Insulated basement floor
I’m seriously considering using “Superior Walls” for my basement. These are factory precast wall panels that are “erected” by a crew from the factory on gravel footings. The panels are cast using very high strength fiber reinforced concrete (5000PSI) with a 1 3/4″ outer “shell”, top and bottom steel reinforced beams and steel reinforced vertical studs (I assume 24″OC).
The manufacturer claims no “damp proofing” is needed and they can supply a faux brick exterior. The top beam’s inner edge and all stud faces are insulated with 1″ of XPS and the spaces between studs have 4 1/2″ of XPS + 1/2″ of foil faced polyiso, the bottom beam is only insulated on it’s top surface. A basement floor and the floor above must be in place before backfill.
Ignoring the top and bottom beams I calculate a whole wall r of about 21 with the supplied insulation and r 28.8 with mineral wool filling the remaining space.
The manufacturer has details that could allow foam under the slab and some thermal isolation of the lower beam but they are quite labor intensive. My thought is to not use sub slab foam or attempt to isolate the slab but instead lay r 10 of high density EPS over the slab and plywood or OSB over the foam (the downside is about 3″ less ceiling height). Has anyone here used them? What do you think of insulation above the slab?
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Replies
Jerry,
Be careful: Superior Walls has a history of R-value exaggerations. They've had more than one version of their walls over the years. One of their versions, Superior Wall Xi, has an R-value (according to energy expert Bruce Harley) of R-8.1. John Semmelhack did THERM calculations and determined that the R-value of the wall was about R-7. Yet the manufacturer claimed an R-value of "R-12.1 to R-12.5."
For more information on the R-value of Superior Walls, see this GBA thread: "What would a weighted average R-value be for a Superior Wall Xi?"
Your suggested approach to insulating the slab -- installing rigid insulation sandwiched between the top of the slab and a layer of OSB -- will work. The method is used for retrofit work all the time. Needless to say, it's important (a) to make sure that the foundation is well drained, and (b) to install a layer of polyethylene under the slab.
Martin,
Thank you! I agree the r values are inflated! Thank you for the link to the previous discussion! One of the posts in the previous discussion included a fairly detailed cross section of the current product along with a VERY flawed set of calculations that claims r 20. using their numbers and (incorrectly) assuming one dimensional heat flow I get r 13.58. Adding mineral wool to the remaining cavity I get r 17.33 (the one dimensional heat flow assumption is more realistic with the added mineral wool, but still optimistic). But adding a strip of 1/2" polyiso over the stud faces along with the mineral wool I get to r24 which meets my goal of r20+ walls.
In the modeling of XPS it's important to use a number no more than R4.5/inch (that number is what is warranteed by the manufacturers, though long term it may be even less), not the labeled R5/inch.
Ignoring the top and bottom beams will lead to a significant overestimation of whole-wall performance.
Putting the floor insulation above the slab will work, but there are benefits to keeping the thermal mass of the concrete inside of conditioned space. The density of the EPS doesn't have to be particularly high- even 1.25lbs per cubic foot Type-VIII roofing insulation would handle residential loads with at least 1/2" of plywood or OSB above the foam to distribute the weight.
Dana,
Thank you!
I got some misleading information, the Superior Walls do NOT use XPS but EPS and the assumed r/" needs to be 4.0. Putting the foam over the slab essentially eliminates the thermal bridge at the bottom beam but the top beam remains, along with the floor structure above the basement wall.
Their documentation indicates they use Dow Styrofoam, which is XPS. Fully depleted it's R-value would be about R4.2/inch. Dow's warranty indicates R4.5/inch minimum.
https://www.superiorwalls.com/superior-walls-products/xi-wall-superior-walls/
Check out Composite panel systems out of eagle river, wisconsin. I was about to the pull the trigger on superior walls when I found them. My wife and I decided to use them instead, I felt they were 'superior' to superior walls and with all the concrete problems having to do with high PYRRHOTITE content in the aggregate plaguing our area (NE connecticut) we were looking for a concrete alternative. Stellar company and great people to deal with, no concrete other than footings and our foundation was set in a few hours with a light duty crane (after I DIY'd the concrete footings), its made entirely from foam and fiberglass. Inherent Vapor barrier due to the fiberglass. I have been super happy with the decision thus far. Also decent R-value over a typical concrete wall, and fast. Its worth looking at in my opinion. Can't wait to move in and really test it out. I also put 4" of foam under the slab.