what are the best choices for insulating under a concret slab? (new home) I will need a vapor barrier as well, and am installing a passive radon system. I’ve seen rigid blue foam board used in either 1 or 2″ and I’ve heard about a blanket that acts as an insulator and vapor barrier in one…but I think that’s typically used in commercial work.
Any info would be of great help, thanks,
KB
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There have been several posts here as well as articles in FHB and other pubs on slab prep. Suggest you do a global search within FHB. Where are you located? Major issues are soil type and normal moisture content, (frost?). A good practice is gravel and sand layers compacted with a moisture barrier (6-20 mil poly). Some folks also add the 2"
blue board, but sometimes conflict with the chairs on the rebar placement. See waht others add.
I haven't used the blanket, but just poured a 900 sq. ft slab over 2" Dow foam board last week.
In either case the foam board or blanket should be placed over a compacted soil and 4" (min) of gravel. In the case of foam, I place a 6 mil poly vp on top of it, and then the ww mesh.
A good way to keep the poly in place as you place the wire is to use plastic cap nails pushed into the foam board like big thumb tacks. I did not tape the seams in the board or the poly, but in your case with radon issue, I probably would. Plumbing penetration and floor drains would also be cut tight, and the poly taped tightly around them.
Dave
I chose to layer (from bottom to top) crushed gravel (12"), vapor barrier (6 mil), XPS (2"), wire mesh, PEX tubing (1/2") and concrete (4"). The thickness of the slab insulation is really dependent on your local soil conditions and the climatic realities. For example, we are in a cold climate with heavy clay soil. As a result, thick insulation is pretty necessary.
In areas where the soil drains well (sand) and that are warmer (Florida), nothing more than a thin thermal barrier (1/2") is probably warranted. Take a gander over to buildingscience.com, they have a great deal of free resources on the web that explain the benefits of insulating your foundation walls and slab on the outside.
IMO, the blankets and wraps like Insultarp, et. al. are basically nothing more than a thermal break. Their insulation value under a foundation is about R1, if that. Thus, I wouldn't bother with them unless they were significantly cheaper than comparable thicknesses of XPS.
If you have a lot of gravel below the foundation, gathering the radon is very simple business. We have a 4" perforated PVC pipe running within the gravel that terminates in the mechanical room. With the amount of gravel we have, the radon has no problem migrating a good distance under the slab to the pipe. The pipe itself had 60x the allowable Radon limit in it when I tested it while capped.
Lastly, if you're pouring a slab, I would incorporate PEX tubing for future heating purposes even if you don't intend to use it now. Dan Holohan has a nice primer for sale at heatinghelp.com that will allow you to determine tube size, spacing, loop lengths, etc. so that you can have radiant heat in the future as an option.
http://www.r-control.com/
This stuff is treated with borate to keep the bugs from eating it.
Cheaper than blue or pink board, not quite the R value per inch.
Joe H
Rigid (foam, eps) inulating boards, 2" is good for r-10, in most cases.
I recommend the CBF product made by TVM building products. It is a layer of reflective foil sandwiched between two layers of polyethylene bubble insulation. This product comes in 16"x125 ft or 48"x125 ft rolls, has a compressive strength of 140 lb/in2, form an exceptional vapor, radon and methane barrier as well as providing an R-10 layer of insulation.
Tim,
You must be a bit fuzzy about the details of building insulation if you believe that a single layer of 5/16"-thick bubble-wrap+foil sandwich can achieve R10 equivalence when used under a slab. :-P
Even the manufacturers of the foils have taken those claims off their product spec sheets ever since real-life testing in Canada showed that the R-value of this kind of glorified bubble-wrap is maybe R1. That is not to say that R1 under-slab insulation doesn't have its place in certain environments but R10 it ain't.
For a in-depth article on the measured performance of these materials and the follow-on discussion, I invite you to this old thread on the Wall at heatinghelp.com. Cheers.
Edited 8/18/2005 8:05 am ET by Constantin
You are right, the link provided some very good information I had not seen, and I will certainly look into my suppliers claims a little more rigorously. Though this is a popular (and profitable) product, I will have to amend my recommendations.
Are you familiar with the product "InsulTarp" ? I have had a few requests from installers looking to buy such. I believe the product to be very similar to the CBF, though I have no direct knowledge of it. I cannot understand the resisitence to the use of rigid foam products.
Hi All,<!----><!---->
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Would suggest reading the November 2003 issue of Energy Design Update where several of the major manufacturers apologized for the erroneous published information provided on their products.
<!----> <!---->Some of the better political retractions from four different manufacturers…
“…apologies to anyone confused by the statement”
“This was an oversight on our part”
“we realized it was erroneous”
“apologize for the misconstrued quote”
<!----> <!---->
It’s too bad that here we are approaching 20 years since my first exposure to this stuff and two years since the article and the critical mass momentum that started is still rolling along…what’s the saying… “if it looks too good to be true…” Its kinda of like the longest running bank scam from Africa that’s been stealing millions since its started showing up decades ago… you know the one….Dear friend…you don’t know me but my late rich Uncle King Whatshisname has been exiled with billions of dollars that need to divided amongst his children and we need your help and bank account number - for your troubles we’ll throw in a roll of bubble foil for your next radiant slab project.
<!----> <!---->
RB
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
Edited 8/23/2005 11:21 am ET by RBean
Tim, Insultarp may be a better bubble-wrap than average as some contractors on the Wall seem to like to use it for things like slabs in temperate climates. However, as Robert points out, the performance of these kinds of wraps under slabs is marginal at best. If real insulation is called for, a layer of XPS is probably still the safest bet due to its high compressive strength and low moisture uptake. When pre-treated with borates, it may even survive in termite country.Robert, as usual, many thanks for posting the responses of manufacturers when caught with the hand in the cookie jar. How any of them could have believed the "science" behind a R10 bubble-foil under a slab still eludes me. Unfortunately, I doubt that justice will be served when it comes to restitution for the damage these products have caused by being intentionally misapplied.
When the internet wasn't around this kind of activity moved around by horse and buggy when the snake oil salemen were discovered and uncerimoniously chased out of town by an angry mob...today what is absolutely amazing ... is with so much realiable information readily availble and free...that the scams still continue - years (decades for the bank scams) after they've been exposed.
One of my favorite websites is The Museum of Unworkable Devices....enjoy: http://www.lhup.edu/~dsimanek/museum/unwork.htmRBean
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
Greetings to you KB. I happen to favour Dow Corning's product HI 60, a high density Extruded Polystyrene insulation. Although it is a good vapour barrier in it's own right, the many seams seem to be problematic for me to get sealed properly. I therefore try to use a sheet of poly on top for a vapour barrier/slip sheet. (I've been able to buy poly in rolls up to 2000 ft2 , 100' on a side, which can reduce the number of seams.) I seal my service penetrations through the XPS with canned polyurethane foam. (I figure anything that difficult to get off my skin has a good chance of staying stuck once I bury it for the next 50 yrs.!)
Although mfgr. claims for compressive strength of their products are usually more than adequate for normal floor loading, I'm usually more comfortable spec'ing a higher-than-req'd product strength to compensate for the long-term effects of material creep and initial compression bound to take place.
I've found that a layer of clean sand directly under the insulation is the easiest for me to grade. I use a large metal straight-edge (10') to get my sand bed plane before I damp compact it just prior to laying down the XPS. You might be able to purchase a heavy gauge steel wall stud to use for this.(I usually have to re-grade my base after compaction to keep it plane.) Most below-grade insulations have little ability to bridge any large voids in a poorly graded base and will deflect when your floor receives it's 1st serious load test, often allowing cracks to happen. (An added bonus to the care taken in grading your base well is the accuracy of your material take-off when ordering the concrete, I try to never run short of concrete during a pour. You also get a more consistent floor thickeness of uniform strength throughout.)
I also favour some sort of steel reinforcement in my concrete floors. For me, it seems best to use deformed steel rebar on top of cubes of concrete spaced out on my poly slip sheet with embedded wire to tie in my rebar grid. If my spacing isn't too great, I can walk all over the rebar grid during the concrete pour without fear of driving my reinforcement to the bottom where it loses it's needed concrete cover. I've found that if I take care to tightly fit my insulation, it doesn't shift much at all while my pour takes place. (I cringe a little at punching holes through that nice insulation to skewer it to the bed below. With all the weight of my concrete cube chairs and rebar, I seem to have plenty of ballast untill the liquid stone starts flowwing!)
I would also echo the suggestion to visit the archives, the way you treat the interface between wall & floor could use some consideration. I usually use some sort of isolation joint here with a flexible caulking (polyurethane high performace) placed on top of my joint material. I've read of many other ways of handling this however & would suggest you draw your own conclusions as to suitability.
My feeling is that a floor system designed with a life expectancy measured in decades shouldn't be compromised by a design philosophy that doesn't address the long term effects of material degrade. (Such things as material outgassing as an example, can reduce the blowing agents within the foam insulation and allow it to compress over the long term).
For me, a strong, flat (or plane) floor is a thing of beauty and the basis for everything that goes on top of it. (I pay particular attention to the bones below the surface but really like it best on top!.)
Lots of luck!
try http://www.insul.net