I have read reports of FB enhancing sill rot. I think that is exactly what Dana is saying about the open cell. This house does have vinyl siding(with fanfold). Given the high ground water, do you think that adds risk to 5" of open cell on the sills?

Posted By WindowsonWashington on 27 Nov 2011 12:19 PM
Dana,

Do people ever see any rot at bands behind the fiberglass ever? I am assuming no in this case because the FG allows for so much air movement and therefore keeps the band above dew-point.

Is the concern with the open cell that in a climate like zone 5 or above that it will allow too much diffusion, hit the cold band, condense, and foster rot? Obviously the bulk moisture needs to be managed from the exterior because the application of a CC foam is going to eliminate any drying to the interior in this case.

What would be the minimum required thickness of CC to avoid condensation on the interior surface of foam and get to the 1-2 (or below) perm rating? I am guessing about 2" yielding about an R-12. If you wanted more in the wall at that location, could you then apply the cheaper open cell foam or does and R-12 not give you the proper resistance to dew point surface temperature at the close cell/open cell connection?

Rot at band joists under poorly sealed or unfaced fiberglass insulation happens all the time, but more commonly you just end up with mold growth (in the fiberglass and on the wood.)  If its so poorly installed that the band joist stays above the dew point it means that it's providing essentially no insulation value.

Open cell foam can indeed allow sufficient vapor diffusion to cause rot issues. 

Generic cc foam does NOT eliminate drying to the interior unless you go too thick. At 2" (R12) most cc polyurethane is still >>0.5 perms- more vapor open than kraft facers used on batts, and still a reasonable drying rate.  If one used 3" of the higher-perm/lower-R Icynene closed cell goods (MD-R-200) you'd be at R15 and 1.3 perms- pretty near ideal for zone 5.

At just 1" most cc foams are ~1.2-1.3 perms, which sufficient to be protective for zone 5, and you could even use fiber insulation to the interior (flash & batt or flash & spray) with low risk.  It protects in multiple ways:  With flash-foam the foam is non-wicking, and becomes the condensing surface, thus even when the interior face of the foam is below the dew point it doesn't wick toward the wood.  The flash foam is also a non-air permeable R value, reducing the number of hours it's interior face is below the dew point.  And by being only semi-permeable to water vapor, it slows the water diffusion reaching the now colder band joist, allowing it to dry to the exterior.

Fan-fold XPS underlayments have thin polymer facers that are typically ~0.6-0.8 perms, which will protect the sheathing & band joist by slowing exterior moisture drives, and by adding ~ R1 to the exteior, modestly reducing the number of condensing hours on the inteiror face of the band joist. (If the fan-fold is a perforated variety it's very high perm though.) Vinyl siding is inherently back ventilated and can't saturate with water, so the drying capacity toward the exterior will be pretty much defined by the facers on the fan-fold. Under back-ventilated siding with a more-permeable underlayment you could use open cell foam to insulate the band joist since it would have significant drying capacity toward the exterior even in winter,  but with un-perforated fan-fold facers you'll likelyi still need something lower-perm on the interior to keep it from loading up with a bit winter moisture. Having 0.5-1.5 perms of drying capacity toward both the interior & exterior is plenty, and sufficiently low perm to protect, both summer & winter.

Putting down 1" of cc then switching to oc to fatten up the R value is usually false economy. By reducing the board-feet volume of each you end up paying more for each, and more in total than simply going with more closed cell.  Most closed cell formulations are optimized for spraying in lifts of 2",  (going thicker than 2" in a single shot increases the likelihood of shrinkage & bonding issues at the edges, and can even be a fire hazard) so that's really the "right" thing to do here if you're going for an all spray-foam solution.  If you want to go higher R from there you can either use unfaced batts & high-perm sheeting such as housewrap for the interior air-barrier or 3"+ of wet-sprayed Spider super-fine fiberglass blown in netting (which would qualify as an ignition barrier.)

OK, 2 inch of cc it is. I'll see if the contractor's comfortable with the fire issue. Any inexpensive or floodproof ideas for an ignition/thermal fire barrier are welcome.

Mineral wool stuffed overtop would be a great ignition/thermal barrier.

What Dana was referencing when he said:

going thicker than 2" in a single shot increases the likelihood of shrinkage & bonding issues at the edges, and can even be a fire hazard

was that closed cell foam is highly exothermic when it cures. Putting on more than 2" lifts can trap the heat in the foam and actually catch fire.

I like the mineral wool idea too but am having a hard time seeing the mechanics and air barrier. I think adding wood framing(even pt) is a bad idea with the flooding. Maybe horizontal furring. Do the ignition coatings for foam reduce vapor perm much?

It think the purpose of the air barrier in that particular application (i.e. batts) would be to lessen convective looping in the insulation.

I would just go with Roxul.

IMO, when considering a design, talk to a local home inspector. A good one has seen it all - rot, mold, flood damage, etc.

Greetings,
QUOTE> Rot at band joists under poorly sealed or unfaced fiberglass insulation happens all the time, but more commonly you just end up with mold growth (in the fiberglass and on the wood.) If its so poorly installed that the band joist stays above the dew point it means that it's providing essentially no insulation value

AMEN, plus I've seen little critters and condensation with faced prods too.

Kraft face is a joke. The paper breaks down. the pitch loses eff., it holds more moisture. see above.

QUOTE>Do people ever see any rot at bands behind the fiberglass ever? I am assuming no in this case because the FG allows for so much air movement and therefore keeps the band above dew-point.

If you have this much air movement you have problem that's as great or greater than condensation.

Using RI you will not have condensation problems.

Posted By WindowsonWashington on 29 Nov 2011 11:48 AM
It think the purpose of the air barrier in that particular application (i.e. batts) would be to lessen convective looping in the insulation.

I would just go with Roxul.

Yes- you need to cut the convective transfer of air through the fiber or it can't perform to spec, and if you don't have sufficient foam (or none at all) against the band joist the convection will move 100x more moisture to the band joist that it would via vapor diffusion alone. 

If you have enough foam between the fiber & band-joist to keep the foam/fiber interface above the interior-air's dew point more than 50% of the mid-winter hours you'll never get enough moisture through even high-perm air barriers to cause any issues with the fiber.  By making the condensing surface foam, it never wicks toward the wood even when it's below the dew point, and it's performance & durability isn't affected by the fog-film that might occur overnight on the coldest nights.

BTW: Roxul held in place with Tyvek would not qualify as an ignition barrier- it has to be more mechanically secure than that. There are wet-sprayed rock wool, f.g., & cellulose with water activated binders that DO qualify as igntion barriers at 3" or more depth. (JM Spider would give you the highest R, cellulose would probably be cheapest.) Half inch gypsum or half-inch ply/OSB also make it.

The wet sprayed fibers are going to have to be done via an insulation contractor at that point. That is not a DIY'er piece of equipment.

Most code officials don't seem to mind the exposed foams in small quantities.

I prefer the cellulose or roxul over fiberglass in most applications because of the lack of VOCs in them and their lessened worker safety issues.

FYI, make sure that people are spraying the sill plate as well in these applications as compared to just the band board. The relatively low R-Value at the sill will certainly allow for a condensing temperature.

Yes- the band joist & sill should be spray-foamed together, and foam-sealed to the top of any rigid foam you have on the walls.

And no, wet sprayed fiber isn't a DIY project by any means (but it does meet code as an ignition barrier.)

I'd never use cellulose in a flood-prone basement- it takes on too much moisture and takes forever to dry.

Posted By Dana1 on 29 Nov 2011 06:59 PM
Yes- the band joist & sill should be spray-foamed together, and foam-sealed to the top of any rigid foam you have on the walls.

And no, wet sprayed fiber isn't a DIY project by any means (but it does meet code as an ignition barrier.)

I'd never use cellulose in a flood-prone basement- it takes on too much moisture and takes forever to dry.

+1

I forgot the original posters provisions.