you keep referancing passive solar. How dows this work in conjunction with the slab in a full basement ? I can see how it might work in a daylight basement if the topography and setbacks of the property allows for the home to face the correct direction. But from what I have read aksmith is building a full basement.

"I would answer that the fatal mistake in such places is digging a basement at all." If you think granite countertops add value to a home, what about more square footage? Build the basement and insulate it so the wet soil doesn't "soak up heat at such alarming rates"

The floor heats the soil. The heat in the soil goes almost nowhere. Delta T drops by the day. A coefficient specific to the soil provides a value for "almost nowhere." Without it, you can't estimate heat loss.
Seriously, 9600 btu/hr and REScheck ignores it?

Again, I think that you are trying to take something that is simple and make it complex.


Yup. They should get with the program. Here is a spreadsheet from the Pikes Peak Building Department which doesn't ignore the slab.

Actually, I am trying to build a passive solar house on a frost protected shallow foundation with maximum thermal mass, including the soil under the slab. By Panelcrafter's math, that heat will just zip through on its way to China. So I'll stop back in late '09 with whatever apologies are necessary.

No apologies necessary(I did catch the sarcasm). But, if that is your goal, maybe you should consider over excavating, putting down Styrofoam, covering with a foot or 2 of rock, and putting down your slab(or pavers) above that. Bobby 'Boom Boom' Villa did just that for a passive solar greenhouse in his house.

2 inch XPS insulation cost me less than $1 per square foot.  If that was a financial burden, I guess I could have made the house 4 square feet smaller to cover that cost.  If I can't afford to properly insulate it, to me that's the first sign that I'm building too big.

Reaching a "steady state" temp in the "heat sink" area of an uninsulated slab doesn't really tell me anything.  The difficulty is determining how much heat is needed to keep what is erroneously called a steady state.  For that, I'd need to know ground water movement, soil R value, etc., or else devise some type of test (probably by building two slabs and comparing them).  Or, for a couple hundred bucks, you can insulate the entire area and put in a 6 inch slab if you want thermal mass.  Then, you have a known mass, slab R value, and insulation R value. 

Mark

Or you could listen to engineering professors who have spent decades devising models and testing them against real basements. Turns out Basecalc, the Canadian government's basement heat loss modeling software runs fine under W2000, and XP as well, except for some extra steps necessary to launch and exit the program. A couple of runs blows a basement-sized hole in the conventional wisdom on this forum.
I modeled a 1,000 SF basement with 8 inch poured walls, and a 4 inch poured floor five feet below grade (roughly translated from the metric measurements preferred by those pesky Canadians.) I conditioned the basement to a year-round 70 degrees. I modeled heat loss using the software's soil and weather profiles for Wash DC, Minneapolis and Denver. Minneapolis makes the strongest case for insulation, not surprisingly, but the others were in line, so we'll stick with the worst case in Minnesota.
With no insulation, our Minneapolis basement bled 61.5 gigajoules of energy a year. A GJ is roughly equivalent to an MCF of natural gas, so it is fairly easy to adjust the loss for furnace efficiency and translate it to dollars. That would be north of $1,000/yr in the case of no insulation in Minneapolis and natural gas at $15 per MCF.
In the second pass, I added two inches of EPS II to the basement's exterior walls in their entirety. The loss fell by more than half, to 28.8 GJ, for savings of $500+ year that are easily justified.
But that would be the end of the no brainers. Adding 2 inches of XPS under the slab reduced annual energy loss to 26.4 GJ from the 28.8 GJ above. So XPS at $1/SF saves $40/year, assuming $15/MCF gas burned in a high efficiency furnace. The payback is an emphatically unacceptable 25 years. It should be noted here that fuel oil at $5/gallon reduces the payback to 10 years, or just within reason. I am not saying don't insulate. I am saying think about it first, including how that $1,000 could be better spent.
Why XPS disappoints is an excellent argument for studying problems rather than throwing money at them. Butting wall and slab foam to the footers leaves thermal bridges at the precise points that soil can diffuse a fair amount of heat: along foundation edges and most particularly at corners.
In a fourth run, I moved the EPS to the interior of basement walls, put a thermal break (RSI=1) between the wall and the slab, and placed 2 inches of XPS under the outside edge of the slab, insulating its first 3.28 feet. The energy loss drops to 23.5 GJ, for a equivalency savings of 5.3 MCF/yr over exterior EPS alone, or $88/year.
In a final run, I extended the two-inch XPS in the model above to full coverage under the slab, vs about 40 percent coverage in the edges-only scheme. Full coverage reduced energy loss to 21.1 GJ, for savings equivalent to about 2.4 MCF/yr, or $40/yr once again. While the payback on the extra $600 of XPS is better at 15 years, it still isn't good. Clearly, basement floors are not major energy concerns, even in Minnesota.
Lest you think I am fudging this somehow, I refer you to a 2005 study by a Canadian goverment task force: "Performance Guidelines for Basement Envelope Systems and Materials" http://irc.nrc-cnrc.gc.ca/pubs/rr/rr199/rr199.pdf The 193 page study presents best practices and uses Basecalc to perform cost/benefit analyses. It ignores slab insulation except in a single word on page 16: "(Optional)"

I made a note on my calendar to come back to this discussion in three years.  "fuel oil at $5 per gallon."  That's going to be really, really funny.  Can I buy 40 years worth of that right now?

How do the slab heat loss calculators handle groundwater issues?  Here's a project that had a spring on one corner of the foundation (probably 3gph in the summer).  The water didn't show on the surface in other areas, but there certainly was water flowing under the slab.

Both seasonal water tables and future energy costs are the elements that make present day "calculation" a bit iffy.  The calculated numbers are better than pulling numbers out of thin air.  Well, maybe.

I would agree that ground water and soil type make a HUGE difference in how these numbers would change. Also does that software take in to account if you’re heating the space via the slab? The loss "out the bottom" would be much higher in that case. So if you intend to heat the slab, why let any leak out the bottom?

I am with Mark, if I can't afford to insulate properly then I will have to cut something else so I can. I can always swap my kitchen countertops out in 10 years, but I can't go back and insulate under the slab. We did a lot of things in this house that we may never use, but it costs "X" now or "5X" later.

The model accounts for depth to water table. It is appears not to be seasonal. (Basecalc does a poor job of defining its terms.) Obviously it doesn't have a "backhoe hit underground spring" factor. Nor should it. Extraordinary problems are by definition not ordinary ones.
I am not looking for precise answers from models. Their value is what they can teach me about how systems work, and how I can use them to create priorities through the very useful process of trial and error. The folks who are worried about oil prices, and I am one of them, should be devoting their resources to alternatives that will break them free of OPEC. They shouldn't be wasting them by gilding subterranean lilies.
I am checking out for real this time. At this point, you are either willing to investigate underground heat loss as a different kind of animal, or you aren't.

Sorry folks,
I really don't care what the numbers say. Basement floors without insulation are cold and damp, floors with insulation are warmer and drier. Now cold and damp when its 90 deg. outside is OK, but not when its 20 deg. outside. I'll restate my earlier point. If you can't find a few hundred dollars in the budget for insulation, then change your budget. If you actually intend to use the basement for any purpose, it will be worth the investment.

toddm,

Thank you for taking the time and effort in your very interesting post.

The model is based on the cost advantages of the structure.  By any chance, is there a basis to model the same question on occupant comfort rather than cost?  I know that my wife takes a jacket into the market because she's uncomfortable in the freezer areas.  What temperature would the floor be at without the insulation, and would that be enough lower to reduce comfort?

Very respectfully,
Larry

Comfort is a consideration, but I would expect the problem to be humidity in the summer rather chill in the winter. Basecalc puts the peak heat loss in my model Minneapolis basement, with wall insulation only, at 5700 btu/hr in January. With the addition of 2 inches of XPS under the floor, the heat loss only drops to 5300 btu (because 40 percent of the heat is lost above grade, and most of the rest is lost at foundation edges and corners.) That modest difference of 400 btu suggests that an uninsulated basement floor is so close to 70 degrees you won't be able to tell the difference.
That said, I worry about summers in my prospective passive solar home in central Pa. UCLA's Heed energy modeling software predicts that my airconditioner will rarely run. http://mackintosh.aud.ucla.edu/heed/download.html Under a passive solar regime of windows open at night during the summer and closed during the day, I may be running multiple dehumidifiers instead.
You won't have extra humidity in your basement if its vapor barriers and drainage systems work, but there will be longer stretches when the air conditioner won't run. I would expect the basement to seem marginally wetter and staler as a result. It is still a question of degree. Basements are always cooler than the house. Insulation or no insulation, nothing happens quickly at five feet below grade.

Thanks toddm; that makes sense.

Very respectfully,
Larry

It's pretty obvious, that you weren't going to spend the $$$ for underslab insulation, and that you tried to find some basis that supports that decision. Multiple professional opinions disagree. Models are models, do you like cold feet?

What about underslab insulation in climates where the cooling days outnumber the heating days? We're building a passive house in the Central Valley of California (climate zone 9b). It will be on a slab. There will be a partial basement for storage, including wine cellar/root cellar so I will leave that uninsulated underneath. But what about the 1st floor?? It seems pretty clear that a VB of 6-10 mil poly is necessary to control moisture, but arguably the heatloss in the winter won't be so bad as to outweigh the cooling capacity of an uninsulated slab in the summer. It seems that one of the following strategies would make the most sense in our situation:

- uninsulated slab with perimeter insulation to the bottom of the footer (no frost line here so that's quite shallow-- 15") and
wing insulation out to 2'.

- uninsulated slab with perimeter insulation PAST the bottom of the footer-- say to 24"-- and then wing insulation out to 2' (seems a little dodgy from a structural perspective to dig down lower than the footer to insulate to 24")

- insulated 6" slab-- would lower heat loss in the winter-- we do get SOME cold days here-- and provide a larger heat sink in the summer.

FYI, nights here in the summer are almost always cool enough to open the house and we'll have a whole house fan to bleed off the heat soaked up by the thermal mass in the slab and other areas. We'll have a couple of small areas with electric resistance radiant heating (bathroom floors, maybe 1-2 other small areas) for supplemental heat/comfort when getting out of the shower.


Any thoughts, suggestions and critical comments are most welcome.

Thanks all.

Pink or Blue it's all XEPS. The only difference is that you no longer have to pay $1.00 per sq. ft. We supply builders with XPS for $ .25 - $ .35 per square foot. No, this is not a con, we sell "reclaimed" foam board. Yes, it's been used, but still keeps a very high R-Value, and at 40% - 70% savings, anyone paying a builder to put in insulation should have them contact me. We have 35 warehouses throughout the country. Good luck.

David.......any of your warehouses in the Charlotte, NC area?

I should probably finish the thread first, but I've read enough and I'm with you.  At the relatively low cost of insulating the slab, I would want to insulate the slab. 

Just a few related thoughts to hopefully add to the debate (and I haven't read every entry yet, so sorry if this has been covered):

There is a difference between heated air and radiant heat.  You can be comfortable several degrees cooler if your body is exposed to radiant heat.  Surprise... Surprise... Surprise... such thermodynamic issues go the other direction too.

When the slab is colder than the room, you will definitely "feel" the heat radiate from your body to the slab.  This is what gives you "cold feet" that the sophisticated heat analysis isn't showing.  And it leaves you feeling cold in a "warm room" just like radiant heat leaves you feeling warm in a "cool room."

On another note:  With a radiant heat system embedded in the slab, I would definitely not want to waste energy trying to heat the infinite capacity of the earth's heat sink.  It will not return nearly as much as you put into it, because most of the mass of the earth that you are effectively heating is not interacting with your home's slab!  The earth's thermal mass is part of an open system, unlike the closed system thermal mass inside a home's envelope.  The software likely has boundary conditions that limit the energy flow out into the infinite earth.

I've just registered so I can reply to this thread.
First a big thank you to toddm for posting so much useful information

I've downloaded BaseCalc, hoping to be able to run some tests on foundation designs I have, but on my Windows XP machine I get a "end of file" message. If you know how to fix that I'd be grateful.

Re the conversation on this thread, this is my view:

1. When considering insulation in cold climates, one must consider where the cold is, and what is between it and the conditioned space. The coldest place is the air outside. That will be cooling the ground outside the foundations.

2. Soil acts as a conductor of heat, but as toddm says, it also warms up, and with steady state conditions inside and outside, reaches a steady state condition, with a temperature gradient between the inside and outside temperatures.

Temperature gradient picture

3. The most important part to insulate is the short path between foundations and ground/air outside.

4. With a slab-on-grade or basement, the distance between the underneath of the slab and the outside is greater, and so the thermal resistance greater. Again as toddm says, that means the heat loss will be lower. It also means the temperture gradient will be much less steep, and so the top of the slab temperature will be quite close to the inside air temperature. The addition of insulation can only affect the floor temperature by that difference, and no more.

5. It may well be more cost effective to increase the wall insulation than put insulation under the floor slab.

IMO the decision on how and where to insulate, and how cost effective it is, should be based on well understood science, and the use of modelling is vital. Good building design is too complicated to be based on just gut feel.
Steve