net zero | passive house, maryland: foam-free, mineral wool underslab insulation installed

October 9, 2014

Two 5″ staggered seam layers of Roxul RockBoard80 have been laid as our under slab insulation. The total 10″ of insulation was decided on because it simplified foundation construction and ordering. Only 8″ was necessary per dynamic modeling.
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LESSON LEARNED:  Next time, we will use the higher 11 lb density Roxul ComfortBoard CIS under slab due to construction traffic issues, and lay boards across for walking paths.  It would be advisable to also immediately cover the mineral wool to protect it from getting wet.
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After all mineral wool was in place, our vapor barrier – Tu-Tuff by Sto-Cote – was installed.  Tu-Tuff claims to be an environmentally preferable product and is a cross-laminate, virgin polyolefin (rather than standard polyethelene).
LESSON LEARNED: Tu-Tuff is a 4 mil product, and we tried to tear and puncture it to no avail – it actually was tuff (sorry, Michael) and meets all the technical requirements.  Lesson(s) learned, 1) their proprietary tape is not up to snuff when it comes to sticking for air sealing purposes and we had to scramble to find tape to replace it – always have some Wigluv or Tescon Vana on hand!; 2) you can’t tape when surfaces are wet with dew; 3) Tu-Tuff being only 4 mil and thin did not stay smooth, but rather preferred to stay wrinkled.