Marmox required? Timber plate to steel.

[DragsterDriver]

Evening  

 

building timber frame onto steel beams bearing on raised piles. Does this need marmox or similar- or would continuous pir internally eliminate any use for it?

 

this detail is very similar- I’ll have 50mm pir,   Vcl and 25mm service void internally.

 

[Iceverge]

It's an intriguing design. I modeled it in Therm and came to the conclusion it'll be fine without.

 

A couple of other improvements if you're chasing easy wins.  

 

The sole plate looks like 140x140. 140*45 would be better thermally. 

 

I assume you have 150mm PIR in the floor?  If you were to move the entire floor up until all your insulation was above the steel it would help. Alternatively save some cash by replacing 150mm PIR with 200mm EPS. It'll raise the floor level 50mm further from the steel and improve the thermal bridge. 

 

I would get the best timber treatment I could for the soleplate and especially the vertical timber in the web of the steel. These will be cold and gather condensation.  

 

If you were to glue something like this between the timber and the steel it might help reduce the condenstation risk on the timbers. Someone will be along to tell me off no doubt. 

 

[DragsterDriver]

6 hours ago, Iceverge said:

It's an intriguing design. I modeled it in Therm and came to the conclusion it'll be fine without.

 

A couple of other improvements if you're chasing easy wins.  

 

The sole plate looks like 140x140. 140*45 would be better thermally. 

 

I assume you have 150mm PIR in the floor?  If you were to move the entire floor up until all your insulation was above the steel it would help. Alternatively save some cash by replacing 150mm PIR with 200mm EPS. It'll raise the floor level 50mm further from the steel and improve the thermal bridge. 

 

I would get the best timber treatment I could for the soleplate and especially the vertical timber in the web of the steel. These will be cold and gather condensation.  

 

If you were to glue something like this between the timber and the steel it might help reduce the condenstation risk on the timbers. Someone will be along to tell me off no doubt. 

 

Thanks!

 

it’s probably more like below- I’m just wondering any merit in a thermal break or if it’s not really worth it  

[Iceverge]

To be honest you're pretty good as you are. I've attached some pictures of the Therm drawing. You can see how significant the steel is. It really is a tremendous conductor. You should aim to keep it as far away as possible from the inside corner. I've done a slight modification in the second two drawings by putting an extra 50mm of PIR between the screed and the wall. and putting 150mm x 25mm of PIR in the bottom of the service cavity. 

 

I reckon this reduces the internal thermal bridge from 0.0124w/mK to 0.0042w/mK or about three times better.

 

However it only would reduce your maximum heating load by 10w if your house had  a 50m perimeter! Like I said your design is fine as it is. 

 

 

[Iceverge]

Out of interest if you were to add a 45mm thermal break under the soleplate it would make it worse. Your internal thermal bridge would go from 0.0124W/mK to 0.0129W/mK.  Hardly significant , about 1/2 a watt extra power peak heating load but really the definition of throwing money away for nothing. 

 

With your existing design (unmodified) the external thermal bridge is -0.02w/mK which satisfies the Passivhaus criteria  <0.01w/mK. 

 

 

Edited November 23, 2021 by Iceverge

[DragsterDriver]

6 hours ago, Iceverge said:

To be honest you're pretty good as you are. I've attached some pictures of the Therm drawing. You can see how significant the steel is. It really is a tremendous conductor. You should aim to keep it as far away as possible from the inside corner. I've done a slight modification in the second two drawings by putting an extra 50mm of PIR between the screed and the wall. and putting 150mm x 25mm of PIR in the bottom of the service cavity. 

 

I reckon this reduces the internal thermal bridge from 0.0124w/mK to 0.0042w/mK or about three times better.

 

However it only would reduce your maximum heating load by 10w if your house had  a 50m perimeter! Like I said your design is fine as it is. 

 

 

that’s so impressive!

 

I've not really found anybody with a similar build to mine so there’s very little to reference.

 

thanks!