What are people's opinions on which is greener: slab or basement?  Or is there really a difference with respect to energy efficiency between the two.  The site would be in Southern Ontario and space is not an issue.  The lot would be on flat ground.  The comparison would be between building a one storey house with a basement or a two-storey house on slab.  You could think of the two storey house as a one storey house with an above ground basement if that helps make the comparison more equitable.

I've read through some of the discussions in this forum which were very educational on the pros and cons of basements versus slab with respect to ease of build, ease of usage, resale value, etc. but I didn't get a clear sense if one method provides better energy results than the other.

In my opinion, frost protected shallow foundations, insulation and air sealing techniques are now good enough that there is no net advantage to building anything underground (aka using dirt as insulation). But a more direct answer is that either one can be insulated to any level of energy efficiency and it is a cost question.

Is building on FPSF, then, in this particular example where energy efficiencies are equal, generally cheaper than building with a basement? I'm trying to get a sense of how to prioritize, for myself, energy efficiency considerations (vs other considerations like ease of build, resale value, etc.) when it comes to deciding whether to build on slab or build a basement.

I think it more a question of how you want to live and how a basement suits the lot.

As for efficiency, run both designs through a load calc program and compare results

Something that must not be forgotten or overlooked is the water table. This will affect costs greatly if the basement route is taken. Backfill, trucking, excavating, etc. Knowing where the water's at would be a big deciding fator for me if I were unsure of a slab or basement build!
Cheers! Hope this helps.

I did build what you are describing, a frost protected slab foundation with a ground floor at level "basement" with the main floor on top. I really wont be able to give good stats on energy efficiency until next year. I am in Zone 5b in the Fingerlakes of NY. Not only is my slab frost protected, but I went a little further. The top 6 inches of the slab is insulated to R30, but I also insulated the entire perimeter with R10 vertically down to 4 feet. My idea being "passive geothermal". I put a soil temp probe in the ground within the vertical insulation perimeter on the north west corner and one on the north side as they would be the coldest spots (prevailing wind out of the west northwest).

The house is not finished yet, so not occupied. I have radiant tubing installed in the slab, but have not installed a heating system yet. I have been keeping the house at around 50 degrees with a space heater. What is interesting is that the ground temp under the house dropped around a degree a week until it got to 50 degrees F on the NW side and 53 degrees F on the N side and seems to have stabilized. We have had one of our coldest Nov/Dec on record with temps averaging 5 degrees colder than last year, and less sun than I ever remember having at this time of year (actually since 15 Sep) in the 33 years I have lived here. This temp stabilization, along with the ease of keeping the house at 50 despite bitter cold weather, is making me feel very optimistic about how energy efficient, that slab and the house are going to be. Time will tell of course, but they are sure getting a good workout pre heating system! The heating system should go in in about 2 weeks, and that will change the slab dynamics completely.

-Rosalinda

Since this is a common optimization question, does anyone have pointers to building cost estimation software that is accurate enough to be worth comparing different designs? With software to model the energy use and other software to model the cost, one could try different designs and pick the most cost effective one for their situation.

I can not imagine not having at least part of the basement. When hell more than the stove and the fly? Of course, I'm in Kansas, you never know when the wind is blowing. A slab has certainly been a challenge for conditioning, plumbing and wiring.

If you compare the extenal wall/roof area for a single story house and a two story house with identical floor areas, the two story house will almost certainly have less external area providing you use simple and compact forms without a lot of architectural gee-gaws (highly technical term). Less extenal area means less energy consumption with everything else being equal (which they never are).

Accordingly, you might add a 3rd option.....a two story with a slab. I can't tell you which will cost less to build, but I agree with engineer that this is as much a lifestyle choice as anything.

If I build a two story house, I need to make provisions for an elevator (more area and $) that I would not need for a single story.

Bruce

I feel that a crawl space should also be part of the discussion since they are a way to avoid subsoil settlement issues depending upon subsoil issues and could have advantages vs. slab on grade.

crawl space also allows future upgrading of mechanical / electrical via the underside of floor framing.

A good discussion of this topic is buried in this thread: http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19438/affordable-cold-climate

Rosalinda, your slab insulation approach could be a problem. If there isn't any insulation under the main part of the slab, the earth (at 55 degrees MAX) will constantly suck heat out of your home. It's doubly efficient at heat sucking if the heat source is in the slab itself.
This strategy is actually a good idea for a garage or a seldom used vacation home, though. The 55 deg. heat will rise into and heat a structure with thermostat setpoint of 50F or lower.
To salvage your slab, you can install carpet with a thick pad, and don't hook up the radiant heat tubes. Install some console heaters instead.

In addition to the perimeter insulation (R30 around the perimeter of the slab itself, R10 down 4 ft vertical), there is R5 under the entire slab.  I should have the heating system in by mid Feb - so we will see if my theories work in practice. 

-Rosalinda

From a green prospective, nothing beats a slab-on-grade home. The heat loss will most certainly be less for reasons previously stated.

If you travel to the Southwest you will see that it is mostly a cultural consideration. It is generally cheaper to build up than out, but every level adds to heat load and detracts from comfort primarily because of the chimney effect. Since most of us no longer keep "roots" we no longer need a root cellar or its prodigy the "basement".

This is old stuff.

FPSF is the ultimate, with no infiltration, no wasted or added materials for the radiant panel/slab, completely invisible and silent. Naturally the tighter the home, the more important the ERV becomes.

As for the insulation, true it is not up to spec. (one of the shortcoming of poor misinformed design, but insulation specs are based on differential temperatures. Covering the floor with carpet is a little over the top. Careful system control strategy can make for a very comfortable and efficient system.

Those thinking of incorporating radiant floor heating into their building plans should bypass the builder/concrete guys and start with a radiant floor design professional. All of these questions are answered with a radiant heat load analysis before the first form is set.

As for crawl spaces, I have rarely seen one I liked. The excuses such as upgrading, repairs, etc. is old woman over their tea.

I built an insulated slab on grade home in 2009 that has out performed any other type of house I have built, and it cost less to build. The slab of the house was not only the foundation but the heating,cooling and finished floor all in one. We finished the floor with acid stain. To enhance the acid stain we used 30% flyash concrete another very green aspect of the house. What the slab has done for the reduction in energy ussage and comfort has been incredible. The house maintains a temperature of between 67-69 degrees with very little fluctuation. We heat the house here in MA with a water-water geothermal heat pump and only use 95F water to maintain the house at these temps. Long slow inputs of energy rather than big bursts one and off make the system operate at a very high performance rate. If I had my way I would build all my houses without a basement. There is a lot of detail to insulate a basement or a slab properly but the slab has much less expose and as a result needs much less sq/ft of insulating materials. For me a slab is a much Greener way to go.

Tom Pittsley
[email protected]
www.eebt.org

Concrete slab is one of the cheaper,yet sturdy foundation options. It is also relatively quick to install. Slab-on-grade foundations are more common in warmer climates. They are also common in locales with expansive clay soils.

It is important for the ground to have proper soil compaction prior to pouring a slab foundation so that cracking or settling will be minimized!

Speaking of expansive clay soils and concrete slabs, some areas in Montgomery AL has clay that is so expansive that either the clay has to be removed or the slab post-tensioned.  Otherwise, it is not unusual for the slab to break.  For years now, the custom was to remove the expansive clay and replace with better dirt.  The trouble with this practice is that basically a bathtub that will hold water has been created under the house since the adjoining soil was not removed.  Post tensioning and piers down to better soil appears to be a better approach. 

I think prospective new home owners should consult with their local code officials about special requirements to meet local conditions.

My understanding is that there are multiple ways to achieve a slab strong enough to work on unstable soil - extra thick concrete with lots of steel, post tensioning or waffle. Or piers and beams (ie, no slab).

I think he meant bentonite. A frost protected shallow foundation, slab on grade, is greener.

1 - the less concrete consumed - in that sense the better hence more sustainable.

2 - a professional engineer should be consulted about foundation design and ideally a geotechnical engineer at most sites.

3 - i did consult both in 2007 on a project in Ft. Worth, Texas on a limestone cliff with about 15 feet of gravel & sand on top.
to achieve design loads for a 2 story home with occupied penthouse we had a waffle slab on 15ft+ piers to bedrock.

4 - my work in Kansas office (www.ecologicalarch.com) usually recommends a spread footing with a crawl space due to
high clay content and non-draining clay soils with moisture driven upheaval.

5 - my work in Colorado also most often recommends framing over spread footings and foundation walls again due to upheaval issues.

6 - for our office slab on grade is rare and must have a very high level of perimeter insulation if done. Perimeter thermal bridinging
can be up to 20% of building heat loss if not using very good perimeter insulation strategies.

7 - finally a vapor barrier beneath slab on grade is also essential to avoid moisture migration into the structure.

8 - only in New Mexico projects have we used slab on grade. however, we have also used a foundation wall and spread footing even in New Mexico and then used an earthen floor which was several layers over a moisture barrier........the earthen floor was spread up to the foundation walls but compacted in multiple layers over a membrane.

It is a primarily a cultural issue, expansive clays can be found in many climates but are by no means ubiquitos. Standard construction practice is adequate for most applications of FPSFs, which came out of permafrost experience.

I worked on the eastern slope of Colorado and found that Bentonite doesn't care if your foundation is shallow or 8ft. below grade.

I have also designed radiant floor heating systems specifying vapor barrier and insulation in most of the northern states of the US and half the provinces of Canada without particular trouble or incident but always with great thermal efficiency and unmatched comfort.