Why Crawlspaces instead of Slab on grade?

[laminarflow]

I have designed a house using my mechanical engineering knowledge and lots of research. The house will be built in the Pacific Northwest on the Olympic Peninsula. When discussing my design with contractors in that area, I was constantly questioned about building on a slab. They kept insisting I should build with a crawlspace. I hate crawlspaces. I just can't stand walking around and hearing the slightest thump thump, cold feet, the added expense of a foundation and a floor, and mainly, the lack of thermal mass.

The contractors and builders would state reasons not to build on a slab such as, moisture problems, hard on knees and hips, can't easily re-model and move the kitchen, and lack of insulation. In talking to forward thinking builders, engineers, and from my own knowledge, I realize that none of these are issues. There are a few homes in my area on a slab, which are properly built that work well. I know the owners. Some are passive solar, but more on that later.

So I ask builders in the PNW why they build with a crawlspace, and have yet to get a real answer. One answer I get from builders is, “that’s just the way we build here”. I’m guessing there are several reasons. Could it be a hold over from when concrete was expensive or non-existent and wood was almost free? Could it be that builders can get more money for a crawlspace house? One reason I heard that makes sense is, “We’re building on crawlspaces, making money, so why change?” I even had one builder state that with equivalent houses, one on a slab, the other on a crawlspace, the slab house will be perceived as a “cheaper” house.

My design consists of a perimeter foundation footing, perimeter walls using ICF’s, and first floor walls made of ICF’s. Insulation will be on the very bottom by the perimeter footings, then about 2.5 feet of compacted fill, then the internal cap slab. This is to capture as much insulated thermal mass as possible with this design. Radiant tubing will be down in the insulated earth box and in the slab. When the solar collectors have satisfied the slab, they then pump heat down in the earth box for later use. Such a design will “coast” through a month long power outage with freezing outside temperatures and should only loose a few degrees a week. And with PV panels to power the radiant pump, a power outage will be a thermal non-issue. The house will look normal but be passive solar in that most of the windows will be south facing. No serious calculations were done, just good sound design practices utilized. So what about the summer? With a passive solar design, there will be minimal gain in the summer due to the high sun. And with high thermal mass, the house will be an average of the heat in during the day and heat out during the night. No forced air needed here.

There is a house on the Peninsula, well actually many, poorly designed and placed with large expanses of glass facing west and on a crawlspace. When seen in a summer afternoon, all windows are covered with shades and I’ll be the AC is running. It’s the sun. It’s thermonuclear. This is with the ambient temperature in the 60’s. And because of the low thermal mass, it’s conceivable that the heater might be needed the next morning. Low thermal mass, big temperature swings, and the more active the heating and cooling system will be.

 

[DRC1]

Getting back to your original question, foundation selection rarely has anything to do with heating. The primary considerations will be the ability to drain soil from the house and at what rate, the strength and settlement charateristics of the soil and the anticipated severity of frost, as well as the cost of construction.
First you will excavate to below frost for your wall footings. Then you will typically backfill the footings and make grade on native soil. If you chose soil replacement to below frost, youu will have significant costs associated with that work. Mostiure into the slab is controled by perimeter drains and a moisture barrier.Site drainage, including foundation drains is very important, however in residential work it is often done poorly or wrong as often as it is done right. The biggest problem with slab on grade is frost. If the ground freezes and the fill heaves, the slab will split in two. I used to live in NY State and have seen it happen. Once that occurs, the strucure generally can not be salvaged.
The othe reason is that if you ever need to access drains the slab will need to be cut. All first floor water will be in the ceiling, again making repairs or modifacations difficult. These are the reasons I belive crawl spaces may be used in luie of a slab on grade. The great thing about this country is that you can do whatever you want. Me, I would go for the full basement. I would suggest you talk to the local building offical about crawl spaces vs. slabs, as there is probably a lot of local history on both.

 

[COEngineeer]

Yeah, we just got hit with 2 bizzards in a row. I really wish I had heated driveway . Took me forever plowing my driveway.

 

[laminarflow]

Hi DRC1, That does make sense but in the Pacific Northwest, it's mostly crawlspace on the west of the Cascades where freezing temps are not an issue. Water is to a degree but not that bad depending on where the location is. On the east side where it gets colder and has less rain, there are more slab houses.

The locals are hopeless as they continue to give the reasons I mentioned in the first post. Could it be as simple as craawlspace is the way they have been doing it and reluctant to change? I have seen very complex foundations for full height crawlspaces with a dirt floor. For just a bit more cement, they could have had a basement.

In my design, and as a few other owner builders are doing, ICF's are used down to good ground and the inside is filled and capped with a slab. The ground and earth box below it will stay very dry. I might even toss in a bit of umbrella insulation too.

 

[TDAA]

I have a passive solar house that was built in ’81 in CO. The solar water panels and system have long been removed. It was designed correctly, from the thermal mass and sunlight respects. A lot of windows on the south side, overhangs that allow winter sun, but not summer, dark bricks on the northern wall for the sun to heat, slab on grade, it is built into a hill (whole thing is a walk out basement), and good cross ventilation for the summer.

I like the SOG; however, the slab curled when it cured, making subtle peaks at the joints. This was a pain when installing wood floors and tile in some rooms, and is noticeable in the carpeted rooms. I have wanted to remodel and would have to pull up the slab to do it (to move sewer), so this is a downfall.

For the heating, there is forced air, with the ducts run below the slab. In the winter, it only runs at night to counter the leaky windows. In the winter the house is up to 85 on sunny days, and the heat is only on from about 8pm to 8am. I think this would be reduced substantially with newer windows. The debate with my wife is “How energy efficient should they be to not reduce the solar effects?”

I was thinking it would be nice to add radiant heat to the main portion of my house, installed over the main slab. I have plenty of headroom with vaulted ceilings (up to 16 feet), and few doorways to contend with. Some of the tank less water heaters I have looked at state that they can be used for radiant heat as well. Some just use isolation so they can do both the water and heating needs.

 

[laminarflow]

I'm amazed that someone would build a slab with forced air ducts under the slab. Why? Forced heat air works just as well with ducts in the walls and ceiling. If you call forced air good. I hate it. It would be much better of course to put radiant in the gravel below the slab or in it. Nothing should be under a slab but what is necessary. Drain plumbing only. I know a sharp engineer who built his own house and has nothing in the slab. The tub and toilet are up 5" for the drain pipes. It's really like being on a throne!

If your getting that much temperature swings between nigh and day, your house does not have enough thermal mass. Is the basement insulated? Is it insulated on the inside walls?

I would suggest looking into insulating the perimeter ground with wing insulation and pouring a cap on the slab with radiant tubes. This would solve your curling slab. What was the reason for the slab curling?

 

[emmgjld]

For the Curling slabs, do a search in the Eng-Tips.
Lots of discussion.

Placing a topping or capping the slab may or may not be effective.

 

[TDAA]

Not so much a lack of mass, but the inflow of cold air. The windows are original, have broken seals, and many do not close well. If we could fix that, we would be much better off. The problem is that most of the windows are nearly floor to ceiling, and need to be tempered. Just for good quality windows, we are looking at over 8 grand (no installation).

Our furnace was out for a few days, and we maintained in the upper 60's at night with 80’s in the day, while the neighbors were continuously running the heat.

 

[mitchelon]

laminar, what is the frost depth in the OP? Around 100"? Also, have you seen a lot of basements in that area?

 

[laminarflow]

Mitchelon, I don't think there is any frost depth here. It is maritime and although we did get 6" of snow in December and due for an inch tonight, the winter temps are usually in the 30's. Not many basements but what you do see is crawlspaces on a slope that are tall enough to be a basement. Really, I've seen the dumbest construction here and there has not been an architect near this place for decades. We saw a slab on grade house last weekend, well it was half a slab on grade. The living room, dining room, and study were SOG and the Bedrooms and Kitchen were crawlspace. Built in the 90's apparently by an owner / builder / contractor, I can't imagine what he was thinking. The slab was not insulated on the perimeter and the ground was wet from the recent rain. It was cold on the edges and the owner was most interested in my suggestions on insulating the edges of the SOG and using plastic and insulation to shed the water away from the edges of the slab. Without insulation, a SOG is just as bad as a crawlspace without insulation. Why do people think concrete has a high R value?

Most basements in this area, even new construction, are concrete right up to the wet earth. What a recipe for mold. Isn't there a test to take before one works for the county or gets a contractor license?

 

[AggieYank]

Your lack of respect for builders and the way things are usually done may come back to haunt you. You dismiss far too easily the way things are usually done on the premise that builders have no reasons other than supreme ignorance or to build a fatter profit.

Crawlspaces are generally chosen over slab on grade for structural reasons. Perhaps a couple questions may help on your quest for knowledge:

1) What is the expected differential movement of the undisturbed soil at the site?
2) What is the expected differential movement of the soil at the site once 2.5 feet of soil are replaced with fill?
3) Will the fill be compacted? Or are you prepared for the settlement which will occur?
4) What is the expected settlement of the soil beneath the exterior footings?
5) Will the expected settlement / expansion of the soil cause problems in the veneer on your house?
6) Will the exterior footings be beneath the frost line? If not, heaving will wreck your exterior walls.

Where I work, there are several different soil types. On one side of the city is clay. When we design foundations there, it is typically more economical to use a crawlspace (suspended foundation). If the owner absolutely insists on a slab on grade, the geotech often recommends 7+ feet of soil removal and replacement with select fill. With this amount of fill, you'll also experience settlement issues. 7 feet is typically the breakeven point for economy between slab on grade vs. suspended. So for this side of the city, you either get a suspended foundation, or a house that will almost definitely have wall cracking problems, especially if someone built it cheap, like one of the national homebuilders.

On the other side of the city, the soil is a bit better, mostly rock, or weathered limestone. The norm here is slab on grade without too much fill.

 

[laminarflow]

"Your lack of respect for builders and the way things are usually done may come back to haunt you."

You bet. Sometimes I feel that I'm being haunted, like right now!

"You dismiss far too easily the way things are usually done on the premise that builders have no reasons other than supreme ignorance or to build a fatter profit."

No, I discussed this with the local builders, and local engineers. Local builders just say that that's the way we have always built. Local engineers like SOG as they design all commercial building that way.

"Crawlspaces are generally chosen over slab on grade for structural reasons. Perhaps a couple questions may help on your quest for knowledge:"

1) What is the expected differential movement of the undisturbed soil at the site?

Not much, glacial till hard as rock with some clay down deeper.

2) What is the expected differential movement of the soil at the site once 2.5 feet of soil are replaced with fill?

Not much as the fill will be compacted. (Hang on, we just had an earthquake.... no wait, it's was my 9 year old chasing my 7 year old through the rental we're in and yup, it's on a crawlspace.)

3) Will the fill be compacted? Or are you prepared for the settlement which will occur?

Yes

4) What is the expected settlement of the soil beneath the exterior footings?

Engineer said everything is OK.

5) Will the expected settlement / expansion of the soil cause problems in the veneer on your house?

Not likely, doesn't on other houses here. Why should it?

6) Will the exterior footings be beneath the frost line? If not, heaving will wreck your exterior walls.

Frost line isn't very deep here but I'd put wing insulation if it was.

But here is a clincher. If crawlspaces are necessary here, why do all commercial building have SOG or perimeter footings with a cap slab? The local engineers all say SOG is no problem and that we even have better soil for it than in other areas that use SOG.

Thanks for your concern.

 

[COEngineeer]

I also want to add sometimes they do crawlspace because it is cheaper. Due to site condition very steep, they might have to import too much material for the basement to work so they do crawlspace instead.

 

[laminarflow]

When the site is very steep, an insulated basement or bermed in insulated wall can be used. But if there is a benifit to getting the house up high, a view or up to capture some sun if the house is on the north slope, then I think I would use an insulated basement and utilize the space. I've seen crawlspaces that are tall enough to park a van in. Makes for an ugly house too.

As far as fill, I've seen new crawlspace construction "push out" a considerable amount of fill which is then trucked out to build the crawlspace. This is to get the house level with the surrounding grade.

Just yesterday I heard of another reason crawlspaces are the most common new construction method in the Pacific Northwest. Their reason was that after WW2, many cheap houses were tossed together on uninsulated slabs down in the wet earth. They were so cold and damp, gee I wonder why, that crawlspaces became the preferred method of construction. I guess that makes sense too.