# Which insulation to use



## Blackswanwood (8 Sep 2020)

Has anyone got experience of Pavaflex wood fibre insulation? The thermal conductivity doesn't seem to be different but I am told it is better at preventing interstitial condensation than Celotex or Kingspan but cannot see why that would be the case?

My construction is going to be brick over a timber frame.

Any thoughts welcome.


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## Woody2Shoes (8 Sep 2020)

The reason it's said to 'prevent' interstitial condensation is that it's vapour permeable - air can move through it, carrying moisture. Celotex/Kingspan are impermeable - mainly because of the metallised paper foil on each side.

Moisture can't be drawn into or out of Celotex, but it can through Pavaflex. Therefore, to avoid problems with moisture getting into the pavaflex and not getting back out again, you need to get the other details right!

PS There's (normally) a temperature gradient across the thickness of any insulation. Where there's a temperature difference, there's a vapour pressure difference too. Interstitial condensation is where air travels along the temp gradient (from warm to cold) and at some point has to drop some of the moisture it's carrying - if that point happens to be inside some vapour permeable material, that material gets wet!


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## Woody2Shoes (8 Sep 2020)

I've just read the data sheet PAVAFLEX - Pavatex
This stuff has roughly half the insulation value (for a given thickness) of PU foam based celotex-type-stuff. It does have a higher specific heat capacity, so it can store heat (and cool) which may increase the thermal mass of a building.
This product seems to be competing with rockwool type batts - they seem to make a big thing about slump resistance, which is probably a weakness of those batts.

If it were me, I'd go for the thickest bit of PU foam-based insulation I could fit - it'll pay for itself surprisingly quickly I expect.


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## MikeG. (8 Sep 2020)

It's not a good comparison, frankly. The Pavatex, which I have used a number of times, is a relatively poor insulator, but is breathable, and (I can never remember the proper word....hydraphylic/ hydrophobic, hydrasomething.....) absorbs moisture readily to release it slowly when the relative humidity drops. It is also a reasonable render carrier for lime render. So, it has its uses, but it is expensive, and unnecessary in most circumstances. 99% of the time good old mineral wool/ fibreglass is a better answer.


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## MikeG. (8 Sep 2020)

Woody2Shoes said:


> .........If it were me, I'd go for the thickest bit of PU foam-based insulation........



Did you mean PU, or PIR?


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## Hornbeam (8 Sep 2020)

Most manufacturers have switched from PU to PIR foams due to the much better fire performance of PIR.


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## Blackswanwood (8 Sep 2020)

Thanks,


MikeG. said:


> It's not a good comparison, frankly. The Pavatex, which I have used a number of times, is a relatively poor insulator, but is breathable, and (I can never remember the proper word....hydraphylic/ hydrophobic, hydrasomething.....) absorbs moisture readily to release it slowly when the relative humidity drops. It is also a reasonable render carrier for lime render. So, it has its uses, but it is expensive, and unnecessary in most circumstances. 99% of the time good old mineral wool/ fibreglass is a better answer.




Thanks Mike. I’m actually thinking of ditching the timber frame and keeping it simple by just doing brick and block with 75mm cavity board.


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## PAC1 (8 Sep 2020)

It is great in my ancient timber frame house. I have used it on a solid 1B wall and I cannot believe the difference it has made. The whole idea is it breaths with the wall, absorbs moisture and releases it. I must admit the act of sticking wood fibre and MDF to a 1B wall was a leap of faith. I expected it to be soaking wet in a week. It coped with one of the wettest winters ever no problem. So far I am impressed. 
That said I would not use it on a modern construction form. There are far better choices to use.


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## MikeG. (8 Sep 2020)

Yep, that's what it is best at.....renovations.


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## Woody2Shoes (9 Sep 2020)

MikeG. said:


> Did you mean PU, or PIR?


I meant polyisocyanurate but I'm afraid that PIR will forever be Passive Infra Red in my small brain!


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