Timber frame spanning

[Pocster]

Hey all,

 

My architect's original spec was for a 100mm timber frame. It would appear all suppliers min is now 140mm. It's not a problem apart from 1 issue.

The steel works at the time etc. were designed for 100mm. So (please see photo's); whilst the timber frame can follow my block & beam but by the time it reaches the stairwell there's just the steel left spanning (which isn't wide enough now). I assume quite sensibly ( I hope! ) that the timber frame can't span that gap unsupported?

I could put a nice 2m lintel across. But as the timber frame is structural I assume I need proper advice on that?

Timber frame supplier hasn't got back too me yet; but I think they'll say 'your problem' .

 

 

Edited April 15, 2017 by pocster

[Pocster]

I'm guessing there are different ways to solve this. My spec confuses me. On the side walls we have 50mm cavity. But no cavity on 2 end walls; why would that be?

 

Does it matter whether the insulation is external to the timber frame ; in the timber frame void or where possible even both?

 

Is it critical?

 

Other timber frame supplier quotes offer different 'systems' i.e. they basically ignore my architect's design for the insulation make-up. (I'm guessing as the timer frame company 'sign it off' effectively with their SE I follow their guidelines....)

 

 

Edited April 15, 2017 by pocster
Because I'm thick

[Pocster]

I'm guessing the 'cavity' only applies when the insulation is outside the timber frame? i.e. thermalite external wall tied to timber frame (where insulation is inside the timber frame voids) does not require an external cavity?

[Jeremy Harris]

My guess is that the reason for you getting quotes for a deeper frame is probably to do with the minimum insulation requirements that the frame companies are used to working to.  A 140mm frame might "just" meet the minimum requirements of Part L1a when filled with insulation, whereas a 100mm frame probably won't. 

Where is the insulation going in your design and how is the thermal bridge at the sole plate being mitigated?  It looks as if the steel is on the "cold" side, to me, which I think could lead to potential problems.

 

The position of the insulation is fairly critical, as the timber studs can thermally bridge the insulation, so reducing its overall effectiveness.  This can be mitigated, but it needs careful detailing to get the integrated design right, particularly critical areas like the wall to floor junction, where there is the potential to both increase heat losses and the risk of sole plate condensation if the design isn't carefully thought through.

[Pocster]

Hey!

 

The timber frame sole plate will sit on DPC on a 140mm wide (140mm deep) brick 'wall' that I have to build.

The majority of timber frame designs  have the insulation in the timber frame void - which is fine for me even if different to the architect's spec.

 

What I am tempted to do here is have the timber frame  (just at the front as in the photo's) tight to that concrete block run (effectively the external wall); then the timber frame would sit on the block work which in turn is on the steel i.e. supported! so insulation internal.

 

If BCO complains; then I could insulate externally. Why though does external insulation (and say then clad) require no cavity?. This is my real concern....

Edited April 15, 2017 by pocster

[Jeremy Harris]

How are you ensuring that the sole plate of the frame can't get cold enough to pose a condensation risk though?  The reason I ask is because this design detail is one that can be problematic, as you can get a situation where water vapour passes from the outside to the timber frame, and can then condense at any cold regions.  There are ways to mitigate this, but it needs careful detailing to get right.  In part it's an issue caused by the need for increased insulation effectiveness, as this can then lead to parts of the frame getting a fair bit colder than they would have done in the past.  It's a well-known issue, as interstitial condensation, and consequent rot in the frame and sole plate, was one of the reasons that one of the major manufacturers got caught out around 40 or so years ago.  Those failures gave timber frame an undeserved poor reputation in parts of England, and caused insurance companies to be a bit cautious about covering timber frame for a couple of decades afterwards, at least in England (Scotland has been building houses like this for years without problems, though).

[Pocster]

I see! (worried look on face).

The answer is ..... I have no idea!. Architect hasn't spec'd anything for that; nor has timber frame company said anything.

 

Is the sole plate though equally OSB/ply clad with breathable membrane? or is it just the 'structure'?

 

Erm, so how should I solve the sole plate condensation issue??? 

[Pocster]

Would MVHR not solve this issue?

[Jeremy Harris]

The usual sort of detail will ensure that there is some form of insulation under, and often outside of, the sole plate.  There are some details here on the forum of different ways this can be achieved, often using materials like thermalite blocks under the frame.

 

MVHR is irrelevant, as the house will have a vapour control layer (VCL) on the inside, to prevent internally generated water vapour from penetrating the timber structure.  The main source of moisture will be from outside, with water vapour moving into and out of the timber frame with changes in temperature and humidity.  As long as no part of the frame can drop below the local dew point temperature for the vapour concentration at any time, then there's no problem.  If the frame can cool to dew point then liquid water will start to condense, and the chances are that it won't be able to dry out, as that requires a significant amount of heat energy.

[Pocster]

Hmmm, yes been googling!

 

I *assumed* I'd have to build my sole plate support structure (block work) not from thermalite . Though I do see

 

https://www.travisperkins.co.uk/H%2BH-Celcon-Brick-Standard-Coursing-Unit-65mm-Pack-of-600/p/753178?googleApiClient=gme-travisperkinstrading&googleApiVersion=3.19&googleTagManagerTrackingId=GTM-KGT66B&siteName=Travis+Perkins

 

as mentioned in another thread.

 

I could certainly squeeze some insulation over the sole plate (i.e. between external wall and timber structure).

 

Sorry; yes MVHR is irrelevant you are quite right!

[Jeremy Harris]

This detail really needs to be right, and will usually have been checked using something like WUFI by the SE that designs it.  It's not easy to just get this right by guesswork.  There may well be a standard detail that you can use, though.  It's not something I've looked at closely, as our frame and foundation/sole plate detail was part of an integrated package that was signed off as a whole by the frame companies SE.

 

TRADA have some standard details, it seems.  You can get a preview of them here: https://www.trada.co.uk/downloads/publications/Timberframeconstruction.pdf plus there is more detail on sole plates here: https://www.trada.co.uk/publications/download/?id=DD978FA1-D08A-4E16-90F8-19298ABB6B1F

Edited April 15, 2017 by JSHarris

[Pocster]

Ok thanks