In the house I'm getting ready to build (ICF) I will have a sealed and conditioned crawl space which I plan to use as the supply plenum. I expect this to cause the floor to serve somewhat as a radiant floor heat system in addition to the warm air circulating through the house. I plan to put insulation and a thin concrete slab over vapor barrier on the crawl floor.

The use of insulation is a no brainer, but how much? I used the heat transfer formula (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatcond.html) to calculate heat loss from the crawl space air to the ground underneath, and then translated those numbers into a SWAG for annual heating cost. I did the translation by multiplying by a factor I derived from the HEED heating/cooling calculation program. The HEED program gives the design heat loss and an estimate for total annual heat loss.

I calculated the heat loss for a concrete slab with no insulation, and then for 4 thicknesses of insulation, 3/4", 1", 1 1/2", and 2". I was looking at insulation material cost only with no labor cost added.

The net result is that 3/4" styrofoam, the minimum I figured to be practical, had a payback of about 1/2 year. The payback for the additional thicknesses were on the order of 11 to 30 years additional. In other words, there's a lot of heat loss through uninsulated concrete, and 3/4" styro will reduce it enough to be very worthwhile. But because the total heat loss from there on is so low the incremental reductions are hardly worth the additional cost of the styro. For my power rates, size of house, and using a COP of 3 for the heat pump system, the heat cost calculated out on the order of a low of $20 to a high of $120/yr, depending on insulation thickness and air temperature assumed. Insulation cost ranges from about $700 to $2000, depending on thickness. I decided 3/4" would be enough in this application.

Incidentally, while doing this exercise I realized why an uninsulated slab never gets warm under ceiling radiant heat. This has been discussed in other threads in the recent past. The heat transfer is so high through a concrete slab that it can easily absorb and transmit the entire output from the ceiling radiant with the concrete surface temperature being only a few degrees above ground temp. My numbers show the heat transmitted through a 2" uninsulated slab is 30 to 50 times that of a slab on 1" of insulation.

Thought I'd just pass this along for whatever value anyone may find in it.

interesting analysis, but did you account for any increase in energy pricing over your payback period?

Also, that heat transfer formula is not very accurate for ground losses. Seigenthaler has a more accurate one for that sort of use.

I do not believe it meets any air quality code I've heard of to use a cavity as an air plenum like that (whether crawlspace, or wall cavity, or joist). That may not be of concern to you, but I wouldn't consider it a very good idea. If the choice is between insulating the crawlspace and insulating some ductwork, it would seem a better choice to use the ductwork..

No, but that was a deliberate omission for simplicity. Besides, the whole analysis is pretty much a SWAG intended to come with a rough idea only so applying inflation would at best be subjective. If the payback to go from 3/4" to 1" was fairly short, say 5 to 7 years, then energy inflation would make the payback shorter and worth considering.

Unless I'm missing something, or there is a misprint in the book, he doesn't show how his formula accounts for insulation under the slab. It shows edge insulation only. And of course, slab insulation is the purpose of the analysis.

The IRC allows it with only a couple of restrictions - no fuel lines and no sewer cleanouts. My building inspector and the state mechanical inspector have no problems with it. The biggest obstacle was finding insulation for both attic and crawl space that have ESR reports allowing their use without a thermal barrier like drywall over it.

You can account for R-value as you would with any other method. You're right it's not built into the formula, which basically attempts to model the "inherent" R value of the different isotherms under the slab.

The only thought I would add is that there may be an extra risk in using a conditioned crawl space as an air plenum when it comes to fire and smoke inhalation.  I know that special wire coatings (teflon/haplon) are required in most plenum uses.

Steve Cox

No doubt there is the potential for greater risk. However, even if the crawl space is not used as a plenum, 4 of the 6 options Lstiburek shows for conditioning the crawl space have air moving between it and the living space. Using it as a plenum does dramatically increase the air flow. On the other hand, by moving the air from the crawl space into the living space if any there is any sort of electrical failure in the heat pump, or heaven forbid, a house wire overheats, the odors will more quickly come into the living space, thus providing early warning.

If I were using a gas furnace I wouldn't install it in the crawlspace, and I'm not sure I would use the plenum as a supply duct either. But I'm using a heat pump so there is no flame and no high temperature air involved.

In another view of this issue, do you know about how much time you have to get out of a house once the smoke detector goes off? It's about 30 to 45 seconds. In my view that time is so short it doesn't make much difference what the air paths are in the house. But now that the subject has come up, I will be sure to put a smoke detector in the crawl space.

Glad to see some studies to back what we have been sayiing about under slab insulation, especailly when it comes to radiant.   The moisture,vapor,and radon protection is also important in our BARRIER products.  Nice to know people are willing not just to use the old standrads BECAUSE that is the way it WAS DONE !

Studies?

Jumping the gun a bit there; this is a back of the envelope calculation, applying an incorrect equation to the situation, using a wildly oversimplified model, and an unheated slab in an air heated conditioned crawlspace in a house heated with a highly efficient and cheap heat source. Not exactly a "study", and not exactly applicable to most people's homes.

Not everyone needs an R10 for sure. This fellow is probably one of those. assuming, of course, energy prices don't double like oil has recently. frankly, I'll bet on the doubling, but at that point we're all guessing.

Backing up a bit; dmaceld, what is the air temp you are using? also, siggy's basement load calculator doesn't use edge insulation; the slab on grade load calculator does which would be appropriate if you are less than 2' buried; also, if that is the case, your assumption that the ground is "ground temp" is way off. I'm curious as well, what temperature air are you assuming in the crawlspace?

I have to agree this is a long way away from a "study"! I guess I would like to think of it as a little bit better than back of the envelope since I did use Lotus 1-2-3, and I ran several iterations! But I admit your assessment is probably a reasonably fair characterization. (How's that for weasel wording? :-) )

All I'm suggesting is that in a conditioned crawl space, or a heated basement (not in-slab radiant) that some insulation on the order of R 4 to 6 is well worthwhile using, but any more than that is questionable. I just wanted to get some sort of a handle on whether I should use 3/4" or 2" of insulation. After all, I get one chance to do it! With warm to hot air in the crawl space it's obvious heat is going to go into the ground, but how much? My nephew HVAC contractor did a quick run with Wright Soft and it indicated there was no difference in building heat load with or without insulation in the crawl space floor. I just couldn't accept that analysis at face value. I tried a new run with HVAC Calc, but it has no provision for basement or crawl space floor insulation. So, as a last resort I decided to use the basic heat transfer formula. After I went through all the machinations I figured I might just as well share it for whatever value someone may make of it.

It was siggy's slab formula that I used for the new calculation, which I did at your suggestion. The outdoor design air temp is 10°F. I used 80°F for air temp. I expect the house temp to be 72°F to 75°F and air will be circulating full time. The air handler output will be on the order of 110°F, I've been told, when it is running, so I figured 80°F is as good an estimate of average temp as any. Actually, the results don't change much whether I assume 75°F or 95°F.

As I said earlier, sig's formula shows a lot lower heat loss than the standard formula does. I just went back and looked at it again, and it finally sank in that edge insulation in his formula includes the under slab insulation around the perimeter. What he doesn't say though, is how far in the under slab insulation extends. Interestingly, when I look at it again, and correct an error I had in the first run of his formula, the payback to go thicker than 3/4" (R 4) is on the order of 16 to 50 years, using the foam and energy costs I have.

Now, just for giggles I decided to see what sig's formula would give me if my house was in International Falls, MN which has a design temp of -32F. I ran a calc with HEED that gives me the design heat loss and the estimated annual energy consumption. I divided the design heat loss into the annual total to come up with a correlation factor between design heat loss and total annual cost for just the slab. I used my current energy cost. His formula shows, contrary to all expectations, a negative payback for edge insulation greater than 3/4". This is because the total energy cost for the heat lost through the slab decreases very little for each additional 1/4" of insulation thickness. I'm still scratching my head over this one!

Explain a little more about what you mean by my assumption of ground temp being way off. I know it'll follow air temp for a crawl space, or slab on grade, for a few feet inside the perimeter, which is why siggy uses air temp. But further in what would it be if it isn't ground temp? I used 55°F in the original calculation. Is that less than what is typically found under a basement slab? Or are you saying the heat that is transferred to the ground through the slab is conducted away more slowly than what is transferred so the ground under the slab warms up somewhat close to room temp?
Looking at my numbers again, if the ground temp is 60F, for a delta of 20F instead of 25F, the payback gets longer for anything over 3/4" thickness.

All my numbers, both the ones using siggy's formula and the standard heat transfer formula, even if it's not the best formula to use and does overstate the heat loss, show that 3/4" thick XPS is cost effective for a slab in a heated space, but greater thicknesses are not, even in really cold climates. But I am mystified why using siggy's formula leads to such poor results for Int'l Falls. Even changing the energy cost to $2/therm and 90% efficiency for a gas furnace, using siggy's formula I still get a negative payback for XPS over 3/4" thickness.

In case you're not familiar with HEED, it uses TMY2 data to calculate design heat load and annual energy cost.

If your slab is less than 2' underground, the ground is lower than ground temp, you're above the frost line. In this scenario, the slab on grade calculation works, and it assumes a 4' perimeter is insulated as well in its calc. But your ground temp can be more like 32 degrees than 50. I wouldn't expect any design temp below 32 to have much effect on a basement loss.. it does have some effect in the calcs, but honestly the coldest you're going to get in the ground is ice, probably. that's my own "shooting from the hip" though.

The lower you go, the more like "ground temp" you get. Below 2' underground you should use his basement slab calculator which does NOT use slab edge values. The load is typically fairly low; the X factor is the soil conductivity and ground conditions which can shift the numbers fairly heavily.

I'm a little confused by your observation with a design temp of 10 and a slab on grade calc like the one you ran. Pretty hard to imagine that 2" of foam on the edge and perimeter doesn't have payback. On a large slab, center of slab insulation is often a lot less important though and I could buy the 3/4" there. Sound possible?

Rob, I'm sure you will be relieved to know things aren't quite as weird as I painted them to be. I found a mistake in my spread sheet. I failed to correct some numbers when I copied a scenario to create a new one.

But my conclusions are still pretty much valid for most crawl space situations. When I corrected the numbers for the Int'l Falls, MN set using $2 per therm gas rate and a 90% efficient furnace there is a payback of less than 5 years for styrofoam up to 4" thick. This would be if my ICF house were in IF.

Using a heat pump COP of 3 with my current power rates, again if my house were in IF, MN, the payback extends out to about 7 years. For the weather conditions here in Idaho where I live, the payback is about 12 years.

These are all for cases using siggy's slab formula. The conclusion still is mostly that 3/4" of Styrofoam is well worth while in any cold climate, but as the design temp for heating gets lower, and the cost of energy goes up, additional insulation may be justified. Not surprisingly, the payback is very sensitive to total energy cost.

thanks for the update dmaceld.