What tolerances are required on grade preparation for an insulated slab floor
AnnaJean
| Posted in GBA Pro Help on
We are building a super insulated shop(4000 sq ft) with a concrete slab floor with radiant floor heat. thermal calculations and recommendations from American Concrete Institute indicate we should put 3″ of extruded polystyrene insulation and a vapor retarder topped by 8″ of gravel under the 6″ floor. What tolerances are really required on the grade flatness under the insulation? Our sub-contractor is is charging a lot for a very tight tolerance. also, would 3 layers of crossed 1″ foam be better and more forgiving than 1 3″ layer?
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I would think you would want the gravel, whose primary job is as a capillary break for any groundwater, to be beneath the insulation. If you did it that way, the grading does not be too precise. The top of the gravel should be flat, but gravel is easy to smooth out. I'm not sure if you wanted the gravel as an additional heat sink, and that is why you have it above the insulation, but 6" of concrete should provide a very high thermal mass...
The reason for the gravel under the concrete was to minimize curling of the concrete - a "blotting layer". But all this is a tradeoff. And we are thinking that what you suggest may be a better option. The gravel doesn't do much to help as thermal mass, contrary to what is often said. We have done our own thermal calculations, but it would be a real service if GreenbuildingAdvisor could find a really definitive finite element calculation of slab floors, thermal mass, vapor retarders and gravel underneath to define best practices and tradeoffs.
the first comment above was meant as answer to the 2nd comment - don't know how it showed up first. I'm the person who asked the question in the first place.
Yes, three perpendicular 1" XPS foam layers will flex better without separation at joints. The gravel (or do you mean crushed stone?) should not be between VP/insulation and concrete, as it will merely create a reservoir to contain water. A "blotter" layer is merely for the convenience of the flatwork installers (so they don't have to wait as long to finish the slab), but will undermine the hygrothermal performance of the floor.
You'll have more than enough thermal mass in a 6" slab. With a radiant slab, you want the insulation layer immediately under the slab to control downward heat loss. And make sure the radiant tubing is lifting into the slab or the lag time (thermal inertia) will be problematic.
Anna,
I'll take a stab at responding to your request for "best practice" advice on this topic "slab floors, thermal mass, vapor retarders and gravel underneath."
From the bottom up: Crushed stone, XPS insulation — as thick as you can afford, something between R-20 and R-40 would be good —, polyethylene, concrete. No blotter sand or blotter gravel layer.
My opinion, perhaps controversial: Thermal mass provides little benefit in a radiant slab floor, and provides some disadvantages (delayed response time, occasional problems overshooting the thermostat setting when weather changes from cold to warm).
If you want the slab to provide thermal mass for a passive solar house, be sure to leave out the PEX tubing. Radiant slab floors are incompatible with the use of a slab floor as a useful thermal mass for a passive solar house. To absorb solar heat, the concrete slab must start out cold in the morning.
What Martin states about the inefficiency of a radiant slab floor for solar gain is correct only in the parts of the slab not in direct sun. To absorb thermal energy by conduction requires that the slab be cooler than the surrounding air.
But a direct-gain thermal mass floor (in direct sunlight) will absorb radiant solar energy regardless of its temperature. Since the sun is radiating at nearly 10,000°F, any object cooler than that will absorb energy and turn it into heat.
You all experience this when you're outdoors on a sunny day. Your skin averages 91.4°F and yet absorbs lots of solar energy.
Robert,
Thanks for contributing; I've learned something.
However, I'll try again: is it correct to say that under these circumstances (a passive solar house on a sunny morning), a warm slab with radiant heat — a portion of which is in the shade — will absorb and store less heat than a cool slab without radiant heat?
Martin, What is Anna's climate?
Does your advice apply to all climates and all ground temperatures? What temperature is typical for the radiant floor heat?
John,
Good question — Anna, what's your climate?
Here's my two cents on insulation:
1. The colder the climate, the thicker the insulation.
2. A slab with radiant heat needs more insulation than a slab without radiant heat.
3. A green home needs much more insulation than minimum code requirements.
Radiant floor heating systems can work with circulating water temperatures as low as 100°F, especially if the home has a tight shell and is well insulated. Slab temperatures may be only one or two degree above air temperatures.
This is not a passive solar house. It is a super-insulated shop with a small temporary apartment. We do intend to try external panels for the hot water for the radiant heat. We are in the northwest near Spokane. Conditions are not good for direct passive solar for heating.
The construction is SIPs - 8" in the walls, 12" roof. triple pane windows, and we are trying to pay special attention to thermal shorts and air infiltration.
We have found that there is a great deal of misinformation floating around, not based on real physics and engineering analysis and controlled experimentation, which is where I think a site like this could be very useful if it "commissions" rigorous articles.