Peter,I replied off list to Rikkert and Joost in Dutch. But now realise seeing your remarks that I should have done so on-list.
I completely support you in your remark that avoiding moisture problems is the main issue in a SB sauna. This is in the case you insist on using SB for the building. My question though depends on whether the object of the exercise is to make a SB building or a sauna. I suppose in the case of Rikkert he wants to build a SB building and a sauna might be a good use for it. If this is the case using a radiant reflective surface on the inside as you and I both suggested is the way to go. My reason for not suggesting wood is that I expect it will require much more work then the (transparent corrugated plastic).
Coming back to the practicality of using SB for a sauna. These are my considerations. Basically a sauna should be easy to bring to a high temperature quickly and not have hot and cold spots which will feel uncomfortable. Specifically this means that the interior surfaces should not be much below air temperature when in use. A structure exhibiting good radiation insulation and a low thermal mass would be the most suitable for use as a sauna. This means that the general practice of a wooden, relatively lightly insulated, out building is not a bad idea for a sauna.
Although I must say that I like the challenge of making a workable SB sauna specifically for our cold and sometimes moist climate. As you say if we could make a workable SB sauna then surely we have proved a point.
Below is the essence of my earlier reply to Rikkert, Joost and additionally a reply to your mail.
Once you place the wooden interior boarding or other radiant heat reflector the mass of the plaster becomes irrelevant. The wooden shuttering will reduce the radiation losses to the plaster mass to such an extent that you will not notice its presence. I suggest you do use a full layer of plaster as it is a serious and important precaution in safeguarding the hay bales against moisture damage. The thicker plaster will be able to buffer the moisture and allow it to be released when ventilated.
Structurally it is also necessary for a LB SB wall to have both faces plastered otherwise the walls will be prone to bowing out to the side with thick plaster.
A curved wall will markedly improve the performance in this respect but is not a complete insurance. I have recently seen a small round LB SB building coated with earth plaster that was very lopsided and very close to being given up as a safe structure. Most probably the main reason for the instability of this building was the fact that the door opening had created a flexure point in the structure. ie the door frame should have been a sturdy box with braced corners thus completing the circle. This was not the case in this building.
At 07:36 AM 7/24/04, you wrote:
I do not quite agree with Rene. 1. thermal mass can be reduced by applying only a thin mud-layer by spraying a slurry. This could be covered with wood, the way, saunas usually look like. 2. principally the bigger problem is the one with the moisture of course. But you use a sauna only a few ours a day, then you can take care about good ventilation. Due to the moisture-problem, especially as it is run mostly during cold periods (large temperature gradient, a lot of moisture, thus the threat of condensation), building a SB sauna really is a demanding task. And if it works (and I think it should), it would be a impressive example to argument with, in discussions with critics.
Rene Dalmeijer