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Poor Man’s Thermomass

ethant | Posted in Green Building Techniques on

The previous conversation got off on a tangent about material costs and PWF… There was not much uptake on my “Poor Man’s Thermomass” idea:

Since drawing that detail, I have thought about replacing the EPS with rockwool, and have done some research on what the backerboard could be. I understand it can’t be Hardibacker or Durock. Perhaps there is another subgrade cementitous board which would be applicable?

I see Finex are OK for ground contact ( I also looked into T-Clear Wallguard ( which is a cement board fused to insulation. They told be that the rockwool version is not suitable to below grade applications, but I wonder if this is in line with the new thinking of below grade rockwool…

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  1. onslow | | #1


    Missed your first posting about poor mans thermomass, but I can see from the image on the link that you are very close to exactly what I have done on my own home. Rather than make a too long post, I would be glad to exchange notes directly if that is of interest. I think if you ask the moderator, he will send you my email. I am a bit hesitant to post it here what with the frequent invasion of sneaker offers and the like.

    A real short version of what I have is 3" XPS salvaged foam shot to the foundation with special ramset anchors, complete incapsulation with Grace bitumen based sheet, and 1/2" hardi board skirting over that to combat rodents and sunlight. I don't believe it is relevant to go with a hard surface over the entire foam sheet all the way below grade. For what it is worth, I think the Thermomass idea has a few critical failings in the way walls must be cast. There is more thermal bridging occurring than first meets the eye.

    I am in a 6500-7000HDD climate and my basement stays at 58 with no heat, a condition I will be taking care of now that I have time to be down there. Approximately 20 percent of my wall area is above grade and sees temps as low as 15 below. (not often though) Ground water is a major concern and I can detail all the efforts I took to control that problem.

    Hope your still in planning phase as some choices will demand finding a contractor willing to do what will seem odd to normal practice.

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