Insulating stone foundation
Hello there, we have an older home with a stone foundation. We have had to remediate for mold twice and really want to avoid any further mold issues.
We have plastic down, two dehumidifiers and are considering insulating the stone walls, but the more research I do, the more confused I am. I don’t want to use foam insulation since that can trap mold since it doesn’t allow the stone/mortar to breath. I have found someone who will build a frame a few inches in from the foundation so the stone can still breath, but wouldn’t the wood frame then be likely to grow mold and possibly the wood rim joists? Would using steel for the frame be advisable? I am hoping someone can advise on what the current research says about insulating stone foundations.
Thank you!
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http://www.foam-tech.com/case_studies/basement.htm
Interesting, thanks for sharing this.
Looks like they didn't spray foam directly on the stone but rather on a membrane that the foam would stick to (looks like Grace ice & water shield)
The stone doesn't need to "breathe". Masonry walls don't need to dry. For a stone foundation, you usually no choice but closed cell spray foam due to the irregular surface that makes it difficult to insulate in any other way.
I would NOT frame out a wall here. All that does is create a moisture trap behind it that is pretty much guaranteed to grow mold.
Bill
Thanks for your reply Bill. I am not interested in using spray foam - not only am I concerned about air flow, but it is a health hazard, especially having two kids with asthma. I dislike that it can't easily be removed to inspect either. Honestly, I would rather have no insulation on these 100 year old walls than spray foam. If you look at the data coming out of the UK, they advise against spray foam and recommend using natural materials such as insulated lime or wood fiber which allows airflow between the insulating material and the stone. Since a huge percentage of their historic buildings are stone and they are concerned about preservation, I feel they can be a valuable resource, although I can't find many in the US who are familiar with the methods and products they use. I am iffy about the frame and appreciate your input about this, may just not insulate for the time being and focus on the attic.
The issues regarding spray foam in the US are not the same as here, there were some specific issues they had related to attics if I remember correctly, which is where the issues with mortgages came up. The UK is generally behind the US in terms of building science, which has been pointed out here on GBA in the past. You DO NOT need airflow to the stone. You'll have more problems with an air permeable insulating material in an application like this (or with concrete or block foundation walls, the air and vapor permeance issues are the same with those too). Using natural fiber materials in a high humidity enviornment provides mold both a food source (the natural fiber material), and the high moisture that it needs to grow. With air circulation, the mold spores can not circulate back into your home which is where the mold/health issues come from. With an impermeable material, you encapsulate any mold so that it can't contaminate your inside air.
Spray foam is not a health hazard IF it has cured properly. The issue is when inexperienced crews do a poor insulating job, and that's where problems arise. This is an issue that I think is overemphasized in many online forums, but it is a possibility. I'll just say that in my commercial work and hundreds of projects, I have yet to see any issues, but I always use a very experienced installer.
Leaving the wall uninsuled obviously doesn't introduce any new issues, but you do still have the moisture issue. Note that you could insulate the rim joists independently from the walls and gain some advantage. I would use canned foam (note that one-part canned foam is NOT the same as two-part "spray foam" and has none of the concerns the two-part materials do) to foam in blocks of EPS against the rim joist. This is often the safest way to go, and avoids the use of spray foam, but it is more labor than spray foam. You do limit the drying ability of the rim joist a bit doing this though, which usually isn't a problem but it is something to be aware of. Stone is less likely to wick moisture compared to concrete or block, but it's still beneficial to have a capilary break under the mudsill. This can be retrofitted with HDPE sheet, but it's an extra step.
Bill
BTW Bill
The link that I provided was a job for the building scientist Joe Lstibrek that he would obviously write the specifications for ...
https://buildingscience.com/documents/insights/bsi-041-rubble-foundations
Here's the BSC project that I believe you are referencing. Pretty great project writeup.
replying to #9 Paul
Yes & figure #3 in that write up shows spraying foam directly onto stone wall ....
https://buildingscience.com/documents/insights/bsi-041-rubble-foundations
Gonna be honest, this looks very complicated and I seriously doubt I could find someone to do this kind of work on our home for less than 20K. The crawlspace is just so low - it's a bit of a nightmare down there. I do think it is interesting how the article mentioned that they made a single zone and connected the short crawlspace to the basement by removing the short demising wall. Our crawlspace has several walls from additions throughout the years and it is very difficult for the dehumidifier to reach all the spaces. We were planning on totally sealing off one of the 'rooms' in the crawlspace and installing a small dehumidifier in there, but I wonder if we should create some openings so we don't have to use 2 dehumidifiers? Our home's footprint is 2100 sq ft.
I like the idea of insulating the rim joist. Would using a material like rockwool board be more likely to allow airflow for the rim joists to dry?
I agree with Bill that spray foam is often the best/lowest cost method for old stone walls, and that the stone and mortar do not need to "breathe." But since you are against spray foam in general, there are some other appoaches that can work.
1. If you can install interior footing drains, then a membrane can work on the walls. Generally, you cut the floor slab at the base of the walls and install stone and drainage system. Note that with stone walls there may be no footing, and the trench must not go deeper than the stone. This might make this approach a non-starter. In any case, the walls are covered with a rugged rubber membrane - generally EPDM or TPO. The membrane is fastened to the top of thte stone walls without overlapping the sills, and the bottom of the membrane extends to the stone and drains below. The trench is backfilled with stone and the slab repaired with concrete. In this approach, any moisture that inevitably seeps through the stone walls is directed down to the drains. Insulation can then be installed on top of the membrane, but it is generally not flat enough for rigid insulation and I agree with Bill that any sort of fibrous insulation is asking for trouble.
2. A second approach is that you can parge the walls with stucco/plaster (aka Render in the UK). For old stone walls, you would use type "L" mortar or softer for the parging. If the wall is generally flat enough on either the inside or outside, it can be plastered flat so that rigid foam can be installed. This approach works equally well on the interior or exterior, though doing it inside avoids the work of digging outside. I would treat the surface of the plaster with a waterproof coating (Drylock, Thoroseal, or others). Then rigid foam can be mechanically attached. You can then cut and cobble rigid foam in the band joist areas and foam in place, or spray foam just the band joist areas.
Thanks for your thorough reply. I like the plaster idea, but the crawlspace is only 2' so it would be very labor intensive for our home. I will do some further research and maybe we could just do the dampest/coldest part of the home.
Generally historic masonry experts advise against drylock or thoroseal as a waterproof coating because it can lead to deterioration of stone during freeze/thaw cycles.
The suggestion of render with Hydraulic lime [type L mortar as you mention] is a good one.
Finding a good restoration mason who will repoint the exterior with an appropriate restoration mortar [with lime] will go a long way to preventing bulk water infiltration through the stonework joints.
Exterior pointing with remedial metal flashing at the top will create a water control layer for rain and snow [i am discounting water table issues]. Air control layer can be remediated as much as possible, though for an old house, it would be a time consuming and intrusive mess to address. If poster is not in favor of exterior insulation, then i'd consider foregoing insulation since it will accelerate stone deterioration and make future repairs to the interior mortar impossible.
Repair and conservation of old masonry is a deep subject. Hydraulic Lime is a good google search term. International Masonry Institute and NPS preservation briefs are good places to start research.
I'd approach the reason WHY there is so much moisture permeating through the wall (and floor?) in the first place. Foundations of any kind should be as dry as you can make them. Proper grading of the exterior area around the house is key, but most important are effective gutters and down spout management. Get all the water away from the house, utilizing french drains if necessary. Spend the time and money outside first, then see what happens.
Nick, That's great advice and should always be the very first thing people do to control foundation moisture issues. In my experience probably north of 90% of bulk water issues are related to poor external water management. That said, old stone foundations will always be problematic. Unless they have been recently treated, they generally have no damp-proofing or other exterior moisture management, and direct contact with damp soil will allow the soft mortar joints to wick water through to the inside where it evaporates from the surface. This is a constant process that causes damage to the mortar joints over time as well as causing the basement to be constantly damp. Running dehumidifiers can address the high moisture in the air, but increase damage to the mortar and cost $$ and energy to use. Once bulk water is properly handled outside, you still need some sort of moisture management in the wall to address the wicking/evaporation issues.
When you say we need moisture management in the walls, are you talking about the render you spoke of before or is there something else we could do? Thank you!
Right. If you address the outside water intrusion and you still have moisture issues, you've got bigger fish to fry, like a high water table or constantly wet soil. That's a much tougher issue to address after construction is complete.
High water tables are almost impossible to solve without mechanical removal. I've avoided them with a vengeance, and pity those that have to deal with them.
If you read the write-up on Joe Lstibrek's website that are linked here in the thread, he uses a "bathtub" metaphor where he has impermeable materials on the floor (foamboard) & walls (spray foam) to keep all bulk moisture out ...
After our last remediation, we did a more thorough encapsulation and installed an extra dehumidifier and it is definitely dryer, but still has a dank smell at times. It definitely has an odor when you go down into the basement although most homes here in Western North Carolina do as well. The previous owner did extensive work on the exterior of the home to mitigate water intrusion. She installed rock around the entire perimeter of the home and installed french drains. We rarely have water in the basement/crawlspace, even after Hurricane Helene hit but in the summer it does stay humid even with the dehumidifiers. We do need to fill some cracks in the mortar that have formed and also create a better seal around the vents. I think we will see if taking those steps and insulating the rim joists helps the humidity levels down there.