Spray foam

I think that is a recipe for trouble. Without air movement under the roof sheathing from vented soffit, any moisture vapor that moves thru the finish ceiling will be trapped in the foam and under the sheathing.....just asking for rot problems many years down the road. That's in addition to the heat build up between the top of the foam and the underside of the sheathing.

My neighbor is building a house and was advised to do just that by the foam installers, and I gave the above opinion. Since he used a scissor truss, he had from 10" on the bottom to couple feet at the peak between the bottom cord of the truss and underside of the sheathing. He scabbed a 1x2 furring strip on both sides of the truss right at the roof sheathing, then cut strips of 1/4" plywood and air stapled them to the strips for a backer for the foam, leaving a 2" airspace all the way up the underside of the sheathing. Lot more work, but less than a possible total roof replacement 20-30 years down the road...and with natural drafting from the soffit to the ridge vent, bound to be a lower cooling low even with the foam.

 

Yup, right now I have 4 selected 2x4s curing in the house below the toe kick of the book case! After all I use a dehumidifier in the house and set @ 55% a board that comes home straight will pretty much stay that way.

Also have some 2x6s in the crawl space as well as 2 1/2 cut plywood in the crawl space.

 

Agree.

 

With more than 2 inches of A&B type foam you risk a fire using wood trusses or studs also all insulation of the foam type loses r- value as it ages as the gas from the bubbles evaporates over years of use. If you need a Dade County rated roof in an industrial setting or a flat roof home its expensive but worth the cost if needed

 

Yesterday I had 6% humidity.
Not too worried about too much moist here.

 

Nope just split wood.

 

I put HVAC in many total foam sprayed houses, the kind applied with a truck. All had great results and most had it applied on the underside of the roof bot\th with and without shingles with no ill effects. The owner of the Sprayseal company applied it under his own shingles as a test many years ago with no issues. If I were to build today I would have it done to walls ceilings and floors. I guess it's just made me a believer.

 

Actually, maybe more than you think. Moisture, like heat, moves to areas with less trying to achieve equilibrium. With a differential inside to outside, moisture will attempt to move toward the outside. (Mine right this minute is 50% inside, wood stove going, and 75% outside)

THIS was the whole purpose behind building papers like Tyvek....not to waterproof a house from the outside, (people generally get that wrong), but to block air infiltration from the outside, and allow water vapor to pass thru so it doesn't dampen insulation and reduce it's effectiveness.

My guess is it will take many, many to fully complete this test.......just like it took 25 years for me to discover Tyvek is crap. And unless the Sprayseal guy has removed some roof sheathing and looked at the underside, he really won't know until it fails.

 

Yeah, But we both know that tar paper works for 100 years..
I'm slowly tearing down an old house built in 44,Tar paper is still good!

 

Pick your poison very carefully I guess.

 

There are different foams. Closed cell foam is quite rigid, adds a lot of strength and is basically water proof and has an R value of 7-8 per inch. @TnAndy would be warranted to be concerned with water entrapment with closed cell on underside of a roof. It also used to be thought that the air space below the shingles was needed to keep them from getting too hot in summer.

There is also open cell foam. Not just water vapor, but liquid water can permeate through it but unlike fiberglass bats it doesn’t loose all its R value if a bit wet. It will also dry. It has R values in the range of 4-5 per inch of thickness.

Fine Homebuilding magazine has had articles and research over the years on this. Foams and long term performance have been studied as well as heat build up on and under shingles. Practice now in eastern and midwest climates is generally closed cell on walls and open cell in ceilings if sprayed directly to underside of roof. No additional vapor barriers needed. If the roof leaks, it will show through on the drywall below and repairs can be made before rafters rot out.

I find TnAndy’s experiences with Tyvek interesting. I wonder if something was out gassing a chemical that broke it down. Was it Tyvek brand or are you using that name generically like Xerox? I have Tyvek brand wrap on my house. Recently had to make some modifications adding some vents and 11 year old Tyvek was as crisp and stiff as the day we installed it.

I also have some on a building that was exposed to the weather and sun for 8-9 years that had very little degradation. Woven poly house wrapping was a very different matter. It fell apart in the weather in just a couple years.

My experience is never use the woven wrap and use actual Tyvek brand wrap. I understand the desire to use tar paper, but that stuff is not water vapor permeable. If you use paper on the outside, you shouldn’t use a vapor barrier on the inside as the wall has to be able to breathe a bit and dry out if condensation occurs. House wraps, foam sheathing, OSB, etc. have water vapor permeability values which a good builder or architect will use to calculate the vapor permeability for a wall system. This is important to be sure the wall will stay dry inside.

In northern latitudes where more heat is used than AC, vapor barriers should be on the insides and in warm latitudes where more AC is used than heat, vapor barriers belong on the outsides of walls. Tennessee is trickier and probably just pick one or the other but never both as a wall won’t be able to dry if some condensation or leakage gets inside the wall.

AT

 

First of all wood twists because it is core lumber or too close to the core . we consider it fire wood.
I am very selective when getting lumber especially for serious projects, if one place has too much core lumber I go some where else.
I worked with contractors that could spot it from the roof we were working on, and tell the driver not to unload, it and take it back .
I am not that familiar with Tyvek, however if most any plastic is left in the sun for an extended length of time UVs will vaporize ,gasses/solvents leave the material and the fabric becomes brittle and falls apart .

 

TYVEK.....by DUPONT (Dupont logo in the oval)......look close at the photos above I posted where it says it right on the paper. I don't think anyone else was even IN the house wrap game in 1984, but I could be mistaken. Now days, everybody and their cousin makes one.

My guess is Dupont changed something in the make up IF it's holding up now days....they realized they screwed up and well, you suckers that bought into it early on are just SOL, "but we got it right now"......or so they say......