Here’s a discussion feed from one of our best-selling courses, Zero Net Energy Homes. In the discussion, one of our students asks Marc Rosenbaum for foam-free ways to retrofit her New England basement.

Here are the alternatives to spray foam discussed below:

  • Expanded cork board insulation
  • Rigid rock wool
  • Mineral wool batt to conform roughly to the rubble wall (with drainage at the bottom) and an installed air impermeable and vapor resistant membrane
  • Drink a LOT of (organic) wine, save the corks, and glue them with a zero VOC adhesive to the rubble, then cover with poly


What are your thoughts about ways to replace spray foam and the chemicals in the built environment? Weigh in with your knowledge by leaving a comment at the end of the article.

Let the discussion commence: 

Student A: Marc, this has been on my mind. I’d appreciate your thoughts and the thoughts of the class [and the HeatSpring community].

I am concerned about the chemicals found in the built environment, including chemical flame retardants (CFRs) in spray foam. CFRs are known to cause human reproductive issues and are potential carcinogens. They are persistent organic pollutants that are bioaccumulative, ending up in our food supply and in our bodies.

I’d like to retrofit my (and everyone else’s) old, musty, typical New England basement with field stone foundation. Do you know of any foam-free way to do this?

It costs a couple thousand dollars to remove the carcinogenic, friable asbestos on the pipes in my basement. I struggle with the thought of doing that and then paying to add a different danger to human health to the same space.

I hope others will consider this, too We need to work together as an industry to develop effective building products that do not make us sick.

Student B (response): 

Expanded cork board insulation might be an option. But it is expensive and has to ship across the Atlantic. The product is air tight so it may do better on the exterior of the stone.

Student A: 

I love the idea of cork! I don’t know the price difference between cork and rockwool, but I have a suspicion rigid rock wool would be substantially less expensive. If air and vapor movement is controlled by a weather resistive barrier on the interior face of the foundation, and the band joist is sealed with just enough spray foam, the air tightness of cork might not be necessary.

Marc Rosenbaum (Instructor, Zero Net Energy Homes): 

Good for you to be wrestling with this, it’s a tough one indeed. I’d rather see the interior air barrier be over the mineral wool rather than under it. The mineral wool has no resistance to vapor diffusion and not a lot to air movement, so interior vapor could pass through and condense on the peel and stick membrane (which itself is a stew of chemicals). If you were able to use a mineral wool batt to conform roughly to the rubble wall, with drainage at the bottom, and then install a air impermeable and vapor resistant membrane over it, that would be reasonably robust.

Thinking a bit about the cork – my sense is that rubble stone foundations reduce our options because they aren’t flat. That’s one reason a spray foam works so well.

Student A: Could installing the rock wool directly against the potentially moist foundation wall cause musty odors? That was one of my reasons for putting the chemical stew between the foundation wall and the insulation.

It was my thought also that a continuous air/vapor barrier between the insulation and the fieldstone foundation would allow the air permeable rock wool to still provide insulation value by reducing air movement through the field stones.

Finally, do you think it is crucial to jack up the house and slide the capillary break between the sill and the foundation wall?

Marc Rosenbaum: I think (and the operative word is think, not “I know”) that the mineral wool is about the most inert insulation you could put there, and I am more concerned about condensation against the membrane which becomes colder with the insulation on the interior rather than behind it. My sense is that this could be a pretty low tech, low investment approach and not at all irreversible – you could try it and see what happens. Hardest thing is to figure how to lightly pin the materials against the rubble. It would be interesting to see if the Hilti IDP fasteners could be made to work.

On the cap break – all these deterioration questions, beyond completely adverse things like continuous water leaks, are rate problems – how fast and how much is a material wetted, and how well and how fast can it dry. So the first thing is to keep the masonry below the sills from getting wetted, for instance by backsplash – use a gutter.

My sense, if you’re willing to watch things, is that you have to jack the house up to fix it if it is a problem, but you don’t if it isn’t a problem. So I might go light on the insulation and focus on air sealing at the rim, with the idea that you’re still trying to provide some drying heat in that location and taking the energy penalty.

My suggestion on the cork is, drink a LOT of wine, save the corks, and glue them with a zero VOC adhesive to the rubble, then cover with poly.

Student C: I’m not a builder, so I cannot advise on this (except that I suggest you drink organic wine!), but agree with you 100% that we need to be much more aware of the chemicals we use/put into our buildings and the environment in general.

Marc Rosenbaum: OK, time to start a new thread on recommended organic wine! Maybe we should all start sending her our corks.

Student A: Thanks for all the feedback! I hear from a colleague that when Roxul boards get wet and then are left in the sun they ‘smell like pigeon poop’. He suggested leaving the vapor control behind the Roxul boards and using a dehumidifier in the winter to manage condensation. Maybe I will do one crawlspace one way and another crawlspace another way and monitor them.

Is there any insulation strategy that uses beer bottle caps? We’re better at collecting those than corks. 🙂

Marc Rosenbaum: I don’t like your colleague’s approach, although it is the standard US one – use energy in place of envelope 🙂 Get some Roxul and try it yourself and report back please.