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What is the best way to insulate a slab on grade home on top of the concrete?

This is a new slab in northern Minnesota.

Asked by Richard Regnier
Posted Thu, 01/23/2014 - 12:14


7 Answers

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It's better to insulate the slab from below than on top of it, which is easy to do in new construction, impossible if it's already poured.

But I'm not clear if that's really what you're asking.

Answered by Dana Dorsett
Posted Thu, 01/23/2014 - 12:24

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I agree with Dana. If this is a new home Minnesota, and the slab has already been poured, it's a little late to be asking the question.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/23/2014 - 12:37

Helpful? 0

The slab is not poured yet. This is for a lake cabin which will be mainly used on weekends during the winter, more often in the summer. The water will be drained in the winter and only heated to about 40 degrees when not in use.
My only issue with installing the insulation below the floor is a cold concret floor in the winter. It will take several days for the exposed concrete to heat up which doesn't work well for weekend use.

Answered by Richard Regnier
Posted Thu, 01/23/2014 - 13:35

Helpful? 0

It's good that the slab has not yet been poured.

First of all, it's essential to insulate the perimeter of the slab with vertical rigid foam insulation -- at least 4 inches, if you can manage the details to protect the exterior foam and flash the top of it.

It's possible to insulate a slab by installing rigid foam above the slab. The usual method is to install one or more layers of rigid foam on top of the slab, followed by a layer of 3/4-inch-thick tongue-and-groove plywood that is fastened to the slab through the foam with long concrete screws.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/23/2014 - 13:50

Helpful? 0

I am planning on installing permiter insulation per the IRC frost protected foundation design. I want to make sure the floor system stays dry and prevent condensation on the concrete in the summer. Do you see a problem with installing with one continious layer of 3/4 inch xps with all seams taped, 3/4 furring strips on top with the space between furring strips filled with second layer 3/4 inch xps and seams taped, then 3/4 inch plywood? This will be about R-9 incuding the plywood. Do you think the floor assembly needs a higher R-value to prevent condensation?

Answered by Richard Regnier
Posted Thu, 01/23/2014 - 14:27

Helpful? 0

Since your house is in northern Minnesota, I would strongly advise you to include more than R-9 floor insulation. It's certainly possible that the home will be occupied during the winter during the future.

At least make sure to include high ceilings -- so that future owners can beef up the thickness of the floor insulation if they choose to.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 01/23/2014 - 14:37

Helpful? 0

Unheated slabs in northern Minnesota are at risk of frost-heaving if the house cools off to below freezing. Make sure to use a "smart" thermostat or something to tip you off remotely when the heating system is down and the house is approaching the freezing mark.

Insulating UNDER the slab will prevent summertime condensation issues far better than insulating above it.

Put 3-4" of Type-II EPS (not XPS- EPS is both cheaper and greener) under the slab, then an inch of EPS above the slab and you'll have an R16-20 floor with NO weekend-comfort issues to overcome due to the thermal mass of the slab. You'll still need the wing insulation to limit the frost heave threat though.

For the above-slab treatment, lay down 6 mil poly under the EPS as a slip-surface / ground vapor barrier. Skip the furring- use t & g plywood or OSB TapConned directly to the slab 24" o.c. wit, the seams staggered a foot or so from the EPS seams. Plywood subflooring on EPS supported by a slab is far more rigid than the same subflooring suspended between joists- there's no point to the sleepers- they'd just be thermal bridges to the cold slab and serve no purpose. Don't sweat the 15psi compression rating for Type-II EPS vs. 20psi+ for XPS either. When fully supported by the slab with a 3/4" subfloor to distribute the weight you could have a month long polka-party with a gang of 300lb lumberjacks in clogs and never damage the EPS.

Answered by Dana Dorsett
Posted Thu, 01/23/2014 - 15:27

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