Replies

1. 
   paulmagnuscalabro | Feb 26, 2024 06:18pm | #1

   mikeolder,

   For your first question, Rockwool can get wet and stay wet (it's basically made of rocks). It is used as sub-grade exterior insulation with some regularity in my area (Montana), with no issues.
   For the second question, I'm not sure but my guess would be that the main benefit of including sub-slab insulation with a shallow frost-protected foundation is so that any heating/cooling in the living space above the slab is more likely to stay in the living space, rather than lose energy through an uninsulated slab.

1. 
   mikeolder | Feb 27, 2024 08:37am | #3

   Thanks Paul, but wouldn't the r value go down if the comfortboard was wet? It could also freeze..
   I talked to my local suppliers who said there is a comfortboard shortage and the prices were high and it needed shipped.. Sounds like I might be forced to use borate treated foam..

1. 
   paulmagnuscalabro | Feb 27, 2024 09:53am | #5

   I'm not sure about the R-value going down when wet, but I suspect if anything it's negligible.

   It can get wet and freeze, no issues. Rockwool rates it for use below grade, so you've got manufacturer instructions to fall back on. We use it on walls below grade on projects at elevation where temps routinely dip below -30°F, and so far it's got a good track record. You're right about the price point though - it is a bit harder to source and it's definitely spendier than some of the blue/pink foam board products.

2. Expert Member
   MALCOLM TAYLOR | Feb 26, 2024 06:48pm | #2

   mikeolder,

   There are FPSFs that rely on heat from the building above, and others that are used for unheated structures. That largely accounts for the differences you see in where they put the insulation.

1. 
   mikeolder | Feb 27, 2024 08:47am | #4

   Thanks Malcolm.

   Can I assume a unheated well insulated structure with un-insulated slab would remain warmer in the winter than the same unheated structure with insulation under the slab? I already know a un-insulated slab would remain cooler in the summer if you keep the doors shut.

3. 
   rkymtnoffgrd | Feb 28, 2024 09:42am | #6

   Not necessarily... If the unheated structure is built with minor passive solar elements, (correct glazing ratio/direction to sf), correctly insulated (only those seeking sub-slab insulation are), with a quality window package; the passive heat gain could will far exceed the geothermal, making the direct contact to the earth a net loss... Im on a FPSF with 4" of subslab and 16" - 24" of edge. My cabin is not heated (yet), but the envelope is tight, and most of the insulation is complete (100%interior, 75% exterior). I don't have electric or heat done yet as the build is on hold all winter... Last month (January), at 10K feet in the Colorado, (zone 7) it has not gone below 40 degrees and typically stays between 48 to 65 daily. Once the shell was tight, It took 4 weeks for the slab to slowly heat up from 31 degrees to 48, which is my average temp.
   Regarding buried mineral wool... Not sure I'd go there, I used mineral wool extensively, inside and out. Rockwool on the exterior of my house was exposed for 4 months before being covered. Know that does compress, holds significant amount of moisture, (if not allowed to dry), and I was alarmed at the disintegration and dirt embedded (at the bottom, from water splashing) by the time I got it covered. I don't think it has the structural integrity I would guess necessary to avoid compression over the years...

   
1. 
   user-723121 | Feb 28, 2024 10:55am | #7

   Cool build, what are some of the other building specifications if you don't mind. Have always thought CO sun and superinsulation would go a long way toward very low heating demand. 10,000 feet, what is your average annual heating degree days?

   Doug

1. 
   rkymtnoffgrd | Feb 28, 2024 12:17pm | #8

   Southern Colorado, 10Degrees east of south facing, (slight emphasis on morning heat) coincidently season norm is around 10k heating degree days. 2x6, 24" OC Advanced OVE framing, FPSF, with subslab air barrier taped to zip, taped and wrapped around top plates, taped to sub truss osb. 5.5" interior & 2" continuous exterior mineral wool, 4" subslab & 16"-24" edge type 2 eps in foundation. Vented attic with R-70 blown cellulous, drywall taped painted and sealed and caulked around outlets. triple pane (two glass, one film) windows. 7 envelope penetrations: all controlled, all horizontal through walls... (one plumbing vent, one Radon vent, one electric trunk, two HRV, one "custom" wood stove exhaust and one feed) with keen attention to sealing those 7 penetrations... Self Built... 2.5 years in... At this point only the exterior continuous front insulation is incomplete and slab is not stained slate black... But if feels good to walk in at 10PM after being gone for a month and see this temp with no human thermal inputs...