Timber frame design principle ponderings…

[Iceverge]

1 hour ago, G and J said:

So, I’m trying to understand the logic behind this build up.  
 

A skin of PIR externally (carefully taped?) presumably does a brilliant job of alleviating cold bridging, and means the frame has to dry inwards.  
 

Presumably the breather membrane is to keep rain out, but could be non breathable given it’s proximity to the non breathable PIR.

Is the intello membrane primarily for air tightness?

 

 

The breather membrane is just to make sure that any water doesn't find it's way into the structure. The PIR if taped correctly is keeping the drips out anyway but belt and braces. 

 

Correct Intello is for airtightness. 

 

 

 

 

[G and J]

15 minutes ago, Iceverge said:

 

The breather membrane is just to make sure that any water doesn't find it's way into the structure. The PIR if taped correctly is keeping the drips out anyway but belt and braces. 

 

Correct Intello is for airtightness. 

 

 

 

 

Now this was in Ireland, I wonder if it would be acceptable in England. If so I think my frame design might benefit from this, assuming I can also get PIR on the outside of the sheathing accepted from a fire resistance viewpoint.  
 

There’s a lot of ‘ifs’ there.   😞 

[IanR]

3 hours ago, G and J said:

and means the frame has to dry inwards.  

 

I'm not seeing any evidence of that, why do you assert that it does?

 

Edited July 20 by IanR

[G and J]

2 hours ago, IanR said:

 

I'm not seeing any evidence of that, why do you assert that it does?

 

Because I understood the PIR to be vapour impermeable.   From what I’ve read you can put PIR outside if the frame if you don’t tape it and maybe leave small gaps, which I would have thought would reduce it’s effective thermal insulation.  Otherwise it won’t be breathable.

[IanR]

53 minutes ago, G and J said:

Because I understood the PIR to be vapour impermeable.  

 

But that doesn't make the moisture turn around and head back the other way.

 

If you look at the Ubakus analysis it finds the moisture will sit at the OSB and take 139 days to dry out. I'd assume by evaporation. Within the standards Ubakus uses, 139 days to dry out is a failure - 90 days is the max. I'd assume that's because there is insufficient drying time in the summer to garuntee it will dry out and is therefore at risk of continuously being "wet".

 

One caveat with Ubakus is that it "does not take into account the capillary conductivity of materials", although with my limited knowledge I'm not sure that would help with the foil covered PIR being the next layer as I'd expect capillary conductivity to be negligible.

 

As mentioned before, a more precise condensation analysis should be used for non-standard build-ups. BC should require it for sign off.

 

53 minutes ago, G and J said:

From what I’ve read you can put PIR outside if the frame if you don’t tape it and maybe leave small gaps, which I would have thought would reduce it’s effective thermal insulation.  Otherwise it won’t be breathable.

 

Really? You've read that the product is OK to use if you do a crap job of fitting it and allow cold air to breach the insulation? Not sure that's a strategy I'd use.

Edited July 20 by IanR

[G and J]

31 minutes ago, IanR said:

But that doesn't make the moisture turn around and head back the other way.

Well, either the house in Iceverge’s link is rotting away inside the walls as I type or things do dry inwardly, so maybe it does.

 

Standard practice can easily become dogma and that’s generally a good thing if it simplifies and ensures that the majority of projects are ok.   But sometimes there might be the possibility of something better if new approaches are tried.  
 

Interesting debate, I do appreciate the inputs.

[IanR]

7 minutes ago, G and J said:

Well, either the house in Iceverge’s link is rotting away inside the walls as I type or things do dry inwardly, so maybe it does.

 

I'm not sure you are understanding what I'm saying, which is evidenced by the Ubakus analysis, so I'll back out of this discussion finishing with me repeating that it does not dry inwardly. That does not mean that its rotting away. It's not as black and white as you'd like to perceive it.

[Kelvin]

I wouldn’t rely on Ubakus for anything non-standard. Get a full WUFI analysis done then you can have more confidence in how the building will deal with moisture over the seasons. Wood can rot quite quickly in the right environment. Intello Pro Clima is for air tightness but it’s also a vapour control (check) layer. 

[G and J]

17 minutes ago, Kelvin said:

I wouldn’t rely on Ubakus for anything non-standard.

I’m using it for thermal performance only.  I’m striving to be as informed a ‘buyer’ as possible, but the final answer will be underwritten as a minimum by appropriate professionals.  
 

19 minutes ago, Kelvin said:

Intello Pro Clima is for air tightness but it’s also a vapour control (check) layer. 

I might be suffering from an inadequate understanding of terminology (amongst other things).   When I tried to look it up I thought it indicated that this was moisture permeable.  
 

If not then I am royally confused.  The good news is that I’m used to being that, but I try not to stay that way.  

[Kelvin]

It’s a humidity variable membrane. Vapour closed in winter and more open in summer to aid drying. They refer to it as a vapour check layer. They also use other terms such as vapour retarder or vapour control. 
 

https://proclima.com/products/internal-sealing/intello/how-it-works#sub_navigation

Edited July 20 by Kelvin

[G and J]

3 hours ago, Kelvin said:

It’s a humidity variable membrane. Vapour closed in winter and more open in summer to aid drying. They refer to it as a vapour check layer. They also use other terms such as vapour retarder or vapour control. 
 

https://proclima.com/products/internal-sealing/intello/how-it-works#sub_navigation

Is its behaviour determined by the ambient temperature it experiences?  And if so, isn’t it always sort of summer indoors in a well designed house?

 

 

[Kelvin]

No. These so called intelligent membranes react to the changing humidity levels throughout the year. They are hygrovariable. It’s why they shouldn’t be used in buildings that are generally very humid like commercial kitchens or swimming pools. 

[G and J]

So more akin to osmosis?

 

 

[Kelvin]

Here’s an in depth study that covers it. It’s by Pro Clima AUS so not exactly independent. It might require you to register to access it. 
 

https://proclima.com.au/wp-content/uploads/2023/04/Pro-Clima-Australia-Study_2023_3.4_online.pdf

[Nick Laslett]

1 hour ago, G and J said:

So more akin to osmosis?

 

 

Here is a good primer by Joe Lstiburek of Building Science Corp on vapour control and how modern “smart” vapour control layers like Intello work. 

 

https://buildingscience.com/documents/building-science-insights-newsletters/bsi-099-its-all-relative

 

 

[Sparrowhawk]

1 hour ago, Nick Laslett said:

Here is a good primer by Joe Lstiburek of Building Science Corp on vapour control and how modern “smart” vapour control layers like Intello work. 

 

https://buildingscience.com/documents/building-science-insights-newsletters/bsi-099-its-all-relative

 

That's a good overview. And then to complicate things further you''ve got Siga's Hygrobrid technology in some of their membranes, which I think works like Goretex/eVent coat fabric to ensure moisture moves in a single direction.

 

I came across it via this case study which says

Quote

[We used] Siga Marjrex [and] it’s cactus like properties mean we can have a lot more internal insulation.

 

[Alan Ambrose]

>>> can't select a sd=7.5m VCL

 

BTW you can creste your own materials in ubakus.

[SteamyTea]

Just a quick note, from much earlier on in the replies, you do not need liquid water for mould to grow and bacteria to multiply. You just need the right levels of relative humidity in the right temperature band.

 

There are two methods used to analyse condensation/humidity risk (statistical or analytical) both have failings. Bur both are better than anecdotal analysis.

It is not as simple as too high a humidity level in the building structure, or even condensation, WILL cause a problem, it depends on the structures tolerance to moisture and how long it can resist fungal or bacterial attack before self healing/repelling.

 

It really is a bitch of a topic, but the good side is that most houses, and specially new ones that are used properly, don't have a problem.