I need your help to straighten out my thoughts about insulation
. I direct my question especially to them of you that lives in your Yurts in cold climates where it usually get down to -20c in the winters but on cold occasions can get down to -30c and in rare and extreme cases down below -40 (once or twice a century or so)... What insulation
do you guys use? If wool felt then what density and thickness do you use?
I saw one company saying that their felt of 6 mm thickness could with stand down to -5c and their premium felt was 16mm thick. When I have read about u-value it said that if you double the material then you double the insulation value. With that logic then 60mm of the 6mm felt mentioned would withstand -50c - but it is really that simple?
In a research paper on sheep-wool I found they state that 40 mm felt with a density of 40 kg/m³ would have a thermal conductivity of 0.034 at +10c (which is the standard temperature of measurement). Mineral-wool with standard density of 25kg/m³ and 200mm have thermal conductivity of between 0.036-0.040 (the lower the better, still air has 0.019 if I remember correct). The research paper also stated that wool of 80 mm thickness but 20 kg/m³ held a thermal conductivity of 0.040.
We are building our Yurt in the north of Sweden (climate similar to Canada and Alaska but perhaps not as extreme and not as wet as the north west US or the UK... Temperatures seems to be a bit similar to southern/middle Minnesota during winter, not sure on snow load and damp etc though). Our plan is to put our wool felt insulation between two sheets of Bison Weatherproof Sheets (that let out the moister to a very high degree but keeps water and wind away), creating kind of a "pocket" or "duvet" closed by strong Velcro, keeping the wool dry but still let it breath but keeping the wind out. Outside of this we have a bubble wrap double coated with aluminium as a heat barrier. But we have a hard time figuring out the density and thickness of the wool felt... We are going with a boiler stove and a water floor heating
system also so we will not only have a regular wooden stove, for a more "smooth" and continues heating
The building standard here recommend 200mm mineral-wool in the walls and 400 mm in roof and floor. So that must manage the temperatures down to -50c (as i can be in the wind in the extreme cases). The question that pops in to mind is what density and thickness of sheep wool felt would give the same insulation value? And how do we calculate it?
If one only see to the mass of the material (25kg/m³ of 200mm thickness weight 5kg) then a wool felt that is 35,5 kg/m³ (that is the same as a 20 mm thick wool felt that weight 710 g/m²) must be 155 mm thick to have the same mass. If we check how much density we must have if the thickness is 60mm to get the same mass then it is 83kg/m³. If we use 100 mm thickness instead then we need to have a felt with density of 50kg/m³ to get the same mass. To give it a comparison; the insulation used before mineral wool hit the insulation market like a bombshell in the 1950s was saw dust, wooden chips and splinters/fibres, often mixed with strips of newspaper, textiles and other fibres such as hair/fur/wool. That mix often held a density between 45-60kg/m³ (the older the more dense it became).
The upside of a high density insulation is that it in prevent natural convection (airflow in the insulation) giving it much better insulation capacity. But what is the downside? The only thing I can think about is that it will not hold as much hot air and hence have lot less insulation capacity... But is that really the case?
Research paper mention above show a relationship that if you have have a high density insulation then you can maintain the same thermal conductivity with a thinner material. With both higher density and higher thickness then the thermal conductivity should decrease (which is what we want). With that relationship in mind it should be the mass of the insulation that matters, not the density and not the thickness. But in another place (information about mineral wool) they said that sure you could compress the insulation and put in more material (to increase the mass and density) to get a lower thermal conductivity in theory but in practice it is the thickness that is important... That got me second guessing everything I though I understood...
So what is you practical experience of this?
Any tip to straighten out those thoughts will be gratefully received.