Anyone know a good product/treatment for the exposed part of the timber frame between the ground and rainscreen cladding?

[maxdavie]

40 minutes ago, CharlieKLP said:

don’t you have some technician friends you can ask to help?

Annoyingly not..... just relying on books, internet, manufacturer's guidelines/advice, common sense and now members of this forum. 

 

31 minutes ago, CharlieKLP said:

I might actually have worked on the episode you’re talking about

Ah really? - sounds like U-build's system? - if its lambswool...think I remember them using that. 

 

33 minutes ago, CharlieKLP said:

 

but you know, there’s no need to reinvent the wheel.

Yes, absolutely agree and this is not my intention. Annoyingly this particular condition calls for something bespoke, so I'm trying to apply all the principles of proven detailing to a new assembly of components. 

 

26 minutes ago, PeterW said:

it will be in a semi-sealed location so it will not work as anticipated

Yes, I was thinking moisture would find its way - forced by vapour pressure - toward the breathable part of the wall. But, as you say, I think I'll have some formal analysis done. 

I

28 minutes ago, PeterW said:

Thermafleece only gained its BBA cert (02/3950 which was subsequently withdrawn) using a fully ventilated cavity on the outside - you’re not doing that with the design you have. 

Hadn't thought about this......will need to investigate further - thanks for flagging that. 

[maxdavie]

35 minutes ago, SimonD said:

I personally wouldn't even trust a desktop condensation/moisture analysis but would be looking to build a test rig in situe with some hygrometers within the structure for at least a full year cycle.

 

Okay. I think I've heard enough. I'm going to have to redesign this little pilot project so that the RC slab and internal FFL can sit above ground level. I don't want to force this if the its just wrong. Thanks all for the advice! Painful to have to return to the drawing board, but seems like its unavoidable. 

[Thorfun]

I'm not as knowledgable about this stuff as those already commented but if you're going back to the drawing board can you not design it with an insulated slab? that way you can sit the cassette on top of the slab as recommended and have loads of EPS insulation below ground to reduce thermal bridging.

 

unless I've completely got the wrong end of the stick here of course.

[Onoff]

Imho you want 150mm min below your timber cladding and block pavers or rain will just splash up and soak the ends.

 

 

[Thorfun]

29 minutes ago, Onoff said:

Imho you want 150mm min below your timber cladding and block pavers or rain will just splash up and soak the ends.

 

 

+1. and this is a Trada guideline as well. (actually I think they say 200mm - 250mm, I'll go grab the book and check!)

[Thorfun]

12 minutes ago, Thorfun said:

+1. and this is a Trada guideline as well. (actually I think they say 200mm - 250mm, I'll go grab the book and check!)

 

ref: External timber cladding - 3rd edition; Patrick Hislop, Peter Kaczmar and Lewis Taylor; TRADA publication

Edited March 29, 2022 by Thorfun
added reference

[maxdavie]

1 hour ago, Thorfun said:

can you not design it with an insulated slab?

This was my original intention, but it doesn't suit my system. I appreciate this approach works well with other frame types. 

Here's the detail I adapted - this is from a book of Passivhaus details. 

 

[maxdavie]

43 minutes ago, Thorfun said:

+1. and this is a Trada guideline as well. (actually I think they say 200mm - 250mm, I'll go grab the book and check!)

Thorfun, OnOff - thankyou! - this is very good to know. 

I'll have to download and read a copy of the TRADA manual 

[Thorfun]

1 hour ago, maxdavie said:

This was my original intention, but it doesn't suit my system. I appreciate this approach works well with other frame types. 

Here's the detail I adapted - this is from a book of Passivhaus details. 

 

I was talking about insulation below the slab. here's an example that many have on here offered by MBC https://www.mbctimberframe.co.uk/passive-house/passive-foundations/

[maxdavie]

17 minutes ago, Thorfun said:

I was talking about insulation below the slab.

Yes - so was I, and that was my original intention.

I don't think its going to work with my system, but I think I'll have another check with my engineers - my timber cassette would have to bear mostly onto the EPS which I can't imagine will fly. Still, worth another look as it might be the better way to do as you say. 

 

In the meantime, here's my re-designed detail incorporating comments. There is a bit of a thermal bridge there still. I'm not sure how much of an issue that will be, but I've decided to get a quote for some U-value and condensation risk analysis. 

 

Thanks again for all input! 

 

 

Edited March 29, 2022 by maxdavie

[Onoff]

😂 I've been playing with a section for a garden room on similar lines. Timber cladding / rain screen not shown...well not finished. 

 

 

Don't forget to add a good insect mesh detail at the bottom!

[Thorfun]

34 minutes ago, maxdavie said:

I don't think its going to work with my system,

I don't get how your system is, in essence, any different to a SIPS panel or a twin-wall system? again to refer to MBC's drawings https://www.mbctimberframe.co.uk/passive-house/passive-wall/

[maxdavie]

28 minutes ago, Thorfun said:

I don't get how your system is, in essence, any different to a SIPS panel or a twin-wall system? again to refer to MBC's drawings https://www.mbctimberframe.co.uk/passive-house/passive-wall/

Yes - you're right. There's not much of a difference.

I abandoned that approach a while ago because my understanding was that doing it like that meant that you had to have inner structural timber studs bearing onto the slab and an outer 'truss' of timber - much like a Larson Truss - to increase the thickness of the whole buildup. 

 

But, maybe I jumped to a hasty conclusion - as you say, if they do it like that with SIPS.....

 

I meet with my engineers tomo and can ask 

[maxdavie]

I'm not sure my plywood cassettes can bear partially onto the slab is what I mean - ref image below. 

[maxdavie]

50 minutes ago, Onoff said:

Don't forget to add a good insect mesh detail at the bottom!

thanks! 

[Thorfun]

9 minutes ago, maxdavie said:

I'm not sure my plywood cassettes can bear partially onto the slab is what I mean - ref image below. 

ok. I guess that's one for your structural engineer.

[CharlieKLP]

That looks so much better!