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

[maxdavie]

Hello, 

 

Does anyone know what is typically used for the element highlighted in red in the detail below? 

Pretty much all the details I've found online just say 'protective board'. Whatever it is it will have to be able to accept paint as it will be visible beneath the timber cladding. 

The detail shows: 

- a plywood timber 'cassette' 

- non-combustible Muliti-pro exterior lining 

- a timber rainscreen (battens, counter battens and Larch board-on-board cladding) 

 

There is a breather membrane over the Muilti-pro board and a DPM but I'm not showing it for drawing legibility. 

 

I imagine what I'm looking for is some sort of waterproof render carrier board that I can adhere to the slab edge and the 'cassette'. Then I'd apply sand/cement render to the small section of exposed surface and paint it. 

 

But, if anyone with experience/knowledge can steer me toward a particular product or key words/phrases to aid my google search or even correct my faulty assumptions that would be very helpful! 

 

Thanks

[ProDave]

Who designed that?  It just looks wrong to me, you are effectively putting a timber cassette down into a trench below ground level, relying entirely on that red board to keep water out, and no way for it to drain if any does get in?

[maxdavie]

Sorry! I designed it, but I'm not showing all the info in the detail, ie, its cropped and I've removed annotations. So your suspicion is totally understandable. 

 

For various reasons on this project I have to have a level threshold, hence the timber frame sitting below the ground level.

The rectangle to the left of the red board is a french drain and runs the perimeter of the building. 

 

 

 

[Thorfun]

there was a good thread if I remember correctly on this subject that gave me lots of ideas (assuming I understand your dilemma)

 

 

[Mr Punter]

18 minutes ago, maxdavie said:

the timber frame sitting below the ground level

 

That is not acceptable.  You cannot have the timber below ground.  Why not raise it up on blockwork?  You can still achieve a level threshold at the entrance(s).

[maxdavie]

Thanks Thorfun! - that's exactly what I'm after. 

 

And thanks Mr Punter for the comments. Its a difficult one. One of the purposes of this small building is to test a kit-of-parts I've been developing (a bit like the one seen on Grand Designs some years back where they'd assembled big plywood cassettes filled with insulation - see pic). So, I haven't got any flexibility on where the insulation goes and I don't plan on wrapping a continuous layer over the cassettes. If, like you say I raised the frame up on blockwork, I'm not sure how I'd maintain the same thickness of insulation. 

 

The building inspector and I thought that if you had a continuous drain around the perimeter of the slab to remove any water before it accumulated and had a DPM and second robust impermeable layer over this then we should be fine. 

 

In the future, I can just raise the entire raft up so the timber can sit above ground. But, for this project its not possible. 

 

But, you disagree - you don't think that combination of protection will suffice? 

[saveasteading]

4 hours ago, maxdavie said:

a continuous drain around the perimeter of the slab

This depends on being designed for storm quantities of rain, both to the drain and to wherever it outfalls, and rigorous maintenance.  Please ensure that for any failure of the drains there is an overflow / slopes that will take the water away.

 

AS Thorfun says, this has been discussed but come back with your own comments.

[Temp]

Fills me with horror but if you are determined...

 

This "red board" really needs to be some form of "basement tanking system" suitable for both concrete and wood (and bridging a joint between concrete and wood). Get one with a long warranty.

 

It looks like after fitting the "red board" you plan to do another concrete pour outside that? Why do that outer concrete pour? It strikes me it will just trap water. Wouldn't it be better just to back fill with free draining material and a drain pipe? I would see what the basement tanking company recommend. 

[MikeSharp01]

I think I get what you are trying to do can I ask a couple of questions. Do you have any housewrap type building paper on the outside of the timber cassette that can lap over the top of the red board to stop and water running behind the red board? (There will be a serious dew point issue behind the gladding anyway.) The red board will need to be impervious and very robust, so plastic come to mind and and joins will need careful attention. Have you considered wrapping the base, up both sides, of the cassette in some sort of membrane although this will be a moisture trap you might be able make it breath  upwards?   

[PeterW]

14 hours ago, maxdavie said:

For various reasons on this project I have to have a level threshold, hence the timber frame sitting below the ground level.

You don’t need your panel to be below ground level though to achieve that. 
 

13 hours ago, maxdavie said:

The building inspector and I thought that if you had a continuous drain around the perimeter of the slab to remove any water before it accumulated and had a DPM and second robust impermeable layer over this then we should be fine. 

that won’t work. And I don’t think you should be trusting the BCO on this ..! 
 

13 hours ago, maxdavie said:

One of the purposes of this small building is to test a kit-of-parts I've been developing

 I’m going to guess you have limited building experience but come from an engineering background ..? Have you looked at product liability insurance …?? 

[maxdavie]

Thanks for the comments everyone - very helpful. But, has also filled me with a deep sense of dread about this sunken timber cassette! (prob a good thing) 

 

 

6 hours ago, Temp said:

It looks like after fitting the "red board" you plan to do another concrete pour outside that? Why do that outer concrete pour?

I think the partial detail hasn't helped the discussion. So, see the full annotated detail below. 

You can see now that the hatched block to the left of the red board is a french drain (not more concrete). 

 

12 minutes ago, PeterW said:

 I’m going to guess you have limited building experience but come from an engineering background ..?

I'm an architect, but quite right Peter, I've not had a great deal of building experience (mostly large projects at pre-planning stages) 

 

1 hour ago, MikeSharp01 said:

(There will be a serious dew point issue behind the gladding anyway.)

I haven't carried condensation risk analysis yet, but may use the BuildDesk software to do so. I'm relying on 3 things in this respect : 1. the building isn't in constant use, any build up of condensation when the heating goes on, should have more than enough time to dry out when it goes off. 2. I'm using sheep's wool insulation which is hygroscopic, ie, it will absorb excess moisture and release it when humidity/temperature conditions allow 3. there is a 36mm ventilated cavity behind the rainscreen, the free flowing air should help carry moisture away. 

 

9 minutes ago, PeterW said:

You don’t need your panel to be below ground level though to achieve that. 

 

Problem I have with raising it up on something (upstand/blockwork etc) is maintaining the insulation thickness. Can't see a way around that. 

 

I have to say this decision is now harder to justify in the face of so much doubt! ........I may have to re-think....

 

 

[maxdavie]

[TonyT]

6mm ply on top of kingspan is going to be a disaster…

[TonyT]

Plywood will be rotten in 10 years….

 

 

build an upstand in concrete as part of the foundation or block to raise it off the floor and alter the internal wall to allow for insulation

[MikeSharp01]

Excellent to see the drawing with its annotation. You probably need to get a big washer (maybe 80-100mm dia) under the cap of the M16 resin anchor to avoid it just pulling through if it ever gets under load and if the timber has weakened as @TonyT points out it will very easily.

[maxdavie]

6 minutes ago, TonyT said:

6mm ply on top of kingspan is going to be a disaster…

Thanks TonyT. Its 6mm flooring ply over an 18mm floated deck. 

[maxdavie]

1 minute ago, MikeSharp01 said:

You probably need to get a big washer

Ah gd point - will run this past the engineers

[PeterW]

26 minutes ago, maxdavie said:

I'm using sheep's wool insulation which is hygroscopic, ie, it will absorb excess moisture and release it when humidity/temperature conditions allow

 It doesn’t work like that - you need to get a full analysis done as if you don’t you have the issue that moisture could go into the fabric not out of it. 

[CharlieKLP]

What did you use as inspiration for your detail there? Is that recommended by the people providing your wall cassette (what even is that).

 

I assume you’re using a timber frame. Just make sure you’re using enough insulation to fulfil the new part L. I also think you might have a warranty problem.

 

Shouldn’t it look something like this:

 

Edited March 29, 2022 by CharlieKLP

[maxdavie]

2 minutes ago, PeterW said:

 It doesn’t work like that - you need to get a full analysis done as if you don’t you have the issue that moisture could go into the fabric not out of it.

 

I wasn't going to have analysis done on this building  - its too small and infrequently used to justify the cost. I will do when it comes to building our home. 

But, from all the reading I've done, I have to say I think it does work like that. 

 

Hygroscopic materials absorb and release moisture from the air therefor lowering the air's vapour content, lowering its relative humidity and it's dew point and minimising the risk of condensation. The combination of the VCL, the breather membrane and the ventilation cavity should ensure its breathable and exits the fabric before it accumulates and causes damage. 

 

It you disagree it would be great to know why with an explanation and reference to facts....

6 minutes ago, CharlieKLP said:

I’m not sure why people make up things that are fairly standard details? What did you use as inspiration for your detail there?

 

Thanks for joining the convo! - it might help to read some of my initial posts for context. 

 

[CharlieKLP]

9 minutes ago, maxdavie said:

 

 

Thanks for joining the convo! - it might help to read some of my initial posts for context. 

 

I did, I still don’t get why you are trying to do here. 
 

you need to have all of your timber above ground, on block. 
Your slab should also be higher than the ground. If you want a level threshold then I can get you a detail for that part.

 

The board you are asking about is on my detail STS construction board. But really you need to sort out the wall frame and put it on block before you stick anything anywhere, and it doesn’t go underground.

 

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

Edited March 29, 2022 by CharlieKLP

[maxdavie]

1 minute ago, CharlieKLP said:

The board you are asking about is on my detail STS construction board.

amazing - thanks. 

 

2 minutes ago, CharlieKLP said:

I did, I still don’t get why you are trying to do here. 

 

See my earlier post - pasted below. 

15 hours ago, maxdavie said:

One of the purposes of this small building is to test a kit-of-parts I've been developing (a bit like the one seen on Grand Designs some years back where they'd assembled big plywood cassettes filled with insulation - see pic). So, I haven't got any flexibility on where the insulation goes and I don't plan on wrapping a continuous layer over the cassettes. If, like you say I raised the frame up on blockwork, I'm not sure how I'd maintain the same thickness of insulation. 

 

 

[CharlieKLP]

Oh yes that does make a lot more sense!

 

I’ve been a part of developing a few similar wall build ups, so I know the difficulties. I might actually have worked on the episode you’re talking about but it was a while ago!  We filled them with lambswool though. In hindsight should have been proper insulation.

 

you can just make the block work as thick as you need for the wall can’t you?

 

but you know, there’s no need to reinvent the wheel. Timber needs to be up on a block with a sole plate as the industry standard.

[PeterW]

(Caveat - seen the use of a VCL annotated on the plans now !)
 

You’re using a hygroscopic material below the ground level and it will have a fairly challenging environment to maintain its breathability and it will be in a semi-sealed location so it will not work as anticipated. Looking at the detail, there is a real need to do a condensation analysis as you have a non standard design. 
 

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. 
 

 

 

[SimonD]

7 minutes ago, PeterW said:

You’re using a hygroscopic material below the ground level and it will have a fairly challenging environment to maintain its breathability and it will be in a semi-sealed location so it will not work as anticipated. Looking at the detail, there is a real need to do a condensation analysis as you have a non standard design. 

 

I think you're putting it mildly. I doubt with the current buildup it's going to perform as intended at all as there are too many additional layers within the cassette wall buildup plus the ground/damproofing etc. IIRC sheepswool can also only absorb up to about 35% of its own weight in moisture before deterioration of performance.

 

This is one of the reasons why European detailing using timber frame/panels with natural insulations often specify a minimum height above ground of 300mm. Below that the detailing starts to become onerous and often requires replacement using EPS, for instance.

 

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.