Polyurethane expanding foam

[Wurlitzer]

My builder is using polyurethane expanding foam to fill small spaces in my build. For example: where the timber roof meets the concrete block walls and not all the timber are flush with the concrete (or simply because the concrete itself is not absolutely flat); between the door frame and the concrete block etc.

I have voiced reservations about the sound isolation of this, and asked if he can use silicon (caulk) instead, but he insists that that's only a sealant and can't really fill large gaps where you have to get it in deep. He says the foam is the only thing for that as you can squirt it in and it expands right to the other edge.

He's happy to use silicon on TOP of that, to seal the edges, and plans to use it on the all the drywall edges, as normal.

Is he right about this? It seems like a point of weakness to me, but I don't know my materials well enough to know whether something else would be better in its place.

He's also using the expanding foam to waterproof the gap between my two concrete foundations (I'm using a split foundation design rather than a floating floor). I'm presuming that the foam in this scenario will not harm the decoupling of the foundations, since it is so light and will act more like a spring. Once it's set, it's kind of like that polystyrene stuff you get in packaging, and people had advised me to use that to separate the foundations when pouring them and just leave it in there, so I presume it's OK.

Am I right? Or should we use some alternative means of waterproofing? If so, what?

[sharward]

You are correct in assuming that the closed cell "Foam in a can" type stuff is very lightweight so it therefore becomes a severe weakness in your mass-air-mass envelope. Although they may be air-tight, you have "holes" where sound can escape through the lightweight foam.

I used acoustic sealant to fill my gaps in my beef-up wall -- lots of it -- 35 28-ounce tubes of it to be exact. I spent more money on sealant on my beef-up walls than I did actual drywall!

Can you take some pictures?

This "problem" will most likely need to be corrected, and soon, before stuff starts to get even less accessible! Yes, this stuff matters -- a lot. Hopefully your contractor will appreciate the fact that you're not just making this stuff up.

I'm glad you're asking about using the stuff to fill the gaps between your slabs. I'm dealing with that question right now. I think I've settled on pretty much doing the same thing you're doing... But stay tuned to my thread to see what the final conclusion ends up being.

--Keith

[Wurlitzer]

Oh bugger.

He's pretty much done around all the eves and door frame now. I suppose the best thing I can do is to make sure those joins are beefed up over the top of the foam.

Sounds like it's OK between the foundations though?

[len-morgan]

Before you get too worried, I think we need to look at why he's doing what he's doing. As I understand it, he's filling in large gaps in the outer shell building with foam and I'm not sure that's a problem in and of itself.

You builder is correct in that you can't substitute calk for foam if you are filling really large (i.e., more than 3/8" or so) gaps. The whole point of calking as I understand it, is to make the rooms, buildings, etc. AIR TIGHT. Sound can't escape in an air tight environment (I know that's an overly broad statement - calking 4 walls and a cieling of wallpaper over studs won't do anything).

I don't believe the calking of wall/ceiling/floor/door opening joints provides any sound blocking on it's own. The whole point is to prevent small air leaks that "weaken" the spring component of the mass-spring-mass equation.

I would NOT recommend substituting foam for calk in the places where it is currently suggested. However, I don't see a problem (and I'm sure I'll be corrected if this is a myth) with filling some of these larger gaps with foam and then calking OVER this to make it air tight.

len

[Wurlitzer]

Thanks Len. Yes, that's pretty much what he said and what he's doing - squirting the foam in deep, then caulking over the top. He made the same point as you that you can't get caulk right into the middle of a deep gap.

I should point out that these gaps are not large or wide at all, they're mostly just the little crevices that always come with cutting and joining pieces of disparate material, no matter how careful one is.

[knightfly]

As long as there are only small gaps this may not be much of a problem; but if he's doing that where rafters meet frame top caps, you're better off angle-ripping 2x lumber to fit those spaces snugly, THEN caulking thorougly. 2x lumber (1.5") is roughly equivalent to 2 layers of 5/8 drywall in mass, where the spray foam is almost ZERO mass.

Keep in mind that the goal is to have two heavy masses with NO HOLES in them, separated by a fairly wide air/insulation space. If the gaps get much larger than what caulk can handle, it's a good idea to find some heavier mass than foam to fill them - for interior applications, drywall mud sets up hard and heavy. For exterior, solid wood and caulk is best... Steve