unforseen cavities/cold bridges in timber frame help

[hotnuts21]

As they say a picture tells a thousand words, so I have attached some photos.
I was scrolling through some pics of my warm flat roof build and realised that the end of the building has two uprights timbers which are effectively a cavity now as they are blocked from inside/outside and now the top. Im wondering if I should drill a hole one of these timbers and fill it with expanding foam??  Although they are in line with the roof/floor joist timbers, they will I presume cause a bit of an effect to the heat loss of the building?  Also above all the doors and windows the erectors uses square metal box section for the lintels instead of timbers (Prob cheaper at the time) and so therefore again another cause for panic for heat loss and cold bridging.

The outside of the TF shell is getting a 60mm thick woodfibre insulation, and I could insulate with PIR the inside of the timber headers (if thats what they are called) but I cant insulate the steel lintels on the inside as thats the plasterboard level. Could I also drill small holes along the steel and fill with expanding foam too?

Thoughts?
 

[Simplysimon]

i certainly don't see any reason why filling steel full of foam would be an issue, certainly no issue behind the timber

[hotnuts21]

This may be a silly question but is there a particular expanding foam I should use? One that maybe has better insulation values or expands a lot to ensure I fill the entire length of the beams?

[AliG]

Someone else may know better than me, but I am not sure that filling the steel with foam will make much difference.

 

The path for heat loss is mainly through the steel itself, not through the air inside the steel. The heat would still travel through the steel.

 

Is there any option to put insulated plasterboard in these areas on the inside of the steel?

 

You would probably see more benefit filling the timber cavities.

 

However, looking at the pictures, putting insulation on the inside of the wood and also taping up the area so that cold air cannot get in from the city might be better. i.e. put insulation in the gap between the last two joists as otherwise the inside timber of the cavity is a cold spot. This is what we will be doing on our warm flat roof, although it does not have the small cavities that you have there.

 

[hotnuts21]

HI AliG, yes that was my worry with the Steel and one of the main reasons I posted on here.

I did wonder if just putting insulation on the inside of the timber upto the flat roof would suffice, especially as i have some left over 150mm celotex. I can bring this right down to ceiling and seal all around it with illbruck FM330 air seal foam, I could also run the vapour barrier across the bottom of the joists as well as up the walls.

I may well have to loose some space inside the room and insulate on the inside of those beams, we are using sheeps wool elsewhere so might use 50mm thick of that in a plasterboard box along the beam, else i may end up with cold spots on the walls.   Its all more work and cost 😞

 

[Mr Punter]

PIR will insulate far better than your wood fibre.  If you swapped for Kingspan Kooltherm your insulation would be twice as good.  You could just do this around the poorer insulated bits.

[AliG]

3 hours ago, hotnuts21 said:

I did wonder if just putting insulation on the inside of the timber upto the flat roof would suffice, especially as i have some left over 150mm celotex. I can bring this right down to ceiling and seal all around it with illbruck FM330 air seal foam

That should work

 

3 hours ago, hotnuts21 said:

I could also run the vapour barrier across the bottom of the joists as well as up the walls.

 

That is what MBC were going to do with my parent's house, but it means you have to put battens under it for lights etc. We are putting a VCL on top of the roof which is the spec for the EPDM we are using. Our VCL runs up the walls then under the ring beam and over onto the roof, to seal things up.

 

However, if you are selling PIR into that space then you just need to run the VCL up the wall and tape it to the PIR/Joists. However, this assumes a VCL above the roof deck. There has to be one somewhere.

 

[hotnuts21]

1 hour ago, AliG said:

However, this assumes a VCL above the roof deck. There has to be one somewhere.

Yep the flat roof has Alutrix600 vapour barrier on the deck, its the bit between the bottom of the joists and the bottom of the deck I was never sure about, whether we cut it round the joists and tape it to the desk of the flat roof.  (warm roof buildup on top of the joists).  Sounds like I might be onto something

[AliG]

MBC seem to have figured that out. We were discussing how to do this as sealing around the joists is awkward.

 

They tape it to the top of the wall plate then wrap it round the outside of the ring beam so the joists are air tight. But it has to be done whilst the walls are going up.

 

Your solution of foaming PIR between the joists was my suggestion before we knew about this.

[hotnuts21]

I think, and I need to research this more, there is some tape that I can put on the bottom of the joists, I can then run the VB up the wall and along the bottom of the joists sticking to this double sided tape, when I install the plasterboard it screws through the tape/vb and self seals the hole, but I need to look into it more, as its only a small area, it means I could also run it up into the skylight well too which is pretty big on a warm roof!