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Breather membrane


Ben100

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this is a very interesting subject for me. I'm in the process of getting quotes for a timber frame and each manufacturer seems to have a different take on the external membrane for the frame. a number seem to go for the Protect TF200 Thermo with some offering it as an upgrade over the stock membrane. another has offered the Proctor Reflectashield TF0.81. 

 

do all these do pretty much the same thing? if so, does it simply come down to the timber frame manufacturers personal choice (or product endorsement for higher profit margin). I'm finding it really hard to determine what the best is or how to easily compare all products. e.g. the TF200 says "Aged Thermal Resistance 0.77m2K/W, the Proctor says "claimed thermal performance of 0.81m2K/W". So I would assume the Proctor Reflectashield is better as it has the higher R-value, but what does Aged mean? and I couldn't see an R-value listed on the specs of the Novia stuff!

 

so, how does one determine the best membrane to use?

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I'm going for the Protect TF200 but without the foil layer for my upcoming build. Its what MBC use on their double-stud builds. 

I am avoiding the foil as I have a concern it will block mobile and wifi signals inside the house unnecessarily and doesn't contribute much if anything to the U-value. Not sure if my concern is warranted.

Edited by Dreadnaught
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I'm not familiar with those particular membranes, but the key aims are to keep the wind out of the insulation and provide protection against rain ingress.

 

For a thin sheet the thermal performance isn't going to make a huge impact, but why not take it if the cost increase is minimal.

 

'Aged' just means the value after a few years of use - the performance will deteriorate somewhat from new.

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it does seem that the TF200 is a popular choice with the timber frame manufacturers and I'll probably end up going that way although I guess it depends on who I choose! it's good to know that there's probably not much difference between the lot of them.

 

14 minutes ago, Dreadnaught said:

I'm going for the Protect TF200 but without the foil layer for my upcoming build. Its what MBC use on their double-stud builds. 

I am avoiding the foil as I have a concern it will block mobile and wifi signals inside the house unnecessarily and doesn't contribute much if anything to the U-value. Not sure if my concern is warranted.

 

the foil may well block mobile signals but I also think the 300mm of cellulose might have an impact as well! so I think even without the foil you might suffer. 

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33 minutes ago, Thorfun said:

so, how does one determine the best membrane to use?

 

Silvered membranes can increase the thermal resistance of unventilated cavities next to them. By default around 0.25m2K/W or higher if tested as those already mentioned (over an ordinary cavity of 0.18m2K/W) It is important the cavity is unventilated as it has no effect on ventilated cavities. Unventilated is deemed to be no more than 500mm2 of openings per metre length of wall.

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54 minutes ago, A_L said:

 

Silvered membranes can increase the thermal resistance of unventilated cavities next to them. By default around 0.25m2K/W or higher if tested as those already mentioned (over an ordinary cavity of 0.18m2K/W) It is important the cavity is unventilated as it has no effect on ventilated cavities. Unventilated is deemed to be no more than 500mm2 of openings per metre length of wall.

 

If using timber cladding then I assume that an external membrane on a timber frame would be a ventilated cavity though, right? as you have the external OSB covered by the membrane with the silver facing outwards into a cavity that has batons and the timber cladding on top. as the cladding is not airtight I presume that's considered a ventilated cavity. by that rationale the only external unventilated cavity would be brick cladding?

 

we are having timber cladding so if that's the case then the silvered membranes won't benefit us at all.

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Correct @Thorfun, there is  however a small additional thermal resistance that can be added to 'rainscreen cavities' such as yours. The cladding lowers the average windspeed in the cavity compared to the outside air. This allows what is called the 'external surface resistance' which is normally 0.04m2K/W to be increased to 0.13m2K/W. The external surface is the breather membrane on the inside of the ventilated cavity. See pages 11 & 12 of the attached pdf.

 

BR_443_(2006_Edition).pdf

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42 minutes ago, A_L said:

Correct @Thorfun, there is  however a small additional thermal resistance that can be added to 'rainscreen cavities' such as yours. The cladding lowers the average windspeed in the cavity compared to the outside air. This allows what is called the 'external surface resistance' which is normally 0.04m2K/W to be increased to 0.13m2K/W. The external surface is the breather membrane on the inside of the ventilated cavity. See pages 11 & 12 of the attached pdf.

 

BR_443_(2006_Edition).pdf 153.24 kB · 0 downloads

thanks @A_L . if i'm reading page 12 of that document correctly (thanks for it btw, I shall read it in more detail when I have the time!) it looks like using something like the TF200 which is a low-emissivity surface will increase the external surface resistance further still to 0.29m2K/W. correct? 

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@Thorfun 

 

1 hour ago, Thorfun said:

it looks like using something like the TF200 which is a low-emissivity surface will increase the external surface resistance further still to 0.29m2K/W. correct? 

 

Correct.

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