Breathable membrane for kit

[davejura]

Hi folks. Our architect has specified Tyvek reflex breathable membrane for our kit. Just wondering if anyone has any other options that are possibly more thermally efficient? Was thinking possibly Protect TF200? We are using Cedral or Hardie planks cladding if that makes a difference. Thanks.

[Russell griffiths]

Why thermally efficient, it’s not there for that. 
it’s an external weather barrier. 

[davejura]

Yes but surely it all helps? That's why I was asking about the TF200.

 

[Iceverge]

  On 24/04/2022 at 20:40, davejura said:

Yes but surely it all helps?

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Very little in the grand scheme of things. 

 

I would just concentrate on getting a membrane that is tear resistant, and does a good job of keeping the rain out. 

[davejura]

Ok thanks. What I have found is that it's obvious that the architect or engineer isn't paying for the build as they seem to specify hi end stuff. (Mine originally had marine grade ply down for the sarking. Soon got that changed!) So the reflex seems to be the most expensive Tyvek as opposed to say the housewrap. Every £ is a prisoner with the way things have gone, so if anyone can come up with a cheaper option that is just as good, that would be great thanks!

 

[davejura]

For example, Novia Reflex.

[Iceverge]

I bought protect vp400 from Screwfix for the garage. Very impressed. Better than the tyvek supro used on the house I think. 

[ProDave]

Another vote for VP400.  Our builders chose it because they said it was rated to be left exposed for longer than most others and ours was a slow build.

[Russell griffiths]

How long will it take to get it all covered up. 

I have  just been on a job where they had to strip the whole sparking off and re do it because it had been exposed for over 6 months. 

It could be a case of using something really cheap for six months and then when you are ready to clad rip off and replace with the best you can get. 

I used a product by proctor on the roof, vent 3 I think it was. 

[ProDave]

  On 26/04/2022 at 08:33, Russell griffiths said:

How long will it take to get it all covered up. 

I have  just been on a job where they had to strip the whole sparking off and re do it because it had been exposed for over 6 months. 

It could be a case of using something really cheap for six months and then when you are ready to clad rip off and replace with the best you can get. 

I used a product by proctor on the roof, vent 3 I think it was. 

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My sun room was left for about 4 years, just the bare frame covered in VP400.  When I eventually got to cladding that, the VP400 was well faded but still sound, and an exploratory lifting a bit revealed the OSB cladding of the frame was as good as new underneath, so I just fitted the cladding.

 

I doubt anyone will actually guarantee their membrane for 4 years exposure, but that was just my real world findings.

[SteamyTea]

  On 24/04/2022 at 20:40, davejura said:

but surely it all helps

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It is too thin to add any insulation value.

Thermal conductivity (k) has the unit W/(m.K)

Thermal resistance (R) is the thickness of the material divided by the conductivity.

m/(W/(m.K), more usually written as l (lower case L).

Then the U Value, the unit we more naturally understand, is 1/R.

Edited April 26, 2022 by SteamyTea

[davejura]

This is what they say about the Protect TF200 thermo. 

https://glidevaleprotect.com/product/protect-tf200-thermo-insulating-breather-membrane/

Had it recommended to me, so thought I would put it out there.

Is the VP400 not a roofing membrane? Or doesn't that matter? Architect has specified foil faced, so not sure how strict I need to be!

[ProDave]

  On 26/04/2022 at 11:21, davejura said:

Is the VP400 not a roofing membrane? Or doesn't that matter?

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Yes it is and that is mainly where is is used on my build, but I used the leftovers to line the walls of my sun room.  If it's good for a roof I can't see why it would be no good for a wall was my thinking.

[Iceverge]

  On 24/04/2022 at 21:50, davejura said:

expensive Tyvek as opposed to say the housewrap

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Designers are often presented a nice "system" from a manufacturer's rep and just paste it verbatim into the plans. It saves them a lot of research time and reduces their personal liability as the big boys (dupont/tyvek) in this case will have approved the design.

 

There is a risk of altering a system without consideration or understanding (Greenfell) but if you know what you're doing and run it by the designer you'll often get a better and cheaper solution. An email to Glidevales technical department in this situation might put your mind's (and your Architects) at rest. 

 

Reflective membranes are primarily for heat protection in sunny climates.

With a good level of insulation and a build-up of high decrement delay they're of limited value in this regard. 

 

They can help to keep the interior a little bit warmer but the effect is very small. They need an airspace of about 50mm to reflect any radiated heat back towards the house. The improvement in thermal performance would be hugely outweighed by putting in an extra 50mm of insulation.     

 

What is your total wall build up as it stands? 

 

 

[saveasteading]

I looked into reflective layers in some depth, as the published benefits seemed unrealistic.

 

In doing so I noticed that foil-backed plasterboard was no longer advertised as aiding U values, just vapour resistance.

Ditto PIR slabs.

 

In summary, the reflective layer does reflect energy, even in the dark. In time it loses some reflectance but still works  a bit.

BUT fundamentally it has to be reflecting energy back where it came from and into a space: an air gap. otherwise it is no use at all.

 

I am all for adding it if the cost is low, and am intending to use an aluminised inner vapour barrier internally, with a service void inside of that. It is a very strong membrane so won't tear, and the shininess costs just a few tens of pounds to the project.

 

However if searching to tweak a U value to satisfy Building Regs, then it is an easy addition.

[Temp]

I also used VP400. It survived being left uncovered for months.

[davejura]

Going to use the TF200. I've had bigger problems to deal with this week that will save me more money! 

Build up is Cedral or Hardie cladding, TF200, 9mm OSB, 140mm PIR between studs and 30mm PIR over the top.

 

[ADLIan]

The reflective breather membranes (and reflective AVCLs) can give a useful reduction in the U-value but only if they face onto an unventilated airspace. When used behind cladding with a ventilated airspace behind most of this benefit is lost and there is unlikely to be any improvement in the U-value