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Homeowner Forum / Construction / December 2005Concrete Slab Insulation
I am planning to build a home, garage, and shop on Whidbey Island next year, and all will probably be built on concrete slab foundations. My question is with respect to insulation. I have read a number of articles stating that a very firm gravel/sand base is required to properly support the concrete, but it seems that rigid foam insulation between the concrete and the base could weaken this support. Is it possible that the insulation board could be crushed and allow the concrete to crack from lack of support? Thanks. In a previous post m2machine@gmail.com wrote... > I am planning to build a home, garage, and shop on Whidbey Island next > year, and all will probably be built on concrete slab foundations. My [quoted text clipped - 4 lines] > possible that the insulation board could be crushed and allow the > concrete to crack from lack of support? Not very likely. Most under slab insulation has a compression strength of between 40 psi and 80 psi. This translates to between 5760 PSF and 11,500 PSF. Since we typically design for an allowable soil pressure of between 1500 PSF and 2000 psf, you can see that the insulation is much stronger than the soil. BTW, it sounds like you are going to use hot water radiant heat. Make sure that you use lots of crack control joints and omit any fibermesh or welded wire fabric. They will do nothing as far as crack prevention. Where you are heating you may need to install some rebar to tie the heat tubes to (I like #3@16" o/c each way) or you can use the plastic grids supplied by some of the heating system manufacturers.  SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA > In a previous post m2machine@gmail.com wrote... > > I am planning to build a home, garage, and shop on Whidbey Island next [quoted text clipped - 24 lines] > Structural & Civil Engineering > Poulsbo WA Thanks for the reply Bob. No, I'm not planning hot water radiant heating, but it seems to me that slab insulation would be a benefit regardless of the heating system. Is that not the case? > In a previous post m2machine@gmail.com wrote... > > I am planning to build a home, garage, and shop on Whidbey Island next [quoted text clipped - 24 lines] > Structural & Civil Engineering > Poulsbo WA In a previous post m2machine@gmail.com wrote... > Thanks for the reply Bob. No, I'm not planning hot water radiant > heating, but it seems to me that slab insulation would be a benefit > regardless of the heating system. Is that not the case? Any particular reason to go to slab on grade for the house if you are not using radiant heat? If not using radiant heat then I recommend that you use a crawl space and conventional framing for the house part. Here's why: 1. The floor will be warmer with batt insulation. 2. A wood floor is more comfortable to walk and stand on. 3. It is possible to repair plumbing. 4. It is possible to modify or add electrical. 5. It is possible to add or modify duct work. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > Any particular reason to go to slab on grade for the house if you > are not using radiant heat? If not using radiant heat then I > recommend that you use a crawl space and conventional framing for > the house part. Generally speaking, is slab on grade construction stronger seismically, "all other things being equal"? Thanks, Wayne In a previous post Wayne Whitney wrote... > Generally speaking, is slab on grade construction stronger > seismically, "all other things being equal"? Not necessarily. At least not one the house I design <grin>. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com Yes, I agree with most of your points, but cost is an important consideration, and I'm sick of squeeky, squishy, creaky wood floors. I've got some of those now. > In a previous post m2machine@gmail.com wrote... > > Thanks for the reply Bob. No, I'm not planning hot water radiant [quoted text clipped - 23 lines] > Poulsbo WA > bob at rlmorrisonengr dot com In a previous post m2machine@gmail.com wrote... > Yes, I agree with most of your points, but cost is an important > consideration, and I'm sick of squeeky, squishy, creaky wood floors. > I've got some of those now. A properly designed and constructed floor will not squeak", "squish", or "creak". Nor is a high quality floor that expensive to build. About half my practice is structural design of high-end homes. Believe me when I say that the owners of those homes do not want floors that vibrate or make noise. Please don't misunderstand me. You can pour a concrete floor and have it work out very well. I just happen to think that a wood framed floor will perform better, and in the long run will give you more flexibility to make changes. In fact, I usually recommend concrete on top of wood vs. slab on grade for radiant heat systems for that very reason. My $0.02 worth. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com > Not very likely. Most under slab insulation has a compression strength of > between 40 psi and 80 psi. This translates to between 5760 PSF and 11,500 > PSF. Since we typically design for an allowable soil pressure of between > 1500 PSF and 2000 psf, you can see that the insulation is much stronger > than the soil. Thanks for the explanation. I have a concrete slab without insulation under it, and I'm wondering about installing a wood floor over insulation over the slab. Will this work? I guess my concern is that point loads on the wood floor could squish the insulation. Will a point load on the floor be distributed by the wood over a circle of radius equal to the thickness of the wood floor, i.e. through a cone of angle 45 degrees? If so, then a 1.5" thick wood floor would distribute a point load over 7 square inches, so it should be able adequately spread a 300 lb point load if the foam's compressive strength is at least 43 psi. Is that how the computations go? Cheers, Wayne In a previous post Wayne Whitney wrote... > Thanks for the explanation. I have a concrete slab without insulation > under it, and I'm wondering about installing a wood floor over > insulation over the slab. Will this work? My office is in what was the garage. I put down treated 2x2 wood sleepers over a PVC vapor barrier and then nailed plywood to the sleepers. I put 1-1/2" rigid insulation between the sleepers. Carper on top. Makes a real nice floor. SignatureBob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA bob at rlmorrisonengr dot com Your description, per se, is much like the floor of a cold storage/freezer facility. Not unusual is 4~8" rigid board with 6" mud on top. with freezer tubes encapsulated intsead of heat. Holds up just great. Dan > I am planning to build a home, garage, and shop on Whidbey Island next > year, and all will probably be built on concrete slab foundations. My [quoted text clipped - 6 lines] > > Thanks. "Not unusual is 4~8" rigid board with 6" mud on top." I presume "mud" is concrete. What in addition to the tubes does the mud have in it to stabilize it - can only imagine the contraction/expansion forces in something like this. wwf (welded wire fabric) I wish the hell I could remember 'exactly' how the layering went. SEEMS like, dirt, then compacted 3/8" minus gravel, then sand, (with the piping/tubing buried in it), then mil vapor barrier, then rigid insulation, then vapor again?, wwf, mud (concrete). The tubing was buried in the sand layer, I do remember that part. Old age has a death grip on my butt and is squeezing my brain that seems to leave less room for informational retainage! The layeing was intended for a 3 part combo, cold storage-freezer-blast freezer building. Ammonia cooling. For all I can remember, the tubing might be heat to keep the cold from penetrating into the subgrade and heaving. (seems logical) But then again, I might have the layering all screwed up. Dan > "Not unusual is 4~8" rigid board with 6" mud on top." > > I presume "mud" is concrete. What in addition to the tubes does the > mud have in it to stabilize it - can only imagine the > contraction/expansion forces in something like this. |
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