advise on combining basement and slab?

15 years ago

OK, I've been lurking here for a couple of weeks and have been truly blessed by all of you and your knowledge!

We are working on our plans and budget. We are considering building a partial basement to house the mechanical items, a freezer, exercise equipment, and some storage. We would like to then do slab under the rest of the first floor and probably have concrete floors in those rooms. We are planning the basement under the laundry, kitchen, and bathroom so that the pipes would be accessible if needed. The major reason not to do a full basement is budget.

We do live in an area prone to shrink/swell, extreme variations in temperature and humidity.

Are we crazy? Advise on how to make this plan work?

Featured Answer

meldy_nva

15 years ago

Yes, it will work (friend did this because of a boulder, the short side was laid over the boulder). One needs an expert in concrete to be sure the joint is tight, and that the rebar is correctly laid.

However, do the math! The primary cost savings is only for the difference in wall height, and the corresponding drainage. Which likely is not very much savings. Actual cost will vary by region, but I read a recent article which computed the difference as being less the $3 sf in my area. That means if you did an additional 1000 sf as basement instead of slab, it would cost less than an addition $3k, which is very little money for a lot of extra space.

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Comments (7)

bdpeck-charlotte
15 years ago
I haven't heard of slab and basement together, but that doesn't mean it can't be done... it should be no different than having my slab garage next to my basement. Call a local foundation company and see what they say.
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I agree on having our kids appreciate what they have. They really don't know how good we've had it! We have 1 incredibly small bathroom we are sharing (that right there has been a growing experience for me - not loving that nor the lack of water pressure - ack). It's an old home - as in no outlets or switches in one of the bedrooms upstairs. They were sharing a room but they've already rearranged. The room without any outlets also doesn't have a heat vent so I don't see my oldest staying in there come winter - but it is also the largest of the 2 rooms. :) We also have no dishwasher and no a/c. Those are basically the comforts that we had that we are now getting used to being without. I keep reminding myself that there is no perfect home and surely there will be things after living with it that we might think are too large, too small or a little off one way or another. Our pantry is pretty sizable - fits our chest freezer which I'm excited about but the doorway is off the kitchen, not IN the kitchen proper. That bugged me for a while but I think it's going to be okay. I was off a bit on my $250k number - dh informed me that it's more like $280k. :( Besides the roofing the other number that came in considerably higher than we thought was lumber. It recently took a jump so that was a bit disappointing. We are hoping to get all of our quotes by the end of the month so we can lock in our I/R before it also takes another jump. We will be doing all priming, painting, laying wood floor, tile, setting cabinets, some trim work, putting stone on fireplace (we have done that before and it's actually a very rewarding project but took way longer than we thought), hardware on doors and cabinets, lighting, ceiling fans. We too will be doing some of those very things you mentioned after the fact. Definitely closets and even the small office we have will be done after we are in. With work schedules and keeping timing on track it may come down to having to hire things out that we originally planned on DIY. We are not moving communities or school districts and are paying rent so time is money to a certain degree. For now though that is what we hope to do in sweat equity. I think you are wise to start working through house plans now. There are certainly many details to ponder. We were hoping to be about 2000 sq feet total for the main and upstairs but ended up at 2400. We don't feel the rooms are excessively large or that we have rarely used rooms so we went ahead with it. I had a few things that I really wanted configured a certain way and made sure they were laid out accordingly. After those boxes were checked I really didn't feel so picky about the rest. Hopefully they will live how I feel they will in my mind! :)
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Putting down XPS foam would also cause a impermeable barrier. XPS is slightly permeable, thus allowing water vapour to dissipate slowly. At the same time, it is unaffected by moisture and the R factor the XPS provides keeps whatever material is above it warm enough to avoid condensation from forming. It's not the vinyl that's affected--it's the adhesive that fails. This piece from Dr. Lstiburek details the problems of water vapour coming through a basement slab. (Though it deals specifically with improperly done slabs, the principles are the same.) Loose-lay vinyl may handle the moisture better. However, your floor will likely require extensive work to achieve a level bump-free substrate. Since you indicated some family members are more than 6'6" tall, I would suggest that tile over a crack isolation membrane might be the best solution. Or you could lay low-pile synthetic carpet without an underpad and just count on replacing it every few years. When I had rental properties, that's what I found myself doing because of tenant damage. Always lots of ends of lines, oddballs and returns available at discount stores and on Kijiji.
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As others have said, yes it is possible to combine slab on grade for a portion of a house and a basement for another portion. That said, the feasibility of basements depends on your land gradients and soil conditions. Lots of rock or a high water table or expansive soils make basements problematic and very expensive. I might suggest that this concept may not be the best overriding concept for the design of a custom home, however... :-)
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frog_hopper
15 years ago
A basement and crawlspace combo is probably more common than basement and slab. Basements are the cheapest square footage available. I think you are making a mistake.
carolyn53562
15 years ago
We had the basement/slab combination at our old house where we lived for 20 years--it was done for passive solar purposes. I bet that most passive solar houses in cold climates are built like that. We live in Wisconsin where not only do you need to worry about getting the plumbing in the slab but also the heating ducts. Our slab was under our living room and family room and the slab and basement settled differently so there was at least an 1/2" to 1" difference between the floors where they met--a problem if you want flooring like hard wood or tile to flow between the rooms. Another problem we had was that there weren't enough heating ducts in our family room and it would have been hard to add them so we just lived with a cold family room--be sure to put in enough heating ducts in the slab. An issue we had, but not a problem, was that we wanted to put in site finished hardwood floor and you can only use engineered over slabs. Everyone has basements where we live and I think that having a partial basement made it more difficult to sell our old house. Plus, I missed having a full basement--one of the reasons we built a new house was that the basement in our old house just wasn't big enough for us. However, combining a basement with slab should work just fine.
shelly_ok
Original Author
15 years ago
WOW! Thanks for the speedy replies.

We do have free natural gas and I thought we would do radiant heat in those concrete floors.

We only have estimates from one builder so far - he estimated $7K for the radiant heat. His original estimate was for the partial basement, we have asked him to give a number for the full basement also, which would of course also increase the flooring costs. He said he'd work it up but he thought $35K (for basement and changing the floor), which would give us a little over 500 more square feet in the basement. We do live where we need heating and air conditioning, so the duct work would have to be run through the slab even if we did radiant heat also.

It is awfully tempting to put in a full basement, but we have not found a money tree on our property so we'd need to massage the plans a bit to fit it in the budget.

Would love to hear more!
frog_hopper
15 years ago
Ducts do not have to run through the slab. You can put them in the attic or in soffits. Air conditioning only supply registers should be in the ceiling or high in the walls. The supply duct can run from the basement to the attic through a chase, or the air handler can go in the attic (ugh).

With radiant flooring, you would not want the supply ducts in the slab. Return air ducts could be in there. They could also be run in chases and soffits or the attic.

It may also be possible to locate the returns over the basement portion, so it becomes a non-issue.
carolyn53562
15 years ago
Whether you can put the heating ducts in the attic depends upon the design of your house. We couldn't put ducts for our family room in the attic or the soffits because of the house design, they had to go in the slab. I think that radiant heat is a great choice for a slab. We considered putting it in the slab for our family room 20 years ago when we found out that the family room was cold, but couldn't find anyone in our area who installed radiant heat. Good luck.

advise on combining basement and slab?

meldy_nva

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advise on combining basement and slab?

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