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Community and Q&A

Basement insulation – dimple mat and fieldstone

bucksbear | Posted in Energy Efficiency and Durability on

I have a question about insulating a basement in our late 19th century house in Climate Zone 5 (Western MA). We recently finished a small addition (now 2400 s.f. total) and switched the entire house’s HVAC to a ground-source heat pump and the domestic hot water to a heat pump water heater (HPWH) located in the basement. The addition has good insulation (R49 in the roof, R23 or so in the walls) and the rest of the house has average insulation (R49 on the attic floor and perhaps R15 or so in the walls). 


We are deciding whether and how to insulate the basement, which will remain unfinished. The reasons for insulating are to keep the first floor warmer, improve the efficiency of the HPWH, slightly improve the performance of the (already insulated) air ducts that run across the basement ceiling, and perhaps reduce the need for basement dehumidification in the summer. The basement has three rooms – one with mostly fieldstone walls, one with brick walls, and one with cement block walls. As I understand it, a normal approach here would be spray foam everything or at minimum spray foam at least the fieldstone portion while using rigid insulation elsewhere (our insulation contractor recommended Thermax) to R13 or R20 (2-3 inches). There are two complications with these plans – (1) we would like to avoid spray foam if at all possible (we have very young children and my partner is an environmental engineer who will be hard to convince otherwise) and (2) we get a small amount of water on very rainy days (though we have internal perimeter drains and a sump pump that is functional but honestly does not seem to get used very often). We did external grading and gutter work already. I have read that you can use dimple mat on the interior walls of a basement (exterior is not possible for us), and that you can attach rigid insulation to that dimple mat, and end the insulation a few inches above the perimeter drains.


I have three specific questions:


– Will this dimple mat and rigid insulation plan work over the fieldstone? Should I be concerned about the uneven surface of the fieldstone creating air pockets behind the dimple mat?

– Does anyone have recommendations on dimple mat products for basement interior applications?

– Will I see much difference between R13 and R20 on the basement walls? Note that electricity is now 34 cents a kilowatt-hour here, so it is more expensive than in most places.


Thanks to the GBA community as always. So much great thinking and guidance here.

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  1. Expert Member
    BILL WICHERS | | #1

    I think closed cell spray foam is probably your best option for that fieldstone wall, but it may be possible to build a sort of airgapped wall with treated furring strips and then put polyiso over that, to allow for a drainage plane behind the polyiso. The problem here is that the surface of a fieldstone wall is almost certain to be far too uneven for dimplemat to work. If you air gap the wall, I would try to keep the air gap small, 1/2" or so, just enough to go even over the majority of the wall surface. Seal the top edge and sides of the gap with canned foam (I'd use Loctite's TiteFoam here, not regular Great Stuff canned foam), then stop the polyiso 1/2" to 1" or so above the floor slab and seal that edge with foil tape to try to protect it from any moisture down there. Spray foam would be easier, and it's very stable once cured. I would recommend staying out of the house for at least 2-3 days or so after installation though, and leave some windows open during that time to air things out. You really don't want to be breathing the fumes that come off of a fresh install, but once those dissipate, the final product is pretty safe and stable over the long term. You get a fully adhered insulating layer that won't care about the irregular surface, and will help to seal the wall from moisture ingress.

    The block and brick walls are easier. I'd use dimple mat here, and then the polyiso (I've used EPS too, but that requires a thermal barrier (usually drywall) on the interior side. I have used DMX 1-step as a sort of extra thin dimple mat for this applicaiton before with good results. That product is intended as a floor underlayment, but it's readily available (the orange store even has it on sale right now), and all it really is is a polyethylene dimple mat so it's works fine to create a drainage plane behind some rigid foam on a basement wall. Put that up, tape the seams, then insulate over that. Wrap the lower edge of the stuff under the lower edge of the foam so that any water that gets in can drain out, or tie it into a perimeter drain system if you have one. If you're concerned about possible mold growth in there, paint the wall with a mold killing primer first and seal the top and sides of the dimple mat, leaving only the bottom open for drainage.


  2. richmass62 | | #2

    We took a different approach and used a product now called BASF masterseal on the fieldstone walls. It stopped the seepage through the wet portion of the wall and we used the white colored version to brighten up the space. We did not insulate the walls. Just the ceiling. To deal with the coldness in the basement we had 4 strategies:

    1) fix the joints in the fieldstone by having masons come in and repoint the wall, squirt in mortar mix, and in one case actually increase the thickness of the wall because it was a tiny section with inadequate support, so we went from a 5 inch wall to a 13 inch wall using concrete.

    2) when we excavated around the house we buried sheets of 3" xps foam to insulate the foundation and some surrounding dirt (did not do much because we did only 50% so far)

    3) big difference: we repaired holes in the sill and then spray foamed the interior of the sill and rim joists above. Even more recently, we have added foam board to the house, 1 to 3 inches thick, that almost completely covers the sill from the outside.

    4) changd the grade outside to bury more of the basement underground (still working on this part)

    Temps rose from about 34 degrees to about 48 degrees after all of this work.
    I suspect we will get the basement temp up to 51 F when we are all done..

    This is a more reasonable temp and allows us to run a heat pump water heater at the basement level for dehumidification. If you look at my profile you will see most of this process documented.

    I am in a similar climate zone, maybe a bit warmer as I am north of Lowell MA.

  3. Expert Member
    DCcontrarian | | #3

    Note that code now requires that basements be insulated. For some reason in the 20th century it was widely believed that only areas that were actively heated needed insulation, the reality is that you need a continuous layer of insulation on all exterior surfaces of your house.

    I wouldn't recommend trying to seal out water, it might work but there's a lot that can go wrong. First, fieldstone is so irregular that it's almost impossible to seal completely. Second, unless there is a drainage path on the outside of the basement wall, sealing it just means the water accumulates. This can lead to hydrostatic pressure which will peel off the sealing, loosen the mortar and even move the stones.

    Bill has a good summary of the options. Either spray foam or frame a wall inboard of the stone wall.

    I believe R-15 is code minimum for basement walls where you are.

    1. richmass62 | | #4

      The basement floor is about 5 feet above the street and we installed a gravity french drain inside to catch the water coming from the hill in the back. The product we used is not drylock, it is a masonry product that becomes part of the wall and can absorb and release moisture. The seepage of water was also reduced when we dug up the 1930 "slab" and re-poured it with a drainage layer of crushed stone underneath.

      I think the idea of putting a wall in a basement with a seasonally damp stone wall is absurd. Even spray foam would be ill advised in our case. Would putting spray foam over the wall prevent the mortar mix from degrading over 20 years due to the potential efflorescence? I doubt it.

      To make spray foam work you are looking at about $10,000 to spray foam the entire basement wall and then you would need to spend another $7000 to waterproof the exterior of the foundation on just the upslope side where it is wet. Otherwise you would be forced to rip out the spray foam every 20 to 30 years to repoint the wall. Also the environment behind spray foam that is laid onto a stone wall is very moists and I presume that insects will like it very much. So this is a case where I hope the code includes exceptions. Insulation inside the basement is not the only way to keep down the energy cost.

      Also the heat pump water heater is stealing heat from 50 degree stone walls outside the conditioned envelope, which is better energy wise than stealing it from the envelope where you are running your heating unit.

      New construction yes you can do the interior wall but the prior occupants of my house had walls installed but it was apparently a temporary thing. The walls were not going to last 20 years in that moist environment.

      So for us we put flash and batt, 2" of spray foam plus 3.5" of rock wool on the ceiling of the basement, with fire resistant poly covering the rock wool. So it is around R25, except in the area below the kitchen pantry where we deliberately omitted the rock wool to keep the food storage area naturally cool.

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