Iam working on an existing home built in the 70s thats located on a steep slope in Chattanooga TN. The home is very chopped up with many angles, intersections and floor elevations. Much of the floor insulation is accessible from below and not enclosed in a crawlspace. The ethelyne VB that once covered the batts and bottom of the floor joists is shredded and dangling in the wind. Closed cell spray foam was selected for its air barrier, adhesion, and water resistant properties and the ability to create a decent layer of continuous insulation across the many transitions. I know the proper way would be to take out the existing batts but Iam tempted to just spray over them and would like to hear opinions from other people on this before I tear them out. Many of them are in good shape and installed very well. Since the plan is to get a good 1" thickness on the bottom of the joists (2" in the field) and there is 1-3" of the side of the joists exposed before the batts, I think there would be plenty for the foam to grip to. Naturally, we would peel back the batts in important areas that effect infiltration. I know the batts could be taken out and reinstalled but then there would be a very tricky air barrier to detail to protect them. Another advantage would be that it would be much easier to rip out the foam if needed in the future. Can I just spray over those suckers? What do you think?

Here are some pics.

I cant give you a scientific answer, but I think this is insane. take down the batts and put them back up below the CC if you want. I would never spray over fiberglass batts. Is that a full basement? Is there HVAC equipment down there? Why waste money spraying below the floor? Insulate your exterior surfaces first.

I hear you. All pictures are from COMPLETELY outside the building envelope (not basement NOT vented crawl). The spf would be the absolute exterior surface, but not if I removed the batts and reinstalled them. I thought the same as you when I first started this project but the more I think about it the more I like the idea of spraying them. Why not spray over FG batts?

It looks like you've had (or have) a mold issue on the exposed ends of the joists, which may or may not have been aggravated by the poly vapor barrier.

Spraying cc foam on the batts is a dubious methodology at best, and may not achieve all of your ends.

If the batts aren't snugged up to the subfloor you have a real potential for thermal bypass currents flowing between the top of the batts and under the sub-floor. At the very least the batts need to be snugged up to to impeded those currents. At the band joist & sills a sealing shot of closed cell will reduce the most egregious infiltration paths that would not be treated by air-sealing at the joist/batt bottom surface.

Rather than spray foam, fill up the remaining joist bay depth with cheap fiber and use rigid foam attached to the bottom side of the joists, supported by furring through-screwed into the joists. Cost-wise R-for-R that's usually comparable to or cheaper than closed cell foam. In Chattanooga an inch of XPS (seams mastic or housewrap-taped) or foil-faced iso (FSK taped) would go a long way toward limiting summertime condensation on the joist edges by keeping them on the conditioned-space side of the vapor retarder, and keep the joist edges warm enough to stay above interior-air dew points in winter, no matter what you use for insulation in the joist-bays. Supported by through-screwed furring 16" o.c. either would be mechanically rigid enough to handle the dead-loading of the fiber insulation in the bays.

At only 1" your thermal break on the joists will be have what it is with your 2" of cc foam concept, but with the additional R4-10 or so of center-cavity R the whole-assembly performance will be similar or better, and you'll have flat, easy to look at surface that won't immediately call itself into question when the inspectors come around. If you're using batts rather than blown, to fill the space completely it's OK (even preferable) to use low density R19s and compress them in there to ensure there is no gaps or thermal-bypass channels. An R19 batt compressed to 3.5" delivers R13 performance- lower R, but higher R per inch (and higher air-retardency too.) If after snugging up the existing batts to the top the bottom gap is reliably less than 3.5", use R11s or R13s to fill it up. If using kraft facers makes installation easier, go for it, but don't skip the foam as kraft facers don't hold up very well over time when exposed like that.

Thanks Dana, great points as always. I suspect the past mold issue was from wind blown rain due to its more exposed location and I havent noticed any other areas even where the VB was much more intact.

If I were to go through with it, we absolutely would snug the batts where needed, pull them back at ends and transitions and even push the ones up that were already snug, just to expose more of the joist's sides for better foam adhesion. The idea would be to isolate the batts to slow down any air and thermal communication between different spaces and seal the framing joints you mentioned.

My first thoughts were to go with rigid foam myself but the floor space alone is ridiculously complicated and would be a geometry, taping, sealing exercise that could take weeks compared to hours with the foam. The R math may not be as good but the real world infiltration issues I think are a bigger problem here. There are many transitions that would be difficult with rigid, canned foam, and tape that would be simple with spf. Thats also not including the difficult and expensive task of adding the new fiber, after ripping out and sending the existing to the landfill.. There are some semi-conditioned areas with unconventional framing that would be quite a challenge to do with batts (these areas had little to no existing batts).

I will be using rigid in some locations like around access doors and behind some utilities that will need replacement in the future.

Just to be clear, batts or no batts, we will be sealing the bottom joist edges with 1" of cc spf which should be achieving a similar result as doing it with rigid foam. A little concerned about appearance and future inspectors. For the most part, it will be such a dramatic improvement to whats there I dont see it being too much of a home inspection issue. Around here I would think most inspectors would see foam in this situation and feel pretty good about it.

The location is not prominent and appearance is a minor concern. Planning on adding a dye to the spf mix. The Foametix rep says he can get a brownish earth tone with half a can of a certain colorant they use but will add the whole can to attempt a darker brown that we would prefer. He assures me that this shouldnt effect the cure but Iam currently trying to talk to a product tech rep closer to the source to get assurance on this.

Surely someone can give me awful reasons to directly spray cc spray foam onto FG batts and perhaps knows of existing results to back up all this concern?

If you are talking with Foametix, why isn't your installer or Foametix answering this question for you?

I've sprayed quite a bit of foam, in a few instances small areas over vertical fiberglass and never had issues, I've never sprayed over horizontal fiberglass but I could see a potential possibility of it not bonding and collapsing after spraying when the reaction stars occuring and before reaching a cured state.

You should try it, and then let me know what happens.

I'd think that installing a mesh matrix of some sort (maybe blowing mesh used by the cellulose & fiberglass folks, or even insect-screening) to the bottom of the joist would give a flatter surface with more consistent bonding that spraying directly onto batts? Whatever you do, getting real buy-in from the foam installer is key- if it doesn't work it can be a miserable job to hack it all before moving on to plan-B.