything different from what they use can't be tolerated regardless of how good it is or how long it's been on the market or that it manufactured and used around the world. Installing RI requires some intelligence a quality craftsmanship mind set which, unfortunately, seems to be lacking among builders today. It wasn't like that 40 yrs ago. Must be a virus.

It was too like that 40 years ago (1971)- the beginning of the end was already over but it still had some to go. Crap construction from the '80s seems to be the bottom in most of the US- it's gotten incrementally better than that since, but not universally so.

rbisys1; what are your credentials?

Greetings,
QUOTE> rbisys1; what are your credentials?

I'm still alive and kicking.
Journeyman mechanist, specializing in R&D work.
Quality control supervisor for a large international company.
Cost estimator for another large international company.
It was shortly after this that I went insane and got into the insulation business.