Looking for recommendations for light, heat-insulating floor tile
July 14, 2013 5:45 PM   Subscribe

I'm looking for recommendations about how to insulate the floor of a sauna on a second-floor porch.

I have a sauna on my second floor porch that's got a pretty serious temperature gradient. While I can get the top 3ft or so to 90-100C, the bottom 2-3 ft is almost the same temperature as the outside (35-40C).

The heater should be more than sufficient (bordering on overkill) for my space (8ftx5ftx7.5ft), so I'm thinking that I may be losing a lot of heat through the floor (the walls and ceiling have insulation in them, but the ground is basically subfloor over the wooden structure of the porch - plywood over floor joists, then the thin pine ceiling of the first floor porch). Anyone have any suggestions for a heat-insulating floor covering?

My contractor has no idea (though he did say that ceramic tile would probably be way too heavy for the structure), and when I asked the local hardware store, they suggested foam insulation sheets (which, printed right below the manufacturer's logo, noted something to the effect of "extremely flammable, will burn" so I figure putting it 8 inches below a heater might not be the best idea).
posted by Seeba to Home & Garden (12 answers total)
Any way to get underneath and cut and put the rigid insulation between the joists below? Even standard fiberglass bats would work that way but the foam board may be more weather resistant.
posted by edgeways at 5:56 PM on July 14, 2013

I'm doubtful that you're losing that much heat through the floor. Wood is a halfway decent insulator by itself. It'll conduct some heat, but not enough to account for that sort of gradient. It's more likely you're losing heat through the ceiling.

I know almost nothing about saunas, but how hot do you want to make it in there? 90C seems... well, potentially fatal.

If you do want to insulate the floor then doing it from below is the way to go.

If the structure can't handle the weight of a ceramic tile floor then you shouldn't be walking up there yourself. On the other hand, ceramic tile is worthless as an insulator.
posted by jon1270 at 6:39 PM on July 14, 2013

if you have access under the floor, use spray foam. I would use open cell and only a couple of inches thick so it could breath a little. The tighter (meaning air movement) you make a damp space the worse problems you are going to have with mold and rot. The even better solution would be to nail plywood to the bottom of the floor joist than rigid foam insulation under that and put in vents so warm air from the sauna can circulate in and under the floor also...This is actually how the roman saunas were built, but they were out of stone and concrete...
posted by bartonlong at 6:41 PM on July 14, 2013

jon1270 brings up a good point, since all that heat is rising it seems reasonable to say that it is actually escaping primarily though the roof.

And yeah, you are def going to need some sort of ventilation because of the moisture content.

TBH a second floor porch sauna really sounds sub-optimal, but I guess you go with what you have.

FWIW - Saunas run between 150C - 212F which in C terms is ~66 - 100
posted by edgeways at 7:07 PM on July 14, 2013

Reflectix over your existing floor with wood tiles over that. Check the seal on the door .
posted by hortense at 7:09 PM on July 14, 2013

The Reflectix sounds good to me. I've previously insulated the floor of a sauna with a similar product (different brand, as I'm not in the US). My sauna was on a tiled floor in an unheated space.
The extra floor insulation allowed the temperature inside the sauna to get 10 degrees C higher. So we could get to that lovely, lovely 90C.

From my experience, you're on the right track.
posted by Too-Ticky at 11:10 PM on July 14, 2013

On second thought, I had foam board (of a flammable kind, yes) below the reflective sheet (similar to Reflextic) and then there was wood covering that. Actual wood, not particle board or anything like that.
posted by Too-Ticky at 11:14 PM on July 14, 2013

Sleep having enhanced my thinking a bit, it occurs to me that significant heat could only be lost through the flooring material if there was a significant differential between the temperatures above and below the floor. If the temperatures at floor level inside the sauna are near the ambient temperature below the floor, then conduction through the floor is not your problem and insulation won't fix it. It could be that hot air is escaping at or near the ceiling somewhere and drawing in cool outside air through leaks at floor level, but that's a different sort of problem.

For those who know more about saunas (Saunas run between 150C - 212F -- Wow!), how do they typically prevent the fact that hot air rises from developing a gradient like the OP is seeing? Do they have low ceilings? Circulating fans?
posted by jon1270 at 4:01 AM on July 15, 2013

They don't. A sauna typically has a low ceiling, yes. But that heat gradient is used: if you like it hotter, go sit or lie down on a higher bench. Some benches are just enough below the ceiling so you can sit or lie on them.

Maybe the OP just needs to install a higher bench?
posted by Too-Ticky at 4:55 AM on July 15, 2013

Response by poster: Thanks y'all, I appreciate the assistance.

To work m'way up through some of the comments: yes, I know it'll have something of a gradient, this is just far more extreme than any I've found at a professional sauna, so I wanted to check into it... and haven't really found anyone in Houston with experience working with home-installs.

Yup, they're supposed to be that hot - I prefer a dry sauna at 90+ (which is why I put it in my home... couldn't find anything at a gym/spa/etc locally, and this way I can enjoy it easily with friends... and second floor is where we had room.)

I will try the reflectix with wood tile, and check out the temperature loss from the ceiling (Makes sense, I just figured "no no, the top's the only part that's the right temperature!"), and if that doesn't work, start looking into cracking the floor for spray or sheet insulation. Thanks!
posted by Seeba at 7:32 AM on July 15, 2013

Another thing to check on before you spend more money is whether the heater is running continuously or cycling on and off. If the heating element is on all the time then either the heater is undersized or you've leaking enough heat / hot air that the heater can't keep up. If it's cycling frequently, consider whether the thermostat (or its remote sensor if it has one) is located in the wrong place.

Edit: what type of heater is it, anyhow?
posted by jon1270 at 8:42 AM on July 15, 2013

Response by poster: Good thing to check (yah, I actually hadn't cleaned it out after daily use for 8-10 months, so when I fished all the now mini-rocklets and vacuumed the two-inches of grit, I figured it'd solve the issue).

The heater is a Tylo Combi 8kW, so with a ca. 300 ft^3 space, it should be pretty sufficient.
posted by Seeba at 10:05 AM on July 15, 2013

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