Mixing spray foam

[Huckleberrys]

I am considering spray foam insulation and my supplier has suggested approx 50mm closed cell and then build the rest up with open cell. Does this make sense? They are suggesting the 50mm will do the air tightness and damp proofing etc but then the rest of the depth to use open cell to save on costs.

[SteamyTea]

Would need to know a lit more details.

 

Insurance companies and money lenders have a big downer on sprayed insulation.

[Huckleberrys]

I'm not too interested in that side of it. It's a timber frame inside a new barn conversion that will probably never be sold. I'm interested in the science behind the idea of the mixing.

[saveasteading]

25 minutes ago, Huckleberrys said:

I'm interested in the science behind the idea of the mixing.

Given that most contractors with this stuff appear to be charlatans, i wonder also about the manufacturers. 

If you (or we) come across tech info then it will need to be checked out.

U values, stability, shrinkage, avoiding gaps, fumes, end of life disposal.

Interesting of course to challenge the norm and find alternatives. 

Checking the cost against mineral wool may be interesting.

 

[jayc89]

I love to concept of spray foam for an all in one insulation and airtightness solution. Like many thinks when it comes to the fabric of buildings, or more specifically older ones, there's a distinct lack of information. Far too much hearsay and "that's how it's been done for years" and not enough data driven conversation. 

I'll add to the above problem by saying, I hear open-cell spray foam isn't much better for use around wood than the closed cell stuff. 

 

I did consider spray foam for my IWI solution, but it was incredibly difficult to find the quantity I needed in DIY form and didn't fancy getting anyone in to do it (for various reasons). I do still plan to use it around a problematic fireplace later this year. 

[SteamyTea]

If the closed cell is on the cold side, and the open cell on the warm side, at least the water vapour  once condensed, will be sponged up.

 

With PU the two biggest problems are shrinkage and moisture. Both cause I consistency problems.

[Huckleberrys]

I have been told...although I haven't seen any calculations...r value very similar to that of PIR which kind of makes sense. I am right for room in the build so to get the U values I want I thought it made sense. I will have MVHR so vapour getting trapped shouldn't in theory be a problem. When I get the price, hopefully tomorrow, I have to take into account that I won't need any damp proofing on the stone wall, vapour or air tightness barrier so there is less work there and thermal bridging will be taken care of in one fell swoop as the studs will be built away from the wall and the insulation sprayed behind and between. Just a baton for a service void then and plasterboard, it all sounds very easy 😀 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

They often do this in the states. Closed cell outboard against the timber frame for airtightness mainly then something cheaper inboard like mineral wool to finish off. 

 

 In 2016 when I priced (walltite, closed cell)!it was €50+vat per M2 for 150mm. K value about 0.026 W/mK

 

Open cell icynene was €30+vat per m2 for 250mm.  K value 0.039 W/mK. 

 

I liked the idea of polyurethane foam in our cavities for a total full fill and airtightness but the shrinkage at 250mm depth put me off. EPS beads were also about 1/4 of the price. 

 

For your project I would proceed with caution. Completely sealing one side of an old (stone?) wall with a total vapour barrier feels risky. 

 

I've suggested this before, I think to @saveasteading, and was shot down for some reason I can't remember. 

 

Plasterboard

50mm service cavities with batt insulation. 

Variable membrane like intello plus.

Studwork stood back from the wall.

EPS beads pumped behind the membrane. 

 

 

 

 

 

 

 

 

 

[saveasteading]

8 hours ago, Iceverge said:

I've suggested this before, I think to @saveasteading, and was shot down for some reason I can't remember

I can't remember either. All comments always welcome.

Not agreeing isn't necessarily " shooting down". If we didn't propose or challenge we would still be in caves.

[Iceverge]

Sorry that was a bit of an over statement! 

 

I might try it myself on some stone buildings I want to refurbish. If I ever get around to it....

[SteamyTea]

18 hours ago, Huckleberrys said:

I won't need any damp proofing on the stone wall

I think you will find that PU foams break down relatively quickly when wet.

 

[Huckleberrys]

As far as I am aware the closed cell foam is completely water, vapour and air tight.

 

 

[SteamyTea]

12 minutes ago, Huckleberrys said:

As far as I am aware the closed cell foam is completely water, vapour and air tight.

 

 

It isn't.

It can take the odd splash, but not prolonged contact.

 

Water is the 'blowing agent' for most polyurethanes, it gets converted into a hydrogen-carbon bond and an oxygen-carbon bond, but it is not a perfect mix, so some of those bonds 'off gas' and some of the polyurethane is not mixed too well initially.  When the unmixed parts come into contact with moisture a new reaction tries to start.  Unfortunately there is no room for expansion in the matrix, so mechanical stresses are created.

It is these stress areas, which are generally on the surface, that start to break down first.

It is why moulded PU car dashboards get tiny holes in them and unused seat cushions collapse.

Cut an old surfboard in half and you will see many places that the chemical matrix has started to break down.

I think I read somewhere that sheathing PU sheet to make aircraft wings is now outlawed.

Edited February 13, 2023 by SteamyTea