Rockwool EWI ?

[Triassic]

I’m trying to plan ahead, as my timber frame arrives in a couple of months time. 

 

The architect has specified a 50mm layer of EWI over the timber frame. He originally had a wall make up of 145mm full fill frame (PIR), with an external OSB layer, a breathable membrane, the additional 50mm of polystyrene EWI insulation, then battens, render board and render.

 

Having read a couple of blogs and posts here was wondering if I could simply use a 50mm thick layer of Rockwool fixed directly over the OSB and breathable membrane, then apply a breathable silicone render to the  Rockwool? By taking  this approach it would reduce the number of layers and will save money.

 

Has anyone got any experience of this approach? Suggestions as to companies who could carry out the work?

[Mr Punter]

Yes I have read that not having a drained and ventilated cavity can lead to damp getting into the frame.  I understand that in Canada they have had real problems with this several years ago known as "leaky condos" which has a Wikipedia entry https://en.wikipedia.org/wiki/Leaky_condo_crisis

[Gone West]

Our house has 50mm Rockwool RW6 on the outside of the OSB3 racking on our timber frame. We then had to have a 50mm air gap before the rainscreen cladding.

[ProDave]

Why not do as we have done, 100mm thick wood fibre board and render direct onto that like shown here

 

Described here https://www.ecomerchant.co.uk/exterior/insulation/wood-fibre-rigid/steico-protect-9611.html

[Dudda]

You're probably getting a guarantee with your timber frame (presuming it's design, supply and erect). While it might be possible to ventilate a timber frame through rockwool/wood fibre and a breathable render would this void the timber frame guarantee? They may demand a ventilated cavity in small print someplace. In addition what happens if someone paints the house with a non breathable or acrylic based paint?

I'd talk to the timber frame company and see what they'll approve unless you can convince your architect to take design responsibility and then use his insurance if it goes wrong.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Triassic]

1 hour ago, the_r_sole said:

can i ask why you have gone with EWI on a timberframe? I always worry about the dewpoint and moisture being trapped in that kind of wall construction... wouldn't be my first, second or third choice with timber kit

This is what the architect designed?

 

Rather naively I thought the architect would design something that was buildable? I've just realised the opening section of the patio doors are too narrow as designed.

Edited October 16, 2018 by Triassic

[ProDave]

2 hours ago, the_r_sole said:

can i ask why you have gone with EWI on a timberframe? I always worry about the dewpoint and moisture being trapped in that kind of wall construction... wouldn't be my first, second or third choice with timber kit

It is important to have a dew point analysis done.  This was ours with the wood fibre and render

 

This is the reason the OSB racking layers are on the inside of the frame (I lost count of how many times a passer by told us they had put the frame up inside out during construction)

 

 

u-wert-berechnung (10).pdf

[Dreadnaught]

8 minutes ago, ProDave said:

u-wert-berechnung (10).pdf

 

That is a very impressive report! Is it possible to get such a thing in English?

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Triassic]

Having talked to the timber frame company, they would recommend the insulation on the inside of the frame. Looks like that's the way we'll go.

[A_L]

4 minutes ago, Triassic said:

Having talked to the timber frame company, they would recommend the insulation on the inside of the frame. Looks like that's the way we'll go.

 

Only if the insulation has relatively high vapour resistance and performs as a vapour check, i.e. PIR/PUR/XPS. Possibly EPS. i.e. not rockwool or any fibrous insulation.

[Triassic]

34 minutes ago, A_L said:

 

Only if the insulation has relatively high vapour resistance and performs as a vapour check, i.e. PIR/PUR/XPS. Possibly EPS. i.e. not rockwool or any fibrous insulation.

They've specified a vapour membrane fixed to the internal face of the frame.

[A_L]

11 minutes ago, Triassic said:

They've specified a vapour membrane fixed to the internal face of the frame.

 

Just to double-check. The extra insulation is on the cold (out)side of the vapour membrane?

[ADLIan]

Most of the insulation manufacturers in the UK will provide a U-value and condensation analysis calculation. Some may also do the thermal response/phase shift numbers.

 

Note that it is NOT accepted good practice to render onto insulation onto the timber frame - see 'Timber Frame Construction' from TRADA, Bldg Reg Approved Docs, TF kit manufacturers details etc. One of the wood fibre insulation manufacturers had a BBA certificate for a direct render system but this appears to have lapsed or been withdrawn. The issue is not so much condensation but liquid water penetration through the render system as referenced by Mr Punter.

[ADLIan]

An additional comment. The U-value calculation from ProDave is wrong as there is no correction for the timber frame creating a thermal bridge (normally 15% bridged proportion) and this would increase the U-value to nearer 0.16 W/m²K.

[Jeremy Harris]

4 minutes ago, ADLIan said:

An additional comment. The U-value calculation from ProDave is wrong as there is no correction for the timber frame creating a thermal bridge (normally 15% bridged proportion) and this would increase the U-value to nearer 0.16 W/m²K.

 

However, in this case there is 100mm of insulation outside the timber frame structure, which mitigates a fair bit of the thermal bridging there would be otherwise.  The sums are easy to do, given the details of the actual structure, but I bet the thermal bridging allowance will be a fair bit lower than 15% in this case.

[ADLIan]

I had included the 100mm external insulation - as you say this does mitigate the effect of the TF fraction. 15% bridging is the default value and includes for the additional timbers around openings, at floor levels, corners, head & sole plates etc and is normally about right, may reduce to approx 12% if you're lucky. Bottom line is the U-value has not been produced in line with UK BR Appr Doc requirements.

This is all a bit academic now as the wall is done! The important point, and where this thread started, is the requirement for a vent void behind the render system in TF walls.

[Jeremy Harris]

Still seems high to me.  I spent a bit of time calculating the possible thermal bridging in our wall and roof structure, accounting for the effect of surrounding any timber thermal bridges with insulation in close contact, which made them effectively only 2D thermal transmission paths, and concluded that the default 15% value was way out for our frame.  I have the spreadsheet on another PC and will try and dig out the exact figure later, but off the top of my head I think it increased the wall U value by less than 2%.  Having relatively thick walls, with only a small cross sectional  area of timber bridging from inside to outside helped. 

 

The system @ProDave has seems to be very similar to the Warmshell Passivhaus build up, but with just 200mm deep frame insulation, rather than 300mm, but a thicker outer layer of rendered wood fibre: http://www.warmshellinsulation.co.uk/document/Products/warmshell_passivhaus_details.pdf

[ADLIan]

15% is about right in my experience for standard stud TF (not larsen truss or I-beam). I've seen it as low as 12% but also as high as  20+%-  a value mentioned by Warmshell in one of their design guides! The Appr Docs refer to BR 443 which gives the 15% bridged value.

I note Warmshellinsulation has a BBA cert for a rendered external wall insulation system but it limited to masonry background. They also mention that they have an applied for BBA cert for this system on TF. It appears this goes back to 2015 and still no certificate issued. As mentioned above a similar certificate (based on wood fibre) was withdrawn or allowed to lapse a few years ago.

I would still refer you to the above documents and the fact that render onto insulation direct onto TF is not accepted good practice.