What are the risks involved in insulating the interior of a concrete foundation wall that has extensive efflorescence present?
I’m a residential energy advisor who is often put in the position of deciding to recommend whether or not to insulate the interior of foundation walls. My region has government rebates for certain energy upgrades, and one such rebate is available for insulating 100% of your home’s foundation walls. When I encounter extensive efflorescence, however, I generally recommend avoiding insulating on the interior until such time as the client can fix the problem (ie with better drainage/waterproofing membranes on the exterior, etc). Some clients still want to go ahead with interior insulation in order to get the energy savings and the rebate. That is their prerogative, but I’d like to know more behind the science of insulating the interior of a foundation walls, for example with closed cell spray foam, when there is extensive efflorescence present. I’ve read every article I can find on GBA and elsewhere on this topic, and while there is lots of good advice, I can’t find anything that directly explains what would be expected to occur if one did insulate over top of efflorescence. Will this encourage mould growth as some sources say? Or would it prevent/slow evaporation off the surface of the wall, thereby preventing mineral deposits from being left behind (thereby preventing the vicious cycle of salt buildup leading to increased osmotic pressure)? Here is what I told a recent client on this topic: ” Insulating over serious efflorescence is not recommended, as the salts will still build up over time behind the insulation, perhaps exacerbating the deterioration of the foundation while obscuring the cause of the deterioration.” Is there anything incorrect about this statement??
Thanks in advance, mad love for GBA, it’s an amazing resource
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Replies
Peter,
If you are talking about a concrete wall, a CMU wall, or a stone-and-mortar wall, I would advise that insulating with closed-cell spray foam is unlikely to cause problems due to salt accumulation. If you are talking about a brick wall, the situation is a little trickier, and has to be assessed on a case-by-case basis.
It is untrue that "the salts will still build up over time behind the insulation." The salts can only build up if there is evaporation from the surface of the concrete to the indoor air. Closed-cell spray foam will stop this evaporation, and will therefore stop the accumulation of salts.
Of course, if the wall is damp, interior insulation may increase the chance that the sill beam will stay moist. The best solution to this problem is to jack up the house 1/2 inch and insert a metal or rubber capillary break between the top of the foundation wall and the sill beam. The risk of sill beam rot has to be evaluated on a case-by-case basis; if the sill is well above grade, and gets sun on the exterior of the house, it will stay dryer than a sill beam that is close to grade and hidden by a bush on the exterior.
I have a client with a huge amount of efflorescence on the inside of their foundation wall, almost like crystalline white fur. I did a bit of research and it sounds like over the very long term, in *some* cases, the structures formed by salts within the concrete can damage it. It seems like you would be slowing the movement of moisture through the wall by placing a vapor barrier (spray or rigid foam) on the inside and might actually reduce the amount of crystal forming within the concrete. I did not find it easy to get conclusive information off the web on exactly what the risks of efflorescence are, perhaps you can do better.
I definitely agree with Martin that the sill plate is a big potential problem. My customer's wall has been so wet that the sills are damp, many years after construction, so we are working on the drainage problems as a first priority. You should certainly be educating customers about drainage and what the solutions are.
If you're going to spray, will you first clean the surface of the wall, and if so, how?
(just to be clear, that last question assumes no capillary break is being installed below the sills)
Hi Martin,
thanks for setting me straight on the salt buildup issue. 98% of the foundations that I encounter are of the first three types, with very very few brick foundation walls in my area, although there are some.
Unfortunately, very few of my clients can afford to jack up a house and install a capillary break under the sill, although I will make sure to recommend this where appropriate. So, it seems to then come down to more of a subjective assessment of the moisture content of the concrete walls, since even if I measured the moisture content, I've never seen any guidelines that correlate foundation wall moisture content with this specific problem of the sill's drying potential when the walls below are insulated and with a vapour retarder and/or barrier. Add to that the shading on the exterior, solar orientation, above grade area... yikes, there's lots of different forces at work here.
One final follow up question (I hope): In cases where there is some concern about the sills taking on more water from the concrete than they can release to the air, does it make sense to insulate the walls with closed cell foam, but avoid insulating the rim joists with the same? IE, is increased drying potential of the sills/joists to the interior in a situation like this a good thing?
I'm guessing that I do want to recommend insulating the sills, as most moisture absorption in the winter will be from humid indoor air, and insulating the rim joists will prevent this....
David: based on what Martin has told me, I think simply a good brushing of the wall to remove any loose material should be sufficient. But I'll await correction, if required....
Peter,
I hate to say it, but the answer to your question is, "This issue calls for judgment." There is no clear answer. There is just a sliding scale of risk.
Bad drainage, high grade, too much shading, and evidence of a damp basement or crawl space -- all are bad indicators that warn builders to be cautious.
Good drainage, low grade (that is, a good distance from grade to wood components), sunshine on the exterior, and a dry basement or crawl space -- all or good indicators that give a builder more confidence that interior insulation is possible.
Hahaha, well here's hoping I have that requisite "good judgement." This discussion has been incredibly helpful. Thanks again!
I would be more comfortable installing rigid foam with mechanical fasteners and strapping than I would spray foam. If there's going to be continued hydrostatic pressure against the outside of the wall, how long will spray foam stay in place?