Hey folks, I've read in numerous places (including this site) that you can spray closed cell spray foam insulation over field stone basement walls to insulate them. Then you can put walls up with metal studs to finish your basement. Few quick questions though: 1) Normally you have to repoint the walls every 25 or so years. Would this still be true or no longer necessary? 2) Normally, humidity goes through the mortar between the stones and into the basement. If the humidity can't get through the foam though, is it trapped in the walls? If so, will this cause foundation issues? (currently we run a dehumidifier 8 hours a day). 3) My walls currently need to be repointed. If I did this would I have to repoint it first, or would spraying it, make it unnecessary even now? 4) Any other issues I should consider? Thanks! This is for a 1910 New England Colonial in MA.

It kind of depends on the particulars- in some instances it can work, in others it's a dubious proposition.

If the basement truly need re-pointing every 25 years (as opposed to every 125 years) there may be too much moisture migration, and if the foundation isn't allowed to dry toward the interior it could lead to rot at the foundation sill. This can be mitigated greatly by putting EPDM membrane between the top of the foundation and foundation sill, which involves jacking up the house a section at a time to be able to slip it in. (Sounds like a worse job than it really is, at least in most cases.) It may be necessary in some instances to dig down from the outside and apply a waterproofing material to the exterior of the foundation to get away with out a membrane capillary break to protect the foundation sill.

Closed cell spray foam has a permeance on the order of 1.2 perms @ 1inch (usually a little higher, but not much), so by the time you're at 2" (R12-R13) the ability of the foundation to dry toward the interior is starting to get severely pinched. Open cell foam has much higher vapor permeance, but it also susceptible to retaining moisture over time.

First things first: The biggest moisture threats to the structure is from bulk water both on the outside, and from the footing wicking up through the foundation. A perimeter drain on the interior draining to a sump that gets pumped in the event of high-water table or flood is a first-line of protection, as are exterior grading, roof overhangs, gutters, and possibly French drains, etc.

You may have to consult a mason to come up with the right mix, but a sacrificial parge over the interior before spraying would give you 100+ year protection on the pointing.

Using a high-perm/lower-R closed cell foam such as Icycene MD-R-200 allows you to hit R15 (3" depth) and still have over 1 perm (1.33 perms, according to the spec) of drying capacity- more than typical closed cell foams at 1" (R6-7). This is plenty of moisture retardency for protecting the foundation sill from wintertime moisture drives from the interior, and if the bulk moisture issues are under control, plenty of drying capacity toward the interior, but not such a high rate that the mortar loses structure quickly (especially if it can be further protected with a parge.)

The floor needs a 6-10mil poly vapor retarder continuous with that wall -foam, and R8-R10 of EPS or XPS is recommended for a MA climate under the slab. Putting R10 of XPS at the slab edge between the foundation and slab provides a capillary break to keep ground moisture from wicking into the slab from the edges, but also a thermal break. Study how they did the foundation & foundation sill in this Belchertown foundation:

https://www.powerofaction.com/media/pdf/DER_CaseStudy_Belchertown_MA.pdf

And this one in Belmont:

https://www.powerofaction.com/media/pdf/DER_CaseStudy_Belmont_MA.pdf

There are other approaches that can work in some instances. Here they did it with channeled XPS to create a micro-cavity for the foundation wall to dry into, with rigid foil faced iso on the interior (see the discussion & photos starting on p.40)

http://www.nyserda.ny.gov/en/Page-Sections/Research-and-Development/~/media/Files/EIBD/Residential%20Buildings/der-contractor-training.ashx

I'm not as convinced of the long-term prospects for doing it this way without a sacrificial parge, but it's not bad.

If the humidifier runs 8 hours/day even in winter you may have a very high water table, and the ground vapor retarder & wall-foam should reduce that by LOT. If it's only in the summer, your problem is most likely to be outdoor air infiltration.

Mind you, in all of these cases they were going for a fairly high-R. In MA the long term cost/benefit of a high-R all-foam wall solution isn't necessarily all that great once you're over R15, but sometimes it can be if using a less expensive approach than an all closed-cell foam method. (I retrofitted my own poured concrete foundation in Worcester using reclaimed R19 polyiso roofing foam at an attractively low cost of goods. There are several sources in MA for roofing foam pulled from commercial re-roofing & demolition at 1/4-1/3 the cost of virgin goods.)

Thanks for the detailed reply! One part caught me:

"The floor needs a 6-10mil poly vapor retarder continuous with that wall -foam, and R8-R10 of EPS or XPS is recommended for a MA climate under the slab. Putting R10 of XPS at the slab edge between the foundation and slab provides a capillary break to keep ground moisture from wicking into the slab from the edges, but also a thermal break."

In the center of my concrete floor is a low point with a drain that the sump pump empties. What I had planned to do was to use a floating sub floor (with the rubber tread on the bottom). This way water could still make it to the center drain and thus hopefully eliminating the need for a perimeter drain and the xps and poly vapor retarder.

Would that work too?

Thanks again

You need to be able to move bulk moisture away from the foundation walls quickly, which is why perimeter drains are preferred to letting bulk water run OVER the slab. A dimpled heavy plastic vapor barrier (or XPS siding underlayment with drain channels, eg. http://greenguard.pactiv.com/residential.aspx?dc14-drainage-mat.dc14-drainage-mat ) would work in much the same way as your rubberized subfloor, and would be preferred, but would still underperform a perimeter drain.

With MA-type subsoil temps (~50F) and summertime outdoor dew points (60F+) the rubbery subfloors aren't enough- you need some R-value between the finish floor and the ground to avoid mold-potential on any rug or wooden sub-floor. There is an economic rationale for R7.5 if XPS, but more if EPS (it's cheaper stuff), and higher still if you used reclaimed roofing EPS from outlets like Insulation Depot (Waverly street in Framingham) or similar. Either EPS or XPS will do, but you can't use iso, since iso is hygroscopic and will slowly accumulate moisture in a slab application. EPS @ 2.5" is about R10, and works great! If you're using an OSB or ply subfloor just stagger the seams of the subfloor with those of the EPS to avoid edge-rocker. Allow ~1/8" on the seams for expansion/contraction from humidity, and Tapcon the subfloor panels to the slab 18" on center, drilling through the foam and vapor barrier into the slab with a hammerdrill. The stackup looks like this:

Finish-floor
-------------
subfloor
-----------
foam (XPS or EPS)
-----------
poly vapor barrier
-------------
slab
----------
drainage rock (hopefully)
-----------
dirt

Take the slab foam and subfloor right up to the stubby perimeter XPS, and the studwall's bottom plate can go atop the subfloor.

If you don't have sufficient headroom to tolarate 1-1/2" or more, the min-recommended in MA to summertime mold potential at the subfloor would be about R3, which takes half-inch XPS.

Again...Thank you! Very helpful! So it sounds like by taking the following few precautions, that it should be fine and safe to put spray foam on the field stone:
1) Jack up the house and put an EPDM membrane between the top of the foundation and foundation sill
2) On top of the existing concrete slab, put a layer of poly vapor barrier and XPS or EPS between the slab and the sub floor.
3) Strongly recommended to install a perimeter drain.

My only question/point of confusion. If I have water seepage that comes up the slab, with the above stackup, wouldn't the water get stuck between the slab and the poly vapor barrier and then wouldn't there be a potential mold issue there (between the slab and poly vapor barrier)? The reason I was thinking the rubberized subfloor, or the dimpled heavy plastic as you suggested, is so that the water has the ability to escape and be flushed out by the sump pump. But without those channels, it seems like it would just get stuck and grow mold/mildew. I'm a total newbie when it comes to moisture issues though, so I recognize that I'm probably wrong...I'm just trying to understand.

Thanks again! Very helpful!

I'd strongly suggest that the poly be placed above the XPS/EPS and directly under the subfloor. This gives you the same vapor management but prevents the foam from being encapsulated in water. (Reinforced poly works much better than standard 6 mil - easier to place and much less slippery, although more expensive.) The perimeter drains need to be several inches below the original slab so that water migrates into them rather than ponding under the foam. Note that if you have the headroom, a 2" concrete slab will work fine above the foam.
One other comment - Joe Lstiburek, president of Building Science Corp, said recently that they have found that in a full basement, where there is sufficient (vapor open) foundation above grade (1'-2'), and where the interior is moisture proofed, moisture wicking up through the foundation will dissipate to the exterior, eliminating the need for a membrane under the sill.

Posted By Bob I on 03 Apr 2012 09:54 AM
I'd strongly suggest that the poly be placed above the XPS/EPS and directly under the subfloor. This gives you the same vapor management but prevents the foam from being encapsulated in water. (Reinforced poly works much better than standard 6 mil - easier to place and much less slippery, although more expensive.) The perimeter drains need to be several inches below the original slab so that water migrates into them rather than ponding under the foam. Note that if you have the headroom, a 2" concrete slab will work fine above the foam.
One other comment - Joe Lstiburek, president of Building Science Corp, said recently that they have found that in a full basement, where there is sufficient (vapor open) foundation above grade (1'-2'), and where the interior is moisture proofed, moisture wicking up through the foundation will dissipate to the exterior, eliminating the need for a membrane under the sill.

This is extremely site-specific. Without knowing the quality & type of the exterior foundation waterproofing below grade, the distance to the water table, the general porosity of the soil, and length of eave overhangs I wouldn't take Lstiburek's statement as a general rule to be applied liberally, especially when dealing with fieldstone foundations. (I've seen instances where 2" of closed cell foam has created sill rot issues, despite ample above grade exposure.  In general it's safer to play toward the more vapor-open end on interior foundation foam when there are too many unknowns.  (Shall we dig up the exterior of the foundation  every time to inspect, re-point and waterproof appropriately in a retrofit?)  On new construction or poured concrete foundations good drainage, no roof valleys creating intermittent localized saturated soil conditions, sure.

If the moisture drive is from below (often is, with slabs poured on dirt rather than 6" of screenings), putting the poly above the foam INCREASES rather than decreases the moisture content of the foam.  If putting the poly above the foam, use EPS only, since it has a higher fraction of closed cells than XPS, and although it takes on moisture in the interstitial spaces between beads (up to ~7% of volume), losing ~10-15% of total R value, it also leaves quickly when the tide recedes.  XPS takes on moisture much more slowly, but takes it on permanently.  (EPS is used in marine flotation applications for it's high resistance to permanent moisture accumulation despite lower mechanical strength relative to XPS.)

Hey everyone,

Original poster here. Sorry to dig up this old thread. After much planning and researching, this project is getting very close to kicking off. There's 2 questions I have though:

1) At the BuildingScience site, they show adding a "Plastic sheet polyethylene butyl composite membrane lining" in between the stone wall and the foam insulation. Is this necessary/recommended? Here's the link (scroll down to Figure 2 in between photo 9 and 10):
http://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations

2) I understand that we don't want the moisture to be able to transfer through the spray foam insulation, as then I could have mold issues. However if it doesn't, won't the moisture get stuck in the foundation wall itself and degrade the mortar between the stones and/or at the worst case have freeze/thaw issues that could destroy the foundation wall? From what I understand, limestone mortar was typically used on old fieldstone walls because it's breathable and you don't want moisture to get stuck between the stones, but it seems like we're doing the opposite here?

Thanks for all the help!

I think the reason Joe L. put the pool liner on his foundation was the history of severe bulk water intrusions:

"Every time it rained the basement flooded. And it wasn’t really a basement."

It certainly won't hurt in homes with less swampy track records, but it isn't a necessity, and probably overkill.

Any time you close up masonry foundation like that it's worth considering a lime-mortar parge to protect the mortar between the stones, but since rate at which moisture will be extremely slow through the closed cell foam (and membrane, if you put in a EPDM or something like Joe did), the mortar that is most at risk would be on the exterior above-grade section, not the goods behind the foam.

Freeze/thaw spalling is fairly rare in MA, and would only occur on the above grade exterior, which could be protected with a sacrificial parge if need be. But that only likely to occur if the moisture content of the stone/mortar is extremely high- more likely to happen in soft porous stones like slate, limestone or sandstone than with granite or basalt (or poured concrete), in a foundation with a poor surface drainage conditions (that could/should be rectified in any case.) Most mortar degradation issues on antique brick or stone is related to moisture movement dissolving and transporting minerals out of the mortar, and you'll often get plenty of warning (several decades) in the form of efflorescence on the surface before it's a structural issue. A sacrificial parge protects, but moving the bulk water away via better drainage is a better solution.

Most mortar degradation issues on antique brick or stone is related to moisture movement dissolving and transporting minerals out of the mortar

Ahh...that's the part I was missing. I thought mere exposure to moisture dissolved the degradation, but didn't realize it was more due to loss of minerals. Makes more sense now.

Thanks for the info on the wall membrane. Probably won't bother with it then. I'll definitely do a sacrificial parge across all of the interior though as well as adding an interior perimeter drain and putting a poly vapor barrier and a lay of xps or eps between the slab and the subfloor. I'll also try to add an EPDM membrane between the top of the foundation and foundation sill to prevent any sill rot.

Thanks again! Much appreciated!