Wall to floor joint and DPM

[GraemeHM]

I hope the attached drawing makes some kind of sense. We are just about to start digging down for our extension. There is fairly straight forward trench foundation and concrete pad. We are planning doing a single pour for both trench and pad. Then a single course ring of 140mm wide blocks (excluding the doors) to build off. 150mm (400mm c/c) stud work filled with Rockwool with the wall and floor build-up shown.

 

My question is about the DPM and upstand on the outside. There isn't much detail on our architect BC drawings and want to check with the collective knowledge on how good (or otherwise) you think this construction is. 

 

Thank you!

Detailed structure x-section 31-3-21.tif

[Mr Punter]

Did you consider using a Marmox or Foamglas Perinsul course under the timber frame to stop cold bridging?  Also, there should be 150mm min to the DPC.

[GraemeHM]

@Mr Punter Thanks. I did think about marmox as it came up on another post, but I looked at the cost and whilst I’ve tried to do a good job on the insulation we are tight on budget.

 

So on the DPC is that 150mm up from the outside ground level and under the sole plate? How important is that?

[Mr Punter]

The ACO will help a bit.  You could bring the cement board down a bit.

 

You are trying to have the external ground at the same level as your internal floor and it won't work out.  The DPM under the Celotex is almost trying to do the job of below ground tanking.

[GraemeHM]

@Mr Punter Do you think the attached would cut the mustard. Following your suggestion I have increase the difference between outside and inside and dropped the render carrier board down. Also added detail about perforated pipe and French drain. I am trying (I suppose) to keep to a single block on the perimeter. Many thanks

 

 

Detailed structure x-section 31-3-21 2nd.tif

[tonyshouse]

I don’t like the sole plate or the thermal bridging it brings. I have had this before and suggested that condensation may happen on top of the dpc, not good . I would analyse the thermals and do something to mitigate before building 

[GraemeHM]

@tonyshouse Thanks for the input. Thermal bridging I understand, but what analysis of thermals are you suggesting so I can find out more. Others have suggested Marmox but it's hellish expensive are there any alternatives that I should look at?

[tonyshouse]

Aerated concrete should work. The problem is with the wood as it is uninsulated and on top of something cold, at the outside face temperatures will be outdoor temperatures and so as this can be very cold condensation can happen. 
 

I recommend adding insulation across the dpc on the outside, if it was mine I would change the sole plates for 100x50 replacing the missing bit with pir

[GraemeHM]

@tonyshouse Thanks for that, I appreciate input. 

 

I have read a bit about condensation risk in timber frame at the sole plate, but hadn't followed up on it. So if I can use an aerated concrete block then this should reduce that risk, right.... so what about this... please stay with me....

 

Earlier @Mr Punterpointed out that I should have a 150mm upstand to ground level, so following his input and yours, I could use two blocks. The base on its side, the send on its edge. So that gives me an extra 100mm to get my upstand. If both blocks are aerated and I face the top block with PIR behind the sts construction board would that reduce my risk of cold bridging from the outside and hence condensation. I appreciate that the concrete slab could (possibly would) cold bridge from the underside through two aerated concrete blocks, but hasn't that got to reduce the risk?

 

Please see attached...What do you think?

 

Cheers

 

On a side note my BC design guy has specified 7.3n blocks which for a single story extension is possibly a bit of overkill?

 

 

 

Detailed structure x-section 31-3-21 two block +.tif

[tonyshouse]

3.5N should be OK especially as they are wider than 100mm

 

the cold bridge through the sole plate may still allow condensation on top of the dpc , need to add outboard insulation to reduce that risk 

[GraemeHM]

So if I understand correctly, you say that insulation on the outside of the walls?

 

Originally I had wanted to insulate on the outside of the walls too i.e. on the outside of the OSB then the batten, render board etc. I was told by the render technical guy that this was problematic. Any thoughts? Should I recheck this info?