I understand my options are to either insulate the floor or seal/insulate the crawl space on my late 19th century home. I'm afraid of seal/insulate space because termites may find the insulation on the foundation an easy path to the house. If I use fiberglass bat between the joists I don't have any sealing and this also invites rodents to nest in it, a real problem around here. Closed cell foam is expensive but offers sealing though one applicator claims an R value of 7, another R-19, for 1" of foam (?). Rigid insulation (Perma-R, 4", R-19) would be the low cost option I think but, again, no sealing. Any thoughts?

The best way to deal with crawls is to insulated the foundation walls, sill & rim joist, and leave the floor joists open, and sealing up any windows or vents (which allow more summertime moisture in than let moiture out, in the humid south). It's less material, and easier to get a perfect seal.

Its possible to deal with the termite issues in that sort of configuration by A: using borate-loaded EPS rigid insulation, and/or B: using copper flashing/foil at the top of the foundation at the foundation sill ,extending into the interior of the crawl space. You can then use spray foam to air-seal & insulated the foundation sill and rim joist (or cut'n'cobble EPS/XPS and seal the edges). Copper is toxic to termites, and if using un-treated foam they could still bore through, but would have to come out and cross the metal barrier, where they would be seen. Borate-loaded EPS is also toxic to termites.

Any rigid foam you use MUST be at least partly-permeable to water vapor so that the foundation can dry toward the interior, otherwise higher moisture levels in the foundation can lead to rot in the foundation sill. Sealing both the interior & exterior part of the foundation with silane-based masonry sealer is also good practice, limiting the uptake of morning dew, stopping mineral-robbing efflorescence, waterproofing it to liquid water migration in while still maintaining decent vapor permeability for drying. No foil or poly faced rigid insulation can be used here- only UNfaced EPS or XPS (the pink or blue stuff.) If you're going for a higher R, an inch or two of foam insulation between the foundation and a studwall an UN-faced batts can work. (2" of EPS + R13 unfaced batts in a mini-studwall gives you ~ R20). Any fiber insulation needs a vapor permeable air-barrier (like sheet rock with sealed seams, or Tyvek housewrap) to work. In most instances building code would require at least 1/2" of sheet rock so that would be the best bet. For foam-only situations you can use furring strips through-screwed into the foundation to mount the sheet rock.

You'll also need to put down a vapor barrier like 10mil poly on the floor or slab (unless you have one embedded in a slab), mastic sealed to the foundation 6-12" up prior to putting up the EPS. If it takes multiple sheets, lap the seams by a foot and seal them with duct mastic too.

Then you'll have conditioned-crawlspace, and absolute humidity will track with that of the cooled/heated conditioned space above. In many instances you'll need to put in some floor-grilles or small amount of active ventilation to guarantee at least some minimal air exchange.

See: http://www.buildingscience.com/documents/information-sheets/5-thermal-control/crawlspace-insulation/files/bscinfo_512_crawlspace_edit.pdf

Dana is long winded, but very correct.
I have used sealed, insulated crawlspaces in my area (KY/TN) for several years with good results.
BTW, if you have a termite contract on your home, the company will scream bloody murder about not being able to correctly inspect with the insulation in place.  I hold the vapor barrier lower than Dana suggests, and leave a 1-2 inch gap between the vb and the insulation.  This is usually low enough to be at or below the finish grade, creating a relatively small problem with overall insulation values, and allowing the inspectors to do their job.  Do not believe them if they say they HAVE to remove the insulation at the sill level to inspect.  They do not.

abh3;

wes and Dana1 are pretty much on the same page and I am in agreement with their methods,
 it is easier with new construction than a retro fit, but PerformGuard termite resistant foam is readily available to insulate the perimeter.

I understand but dread the idea of jacking up the sills all the way around the house to insert the copper flashing. The house is on concrete block 'piles' with cement-board between the piles and then the whole thing stuccoed to give the appearance of a solid foundation. It was moved here in '86 but was originally built on a solid brick foundation, piles seem to be standard practice around here when moving buildings. Another problem I see is the strong chance of burrowing animals going under the cement-board and then through the poly and foam, there's nothing going into the ground except footers where the piles are. I suppose I could trench and concrete between the piles but getting to much of the foundation under the house/decks with the concrete would be a major problem. I'm afraid my foundation isn't very well suited to becoming a conditioned-crawlspace and staying that way unless there is another method...

For the above reasons I'd assumed insulating the joists would be the way to go only 1) I'm confused by the claims of the spray insulation applicators regarding R values and 2) don't know if sealing all the small air leaks that must exist w/ spray ins. is more important than using higher R value rigid insulation, like 3 or 4" Perma R...

Thanks for the help!

Air sealing is absolutely the most important factor in insulating your floor. Spary foam is excellent for that IF you have a good installer. Cellulose also works well there to fill the bays, but if you go that route I'd suggest you go around with cans of spray foam and seal the corners of the bays and any gaps you can find. Two inches of permeable (non foil) foam board under the joists will stop the thermal bridging, and a layer of plywood under that will seal the space from burrowing animals. (They can of course chew through plywood so you'll want to check it periodically.) Covering the ground with a heavy layer of poly taped at edges and seams will keep the moisture level down.

Insulating inside the joist bays with sheet foam will work, but they have to be tight to each other and very well sealed around the edges and movement of the buidling can break the seals. The layer of foam under the bays is still important as is the plywood.

So let me get this straight, closed cell foam sprayed onto the bottom of the floor and then permeable rigid foam over that? Isn't closed cell foam essentially a vapor barrier much like the foil found on some foam board? Or is the idea to not have two vapor barriers in the same system thus trapping moisture between them and creating mold/rot issues? The joists bays are open, the ply to close this in would kill the budget and probably be overkill in DOE Climate Zone 3...

What about permeable foam against the floor and then spray closed cell foam over it for sealing?

I've been thinking about the conditioned crawlspace more. I have quite a bit of very high % CCA treated 2x8s, intended for docks/bridges, that could be used on the ground between the block piles/piers, the poly could be under this w/ can foam to seal. Would it be a serious problem to leave the piles themselves uninsulated while applying insulation to the rockboard between them or would this cause condensation on the blocks? No one in my area seems to know anything about PerformGuard insulation panels, I'll have to call the manuf. for more info. If I used PerformGuard what keeps the termites from moving between the masonry and the treated insulation? What effect will a heatpump air handler being in the crawlspace have? The condensate drain leaves the crawlspace area but if seems it may add a lot of humidity...

Thanks again for all the help.

Posted By Bob I on 23 Oct 2010 12:02 PM
Air sealing is absolutely the most important factor in insulating your floor. Spary foam is excellent for that IF you have a good installer. Cellulose also works well there to fill the bays, but if you go that route I'd suggest you go around with cans of spray foam and seal the corners of the bays and any gaps you can find. Two inches of permeable (non foil) foam board under the joists will stop the thermal bridging, and a layer of plywood under that will seal the space from burrowing animals. (They can of course chew through plywood so you'll want to check it periodically.) Covering the ground with a heavy layer of poly taped at edges and seams will keep the moisture level down.

Insulating inside the joist bays with sheet foam will work, but they have to be tight to each other and very well sealed around the edges and movement of the buidling can break the seals. The layer of foam under the bays is still important as is the plywood.

Sealing & insulating the floor rather than the foundation walls & rim joist is absolutely the WRONG approach for the humid south- it's more mateiral, more labor, less reliable, and insulates the conditioned space from the thermal mass of the (nearly ideal subsoil temp) earth, adding to both cooling and heating energy use.  Any failure of the air-barrier leads to condensation on the sides of the joists too. 

The source of the majority of the moisture in southern  (or even New England) crawlspaces is outdoor air, not the soil.  Sealing and insulating the foundation/sill/rim-joist is of primary importance, but soil gases and biology are also important to control. Insulating between the joists presents a colder joist edge toward the crawlspace on which condensation and mold conditions WILL occur when outdoor dew points are above 70F.  If taking the insulated approach all insulation needs to be below the joists, keeping the entire joist within conditioned space.  In most instances a conditioned crawlspace approach would be superior in almost every way- cheaper/better/more-reliable/lower-energy.

OK, conditioned crawlspace it is. I'm very fortunate this forum exists as I was within days of pulling the trigger on 2,300 sq ft of 1.5" foam on the bottom of the floor for around $4-5k. That sounds like the wrong move given my location! Here's what I'm thinking:

Floor Sealing: a membrane taped to the bottom PT 1x4 between the blocks and Durock belting the foundation, using
a) 10+ mil poly, or
b) Cut what I need out of an unused but time expired 50,000 gallon jet-fuel bladder-tank I happen to have, it's a really tough membrane like used on inflatable boats.

Walls:
a) Perform Guard rigid insulation between floor membrane and sill, taped and can-foam cracks, or
b) fashion a lower sill, either from high % PT 2x8 or concrete, to create a seal against the Durock. Tape floor membrane to this and spray foam up to sill.

What would be wrong with these approaches? Thanks for all the input.

I'm not sure I'm understanding the stackup clearly- are you using the Durock between the foundation and rigid foam?

It would be best to glue the Perform Guard (with foam-board adhesive) directly to the foundation, foaming or taping the seams as you go to make it a pretty-good air barrier, spray foam the top of the Perform Guard to the sill making a perfect air-seal and insulate the rim joist with 1.5-2" of spray-foam at the same time. Then add the layer of Durock through-screwed to the foundation through the foam as the ignition barrier for the foam. If you stagger the seams of the Durock with those of the rigid foam, caulking the seams as you go, the combined foam/Durock stackup will be an excellent air barrier with moderate vapor-permeablity.

If sealed the membrane to the foundation near the bottom, and started the foam + Durock 2-3" above that for a termite-inspection strip you'd probably be just fine. The pressure-treated furring screwed to the foundation with caulk or mastic on both sides of the membrane at that point to makes the foundation/vapor barrier connection air-tight & secure, clamping the vb to the foundation with the furring.

Is the foundation wall poured concrete, CMU or ???

Do you have a vapor permeability spec for the fuel tank bladder? (It's PROBABLY water-vapor impermeable, but leak proofness to liquids is no guarantee.)

The house was moved and now rests on concrete block piers/piles. At one point I decided to enclose the foundation and used two PT 1x4s Tapcon screwed to the outside of the blocks w/ Durock over that with the bottom edge in the ground, then stuccoed. The building exterior is batten on board with a 1.25x10 trim board at the bottom, extending an inch or so below the siding/sill, with the Durock behind it. There is nothing but dirt between the piles at ground level, no beam foundation, nothing...

What I'm proposing is attaching the floor membrane to the bottom 1x4, then putting the Perform Guard against the 1x4s and between the block piles with 'cut and cobble' around the piles. The Perform Guard would be taped and/or foamed at top, bottom and joints. The Perform Guard will be on top of the membrane on the ground, relying on the integral borate salts to keep the termites out. For peace of mind a serious borate application (and other products) would be applied along the foundation before the membrane went down. I know this is all pretty hillbilly but we have to work with what we have here, to pour 250' of concrete sill between many piles to get the insulation off the ground isn't practical. I'm not so much worried about the ground contact of the Perform Guard as I am termites 'tubing' between it and the 1x4/Durock. That being said, annual spraying into the crack above the Durock could be a preventative.

I'm not sure what the vapor permeability of the bladder tank is but I'm betting it's quite low given it was designed to store JP-4 and JP-8 anywhere on the planet. JP-4 has some very light components, it's not just kerosene, I'm sure they didn't need that sort of vapor drifting around a tank farm!

I understand better now- the Durock is the exterior skirting around a piling foundation.

In the building biz the term "vapor" is exclusively about WATER vapor (not air, not radon, CO or CO2, not argon, and certainly not hydrocarbon gases.) Air barriers are not automatically vapor barriers, and conversely. (A sheet of poly with a bunch of slashes in it won't be much of an air barrier, but has lost very little of it's vapor-barrier qualities when applied in a wall stackup, in practical terms.)

Liquid fuel molecules are very large compared to N2 or O2 air molecules, and gia-NORMOUS compared to an H2O molecule (which is about half the size of an air molecule). Methane would be the only hydrocarbon fuel even close in size to a water molecule. Many materials can be permeable to water vapor, yet still quite impermeable to any component of jet fuel.

Exactly, it's skirted w/ Durock.

If that's the case w/ vapor then I think we'll be better off using poly, it'll certainly be easier to handle under the house! What kind of tape should be used with poly, it needs to stick to concrete block, wood and poly and last as long as possible, what's available?

Any thoughts on the insulation arrangement? Today we're leveling the dirt under there and finishing some sill replacement that needed doing for 20 years...

Don't use tape with the poly- use fiber-reinforced duct mastic- it sticks to everything and puts up with wide range of humidity and temperature conditions.

OK, sill replaced, much carpentry done (it's never just a sill is it?), dirt almost leveled, 10mil poly and case of fibered duct mastic on the way. Perform Guard distributor located, now I need to decide on panel thickness and what to do with the block piers. The Perform Guard panels aren't that expensive (it's the shipping) so I'm thinking 4" for R14 +/-... The approach I see with the piers could be:

1) Leave them uninsulated but they will likely have condensation on them when it gets seriously cold several times a year.

2) Insulate between the piers and the durock skirting with either 3/4" Perform Guard (R3) or three layers of double-bubble radiant foil stuff I have a roll of laying around (total of R5?).

3) Cut and cobble Perform Guard around the inside of the piers, I'm concerned this may result in condensation issues as well only it won't dry as readily.

4) Combine several of the above plus add insulation (cellulose?, foam?) down the hollow piers through the crack next to the sills. Laborious but doable...

Any thoughts?