Any feasible method to reduce RSJ cold bridge

[tanneja]

Hi all,

 

We are fairly progressed with our build and have noticed the top steel for the 5m wide patio sliders doesnt have much in the way of insulation.  Sliders will be very good 3G, but somehow the detail of around windows and these sliders hasn't had lots of focus with all else that goes on.

 

We are now at the stage that the steels are in, they have been timber capped and the ceiling joists attached, but the sliders won't get measured for 2 weeks, with render planned after that.

 

What if any salvage work should be done to reduce the cold bridge? I dont know whether the finished ceiling will come lower so that the steel is not visible, that probably needs to happen for regs, or maybe the sliders will be fitted more recessed so that the slider frame is flush with the internal corner of the steel.  In either instance, would one look to a product such as aerogel? We are keen to not sacrifice any of the opening size if at all possible.  Unsure what kind of strength any product would need to have given the enomous sliders.

 

Caveat that I am not technical or on site atm, so this is my basic understanding of what we have.

Edited August 28, 2020 by tanneja

[the_r_sole]

what construction method are you using for the house?

[tanneja]

Hi @the_r_sole

 

To be clear this is an extension, so 300mm cavity block work (100 block, 100 gap with insulation, 100 block). 

 

Floor is 150mm concrete slab, 50mm sand blinding, 120mm insulation, 50mm liquid screed.

Edited August 28, 2020 by tanneja

[Temp]

 

21 minutes ago, tanneja said:

To be clear this is an extension, so 300mm cavity block work (100 block, 100 gap with insulation, 100 block). 

 

Unless I'm missing something the UB/RSJ won't be 300mm wide. 

 

Has someone specified a particular size UB/RSJ for a reason compared to something like an insulated lintel? Whats the span? Heavy doors? 

 

Here is a section google forund..

 

[Temp]

PS The BCO will almost certainly want the beam clad with plasterboard to meet fire regs. So no exposed inner edge should be showing.

[Conor]

We have our beam set inside from the wall (ICF) centre and the doors will hang on triangular brackets welded on the side of the beam. The beam is then packed with EPS same as the ICF and rendered.

[PeterW]

If it’s a standard build up then the steel should be sat on the inside blockwork, leaving you a face on the external blockwork to fill with “something” then render over. 
 

Do you know the steel sizes ..? If there are two (overkill at this point if the sliders are floor to ceiling and it’s a cold roof) then you should have a void between the two steels and you can basically pump this with foam. That isolates the two steels. 
 

if it is as drawn then the steel is in my opinion in the wrong place. 

[tanneja]

1 hour ago, Temp said:

 

 

Unless I'm missing something the UB/RSJ won't be 300mm wide. 

 

Has someone specified a particular size UB/RSJ for a reason compared to something like an insulated lintel? Whats the span? Heavy doors? 

 

Here is a section google forund..

 

Sorry all, this is likely what we have, I have measured since I got home and is approx 100mm steel, with something like a 300mm plate welded on the underside, with block work flush around that plate.

 

On the inside then, is the guidance to, as per the diagram, to have the finished ceiling height lower than the level the steel emerges, presumably due to the encasement in fire rated plasterboard as mentioned? What degree of insulation should be present with that encasement? 

[Temp]

17 minutes ago, tanneja said:

On the inside then, is the guidance to, as per the diagram, to have the finished ceiling height lower than the level the steel emerges, presumably due to the encasement in fire rated plasterboard as mentioned? What degree of insulation should be present with that encasement? 

 

It depends on the height of the roof/celiling. The beam is either "in the ceiling" or "in the wall" if you get what I mean.  Ours are in the wall so the inside had vertical patches of plasterboard covering the beam and then the wall was plastered.

 

It sounds like your beam is higher (or the ceiling lower) so the beam is best hidden in the ceiling as per that cross section. Either use taller ceiling joists and notch them for the bottom of the beam or fix battens to the underside so the plasterboard is a bit lower and covers the beam.

[PeterW]

Ok so that’s a standard detail. Not a lot you can do to mitigate the loss 

[Temp]

42 minutes ago, tanneja said:

On the inside then, is the guidance to, as per the diagram, to have the finished ceiling height lower than the level the steel emerges, presumably due to the encasement in fire rated plasterboard as mentioned? What degree of insulation should be present with that encasement? 

 

Re: Fire protection. 

 

I think this is still current.

 

https://www.planningportal.co.uk/info/200130/common_projects/17/extensions/9

 

Quote

If the beam is steel then it should normally be protected against fire so that it will have 30 minutes resistance to fire (if measured in a standard test). There are different ways that this may be achieved, but the most common is the use of two or more layers of properly fixed plasterboard - the thickness of which will depend on the manufacturer's specification.

 

 

 

 

 

Re: Thermal Insulation:

As much as you can get as per the cross section I posted. Ideally the roof members would rest on top of the beam rather than against the web so there is more room for insulation above the beam.

 

Is there something (like a window on the first floor) stopping you making the roof thicker by putting the insulation above the ceiling joists?

 

Edit: I see your diagram shows a "cold roof" construction that requires ventilation. Could you switch to a "warm roof" construction that puts the insulation above the rafters and UB and doesn't need ventilation?

 

https://www.jtcroofing.co.uk/news/warm-flat-roof-cold-flat-roof/

 

 

 

 

 

 

 

 

 

 

 

Edited August 28, 2020 by Temp

[tanneja]

@Temp I think the limiting factor is next door's flat roof. If we went warm room it would be far higher than the existing pad stone that was there before, and difficult to say was part of planning. I would be in favour of it if we thought it was reasonable.

[tanneja]

2 hours ago, Temp said:

It depends on the height of the roof/celiling. The beam is either "in the ceiling" or "in the wall" if you get what I mean.  

This is gold! I didn't concieve that we could build oit the wall, i had in my head that ceiling height would have to be lowered, i offer my sincere thanks for this jolt to the brain!