Hello -

We are trying to engineer the insulation of a damp crawl space and are considering using Airkrete because of it's being non-toxic, non-flammable, a deterrent to insects (we may have carpenter ants), reasonably good r value, etc.

We have some concerns because this building gets a LOT of vibration from local truck traffic going over cobble stone streets. This is a wood frame balloon construction old house with joists 24" OC. As Airkrete doesn't adhere well to wood and because of the expansion/contraction of the wood vs the stability of the Airkrete, I'm looking for a way to use this material and work with the possible gaps created by vibration and settling.

I'm wondering how much r value will be lost due to the gaps and if it may be acceptable.

I'm reluctant to use spray foam because of flammability concerns, off-gassing, it isn't as much of an insect deterrent, etc. That said, we may override those concerns if it could be used safely or minimally and:

Would it be prudent to combine Airkrete with a spray foam in some way? I realize that the cement is vapor permeable while the spray foam isn't and I don't know if combining those is a bad idea.

If we do use some spray foam, we will do that application ourselves and the Airkrete installer would handle the rest. When I contacted Airkrete they were unable to answer my concerns viz the coefficient of wood expansion and cement.


We are not trying to create an airtight crawl space; in fact, we want to ventilate the space which is @ 700 square feet and 5' high with an earth floor. As it is, the crawl space has a  4'x4' screened opening which we may diminish if we can establish sufficient ventilation.

We do have to deal with the rim joist as well and all the while we want to be able to inspect for insect damage.

We'll probably be putting down plastic over the earth and installing a fan for ventilation.

We're also considering using a reflective bubble insulation in the radiant floor above the crawl space to offset any lacking r value from below.

Dampness is a concern and we live in a so-called flood zone with a very high water table. Our building is somewhat unique for this area in that it doesn't have a basement apartment, hence the tall crawl space, so we aren't attempting a tight seal but rather think that it may be best for the space/building to breathe as much as possible.

Finally, is there any counter indication for Airkrete coming into direct contact with old damp wood (we'll obviously try to air the space out as much as possible before installing) in terms of alkali's harming the wood?

I hope this lay person has provided you enough information and any/all advice and information is much appreciated,

Thanks,

Andrea

Bubble foil is not insulation!

Put a six mil vapor barrier down on the level floor of the crawl space and call me in a week or two. Crawl spaces are no longer vented in cold climates.

Airkrete is lighter and more resistant to heat than a standard concrete mix. You will need a proper radiant heat load and specify all other components from the result.

Sub-floor panels may work best, since they will not shake loose, but again, only a heat load will tell.

I insulate all crawl space walls and sub-floor depending on the individual room heat load and design water temperatures.

Crawl space is traditionally the most misunderstood part of your home. It encompasses many crucial building components of the foundation. A compromised crawl space endangers the structural integrity of the foundation system. Damage occurs when the environment is moist, exposing joists, sill plate, support beams, subfloor, piers, block foundation walls, etcetera, to undesirable temperatures and water vapor levels.

If you're not going to seal up the 16 square foot opening, where exactly were you planning to install the insulation, and what were you expecting it to accomplish? ( under the first floor? The founation walls? The crawlspace floor? or...????)

Was the fan going to blow in exterior air, or exchange conditioned-space air?

Where is this house? Hamburg? Lima? Montreal? Adelaide? The particulars of the approach taken may differ with climate.

In general, air-tigthness is more important than vapor permeability for moisture control, but cementicious or other non-expanding foams aren't inherently good air-barriers. Making the crawlspace IS the right approach, along with controlled amounts of appropriate ventilation.

Despite the marketing hype, bubble pack is not insulation, but it is a powerful vapor barrier (but sheet polyethylene is a fraction of the cost.)

If the crawl space is still vented to the outdoors, insulating and air-sealing the floor is the best option. Depending on climate some amount of rigid foam under the joists may also be in order, which may or may not be used as the primary air-barrier.

In climates with warm humid summers a primary moisture source of crawlspace moisture is outdoor air. If the crawl space is to be sealed from the outdoors Airkrete on the foundation walls works, as does a poly vapor retarder on the floor along with some amount of rigid foam covered by a non-strucutral concrete slab. The high permeance of the Airkrete ensures that the foundation walls don't rise in moisture content to the point that the wooden foundation sill doesn't rot. The vapor barrier on the floor limits the flow of soil gases and moisture from entering the crawl space, but there will still be some ventilation (usually with conditioned-space air) to keep it well -controlled since you'll have the additional thermal insulation under the radiant floor creating a larger than usual temperature difference between the crawl and conditioned space.

Hi Dana and thank you for replying to my crawlspace query. Apologies for lack of specificity &c. Our home is in Brooklyn, New York, in an area with a high water table and in flood zone.  Yes, hot humid summers. As I mentioned, it is an older wood frame structure (1860's), and has already suffered some wood damage owing to water and possible carpenter ant presence. That said, the place has remained generally sound over time with the open uninsulated crawl space and we don't want to do anything to compromise whatever integrity the place has. The damage that has occurred was the result of a deck being pitched in the wrong direction taking water toward the foundation; otherwise no water would have come in contact with wood. Still, it is damp down there and I do feel that this needs to be dealt with. So, my primary concern is maintaining the health of the wood. Naturally, making a more efficient building is the next concern.

As I'm the homeowner, not a contractor, and have had a range of suggestions from insulation companies (using cotton batting, blown-in cellulose) that don't address my concerns regarding moisture, longevity of installation, airflow, mildew, wood health, insect deterrent, etc., I'm trying to educate myself.

We were planning on insulating only the ceiling of the crawlspace and then below the first floor. This in addition to putting down a vapor barrier on the earth floor of crawl space. As I am interested in following "green" practices, spray foam was not persuasive to me. Also, I was concerned about flammability. That it has some structuralizing benefits and would not fail with vibration was a plus and my husband was leaning toward using spray foam and installing it ourselves. I was also worried that it might expand too much and damage the foundation. I'm looking for an alternative, hence my interest in Airkrete. For the ceiling. I am worried, however that it will fail because of the vibration factor. If it settles a bit, that may be acceptable.

I agree that the large opening isn't too smart. That it is currently letting the place air out is why it has remained. I know there are complex methods for ventilating such a space and am looking for the simplest and wisest for this situation. Air flow seems to be key which is why I am reluctant to make it airtight.

My primary question is: Is Airkrete advisable for the ceiling of a crawl space given the amount of local vibration? The company recommends using Tyvec to hold it in place. They say to slit it and the concrete foam is then sprayed into these openings. I don't think that rigid foam offers the same R value as the Airkrete though we may use it beneath the first floor.

If you have any further thoughts with this information it would be most appreciated. Given the amount of misinformation I am sorting through, all sound advice is invaluable.

Thanks much,

Andrea

Forget using Airkrete between the joists- not air tight enough, and it may not adhere well enough everywhere.

Is the crawlspace where the boiler or HW heater lives? Is it used for storage? If yes, it's better to convert it to a semi-conditioned crawlspace, closing off the 4x4 vent and air-sealing the rest of it.

If insulating just the floor/ceiling and keeping the crawlspace vented, a combination of rigid foam below the joists and fiber between them is more cost effective than any sprayed-foam option. The thermal bridging of the R1-per-inch joists is severe enough that the difference in "whole assembly" R between closed cell foam, Airkrete and cellulose isn't very meaningful, as long as the assembly is air tight. (In 2x6 stud wall assemblies the "whole wall" R of R19 batts is ~R14 when the thermal bridging of the framing is factored in, but that only increases to ~ R16 if you used R30 of closed cell foam between the studs. The situation with joists is similar.) But if you add rigid foam to the underside insulating over the joist edges you add a LOT of performance, since it adds the full R of the foam to the low-R framing reducing the impact of thost thermal short-circuits. With a radiant floor over a vented crawlspace, 2" of foil faced iso (polyisocyanurate, ~R13@ 2") in addition to R30 ish fiber (assuming 2x10 joists) makes a lot of sense in an NYC climate.

Before you do any of that you'd want to air seal the band joist and foundation sill with 1-2" of closed cell spray foam. (It sticks better, and is flexible enough not to separate.) By air-sealing the rim joist and foundation sill you prevent air infiltration from bypassing the floor/ceiling insulation, and by coating that colder outer wood with a semi-impermeable and waterproof material (closed cell foam) it protects the wood from interior moisture drives. After installing any under-floor radiant, etc, then putting OSB or plywood ceiling to the crawlspace, caulking all seams. Then it can be insulated to the full joist depth with no gaps by drilling holes in the OSB for and blowing in cellulose. (seal the holes with 1-part foam after the fact, and trim flush when hardened. At that point add the layer of foil-faced iso staggering the seams with those of the OSB. It can be held in place with 1x strapping on 2' centers through-screwed to the OSB, then tape the seams with 2" aluminum duct-tape (FSK tape) for a near-perfect air seal. The foil facers of the iso are a powerful vapor barrier as well- if you go too low an R ratio there is some risk of wintertime moisture accumulation in the the OSB, but in an NYC climate if the R value of the foam anything more than 25% of the center-cavity R that risk is eliminated, and by using cellulose for the cavity insulation any liquid moisture that forms is rapidly redistributed, protecting the structural wood.

Second choice on the fiber would be "dense packed" superfine fiberglass such as JM Spider, or Certainteed Optima, blown at 1.8lbs density. Fiberglass is slightly higher in R value, but it won't buffer moisture the way cellulose does. If plumbing leaks or overflowing tubs are a common occurence it might be the better choice though, since cellulose would take years to dry from a total flood and would have to be replaced, whereas with FG you could drill drain holes to relieve the bulk water (sealing them after it dries. Third choice would be rock wool, which is inherently denser than low density fiberglass, and more air-retardent, but also doesn't buffer moisture. The density of the fiber matters- low density fiberglass is far more susceptible to thermal bypass air currents and convective losses of R value at the temperature extremes than cellulose.

The eco-impact of high density foam is exaggerated and rarely counterbalanced by it's unmatched performance (lowering fuel bills by the inch).

Dana is right about foaming the outer wall of the crawl space and the sill plate. Cover the leveled ground with 6 mil poly and you are all done. When you insulate an unheated floor over a crawl space or basement you may actually find them cooler as the windows dump cold air on a floor without the benefit of thermal mass beneath.

Crawl spaces in northern climes are no longer ventilated.

Hello and thanks all for responses - most helpful.

It sounds like we will do a sandwich of materials rather than any sort of foam. Thanks, Dana, for Airkrete advice. It sounds like a great product, just not appropriate for our needs. We'll do the band joist and sill with closed cell foam and the vapor retarder on the earth.

Question: Will the cellulose settle? We would most likely install ourselves - probably from above as we'll be removing the floor. Would "wet" cellulose be better for this situation? I understand this type of installation is not possible to do DIY.

The crawl space houses no heat plant or any utility other than a sump pump.

The joists are 3x7 rather than 2x10, if that makes any difference in your suggestions.

Also, has anyone had experience with Spaceloft Insulation made by Aspen Aerogels? This is what I originally meant when I mistakenly said bubble foil. Thanks.

I realize that with new construction crawl spaces are not longer ventilated. I also gather that architects/engineers are now considering vapor permeability to be better for the home. It's a matter of giving this particular old structure, which has essentially stood the test of time, the best conditions. We essentially want to provide a barrier at the first floor from the cold and damp of the crawl space, not alter the crawl space temperature for its own sake as it not a utilized space. By opening the grate in the front of the house and leaving the 4x4 opening in the back, we were able to air it out considerably. Because this diminished the mildew, I am reluctant to seal the space. Certainly, if we were starting from scratch we would design otherwise.

As we're in a flood zone, the cellulose may not be a good choice. I won't use fiberglass for a DIY install so we may still be near square one. But, a flood would probably mean removing all but spray foam so we may take the chance with it.

Once again, most appreciative for your help with this,

Andrea

SpaceLoft would be a very expensive way to go about it- I'd only go there if you didn't have the room for something cheaper/thicker with better R/$.

Cellulose "dense packed" at 3.5lbs density absolutely won't settle, even without adding rigid iso, but it's tough to get that density as a DIY with a low-pressure low volume rental blower. (I assume this means you'll be doing the radiant above the sub-floor rather than on aluminum heat spreaders from below?) With a Force-1 or similar single stage blower you can probably hit 3lbs though, if you drill every 6' or so along each cavity and blow until the blower backs up and stalls. Drilling the holes 2.25" in diameter (or just a hair bigger than the hose diameter on the blower you use) ahead of time and stuffing them with rags ahead of time to avoid blowing a lot of dust & insulation out the holes works pretty well. Start about 2' from the end of the joist bay and angle the hose back toward the wall, blow until it stalls, then move to the next hole. You'd be able to poke your finger in, but it'd would be pretty dense, with a lot of pushback. (The dense-pack pros get pretty good at estimating the density just by finger-feel.) At 2.8-3lbs density it may settle very slightly in 20-30 years- it depends on the seasonal humidity cycling, but you'd lose at most 5-10% of the initial R over that period of time. Odds are it won't settle at all if you have 2" of iso underneath, since the cold edge of the cellulose will be warm enough that it won't accumulate moisture in winter.

Drilling the installation holes with a hole-saw results in fewer splits/splinters that way but it's slow going. If the layers going down above the subfloor can stand a few ragged edges and splinters, self-feed bits like this one make for a much quicker (if not as clean a result) work out of the drilling: http://www.milwaukeetool.com/accessories/drilling-accessories/wood-drilling/selfeed-bits/selfeed-bit-2-9-16-in-/48-25-2561 Use a real monster of a drill too (the Milwaukee Hole Hawg would work great for drilling down from the top, if a bit heavy for side or up-drilling.) It's a lot of torque and over the course of dozens of holes lighter-duty drill motors can get burned up.

You might consider letting a pro do it with a better blower and dense-packing hoses that get fed into the installation hole (up to ~ 12'), which needs far fewer holes and will get you somewhat higher density. If insulation is subsidized in your state/city (probably is), it can even be cheaper than DIY by the time you've bought respirators, drills, bits, and rented a blower for several days. Install the OSB/plywood "ceiling" to the crawlspace first (and use screws, not nails), and let them decide whether they'd rather blow from below or from the first floor. (Sometimes access will be easier from below.)

If those are full-dimension 3x7s, the center-cavity R of the cellulose will be about R25-R26, but the thermal bridging of the 3" wide joists is huge, bringing down the "whole assembly" R value of ~R15 or so. Adding 2" of iso would bring that up to ~ R28, which would be fine if this weren't a radiant floor, but since the subfloor will be significantly warmer than a non-radiant floor resulting in more heat loss out of that surface. More R value is warranted. There's a long-term rationale for going at least 3" (~R19) or even 4" (R25-ish) on the foam. Anything over R40 total (whole-assembly, not center-cavity), might be tough to rationalize, even with a radiant floor, and the screw lengths required for installing 4"+ foam are awkward. The screws need to be able to penetrate at least 3/4 into the joist through the strapping (3/4") and the foam, so 3" of foam takes a 4.5-5" screw. Flat head SIP crews similar to these are necessary: http://www.r-control.com/downloads/brochure/R-Control%20SIP%20Screws.pdf They're not cheap ( figure on at least 50 cents per) and you'll be using them 24" on center.