Passive Solar Sauna - Glass Selection

Hello All. I am thinking about building a solar sauna and although I think I have a general understanding of the key principles I do have a question about glass. Clear glass would let in the most energy, but privacy is an issue. So I am considering the textured glass option. Ripples, rain tracks, wavy glass, etc. It would remain essentially clear (not frosted of coated/covered with anything). So my question is would there be significant energy loss: clear-but-textured compared to smooth clear glass? Hope someone out there has come upon and mastered this issue in the past. Thanks.

Hi Tom, welcome to permies!

I have no hard data, but observation of the quite limited shading produced by this sort of glass in various decorative applications leads me to believe the losses vs clear glass would be pretty minimal.

Other considerations that come to mind; finding this stuff cheap, especially in anything but single-pane shower-door sort of sizes, might be hard. Do you need/want double-pane glass for insulation purposes?

Have you considered clear glass with some obscuring media behind it?

Have you considered going with clear class and closeable curtains?

I'm not terribly familiar with the concept of a Solar Sauna, but does it run on active solar rays or thermal mass that absorbed solar energy?

If it's the latter, you could use clear glass to charge it and just close the privacy cover for use.

Clear patio door glass is often free on the used market. Failed seals on double pane windows, can be cut, to produce 2 single panes. It's tempered. A light coating of etching compound, can make it frosted. Much better than those plastic sticky coatings.

Hi All, thanks for the comments. I am looking at shower door panels at Home Depot, etc. (so they will all match) they are pricy. Probably $300 for 4 panels to cover 9 of the front wall 10 ft. Will try a salvage building material store first to see if there is anything useable there. Hope so, so I can afford the pricy wood I will need for the inside (seating mostly). To answer one of the questions: The clear glass is the collection surface and the masonry back and side walls are the storage features. A paver floor should hold some heat also. I am thinking I might need blinds or curtains on the inside to control heat to some degree and could well use them for privacy but the interior will be pretty small so billowing curtains or clunky blinds might be a turn off.

I am thinking about supplemental heat from a passive thermosiphon system mounted directly beside the sauna. There is a convenient and sunny spot there. Does anyone know if such a system can heat-store-circulate hot water beyond the traditional collection-store unit. If able I would like to circulate (passively/no pumps) the water from the storage tank, through the sauna which is lower than either the solar collection box or storage tank, So don't quite know if that is an impossibility or not. Not sure if the passive convection/circulation can overcome a 6ft drop (to the sauna floor) and return to the collection box. If necessary I could run any piping up high and minimize any height differentials. Any insights would be warmly appreciated.

Hello Tom,

[begin of useless explanation]
the amount of light transmission depends on reflection on the surface and absorption. The latter can be ignored unless the glass is colored or very thick.
The amount of reflection depends on the angle in which the light shines on the surface. Perpendicular rays have the smallest reflection.
A modulated surface will increase the overall angle and increase the loss (not too much).
[end of useless explaination]

I would also suggest to use plain glass and put a dark surface behind it (black paper) with enough space for air to circulate.

The thermosiphon requires the source below the sink (unless you build a fancy system that acts as a pump).
You could use a small solar panel and connect it to a DC pump in the same voltage range.

What really matters is the energy balance:
How much radiation do you get into the sauna and absorbed? (Maybe put mirrors at the sides?)
How much heat loss is there? (Heat loss is proportional to the temperature difference.)

For the privacy part: You could keep 0.2m to 0.5m of glass at the ceiling and the floor. It should provide more than enough light and doubles as a path for the heated air.

Heat storage: If the sauna has a high thermal mass, heating up will take longer. A sauna with low thermal mass will heat up faster and reach the required temperatures even when the sun does not shine all day long.

Before buying anything I would highly recommend to get a few numbers and calculate:
- the equilibrium temperature (heat input = heat loss)
- the rate of heating up (thermal mass / [heat input - heat loss] )

You don't need perfect numbers, just some estimates.