Insulating Steel Box section over Bifolds

[STORYTELLER]

Hi,

Can't seem to get a sensible answer on this. SE has specified a steel for over bifolds...see below.

Question is how to minimise cold bridge? Can I/should I just pack with some say knauf 32 or 35 slab...for example? It's about 3.15M Long

 

[noseall]

Hi,

Can't seem to get a sensible answer on this. SE has specified a steel for over bifolds...see below.

Question is how to minimise cold bridge? Can I/should I just pack with some say knauf 32 or 35 slab...for example? It's about 3.15M Long
View attachment 366408

Foam fix plasterboard.

Or, if you have the tolerance on the reveals - foam fix thin insulated plasterboard.

 

[STORYTELLER]

Foam fix plasterboard.

Or, if you have the tolerance on the reveals - foam fix thin insulated plasterboard.

OK, so you wouldn't bother insulating within the steel box...? just the cavity and then plasterboard over on the internal...?

 

[tony1851]

How would you insulate effectively inside the section? The top and bottom flanges will still be cold bridges.

More generally, many inexperienced SEs specify these (expensive) box sections in these situations. They do it because the outer skin pressing down on the projecting bottom plate causes torsion on the beam. While students, they are taught that closed box sections are better at resisting torsion than open 'I' beams; so they dutifully specify RHS beams in these situations without thinking.

But there is a very simple check to ensure that the torsion forces may not be excessive and using an 'I' beam (or an 'H' beam on it's side) can be just as effective, and cheaper.
Its also easy to pack glass wool or similar into each side of the web.

You still have the problem of insulating over the edges so as Nose suggests, just glue insulated plasterboard (PIR) over the exposed parts.

 

[freddiemercurystwin]

@tony1851 Presumably two separate beams can be specified, a box or I beam for the inner skin and an angle for the outer skin?

 

[freddiemercurystwin]

@STORYTELLER I've seen a few threads on here where Mr BCO expects to see the bottom leg of the steel protected from the weather, possible to sheath it in something during construction, extremely difficult to achieve as an afterthought.

 

[STORYTELLER]

@STORYTELLER I've seen a few threads on here where Mr BCO expects to see the bottom leg of the steel protected from the weather, possible to sheath it in something during construction, extremely difficult to achieve as an afterthought.

ah, so you mean wrapping the steel plate with some insulation? even though it will be mortared and blockwork on it on the outer leaf..?

 

[STORYTELLER]

How would you insulate effectively inside the section? The top and bottom flanges will still be cold bridges.

More generally, many inexperienced SEs specify these (expensive) box sections in these situations. They do it because the outer skin pressing down on the projecting bottom plate causes torsion on the beam. While students, they are taught that closed box sections are better at resisting torsion than open 'I' beams; so they dutifully specify RHS beams in these situations without thinking.

But there is a very simple check to ensure that the torsion forces may not be excessive and using an 'I' beam (or an 'H' beam on it's side) can be just as effective, and cheaper.
Its also easy to pack glass wool or similar into each side of the web.

You still have the problem of insulating over the edges so as Nose suggests, just glue insulated plasterboard (PIR) over the exposed parts.

I felt creating and warm knauf bubble inside the steel would slow down the rate of heat loss as cold moves from the plate into the box... similar to how a catnic is insulated with foam... Would this not be the same principle? Ive read that these box and plates can be considered over-engineered.... especially with smaller openings.... but maybe it's partly because it is next to the edge of the new extension corner with the minimum (665mm ish) distance from corner to opening. Either way, am having a pricey (ish) Korniche 3 door 285cm bifold under there so more than happy to pay a few hundred quid extra for some belt n braces stiffness and torsional benefits... tho... if it's turns into cold radiator up there......

 

[noseall]

I felt creating and warm knauf bubble inside the steel would slow down the rate of heat loss as cold moves from the plate into the box... similar to how a catnic is insulated with foam... Would this not be the same principle? Ive read that these box and plates can be considered over-engineered.... especially with smaller openings.... but maybe it's partly because it is next to the edge of the new extension corner with the minimum (665mm ish) distance from corner to opening. Either way, am having a pricey (ish) Korniche 3 door 285cm bifold under there so more than happy to pay a few hundred quid extra for some belt n braces stiffness and torsional benefits... tho... if it's turns into cold radiator up there......

Insulate the hollow section - yes. Fix plasterboards with foam etc.

 

[freddiemercurystwin]

ah, so you mean wrapping the steel plate with some insulation? even though it will be mortared and blockwork on it on the outer leaf..?

I mean the exposed leg of the plate that will be protruding out above the window. Some BCO's expect this steel to be protected from the elements.

 

[aveatry]

If you want to fill it you could use expanding foam - attach a long tube, push it to halfway and slowly draw it out as you squirt then do the same from the other end.

 

[tony1851]

@tony1851 Presumably two separate beams can be specified, a box or I beam for the inner skin and an angle for the outer skin?

Yes, as long as the span and loading aren't too great because angles can of course twist.

 

[Ivor Windybottom]

If you want to fill it you could use expanding foam - attach a long tube, push it to halfway and slowly draw it out as you squirt then do the same from the other end.

I'd worry about trapping condensation and causing rust from within by filling it with foam.

Foam inside it would make very little difference anyway, as there's a very conductive path of metal at the top and bottom.

It's obviously a lot less insulating than block, but there's still the cavity.

Anything metal within the wall is going to create a less well insulated zone, but it's still much much better insulated than the bloody huge slabs of glass below it. Double, triple or coated with magic materials, windows leak heat way more than any wall does. If you were really bothered about energy use then you simply wouldn't be fitting bifolds at all.

 

[tony1851]

much better insulated than the bloody huge slabs of glass below it.

Correct - the quantity of heat lost via the glazing will be far greater than that through the metal bridging. But the point is that that small area of bridging will attract condensation bringing localised dampness and possible mould growth.

 

[Ivor Windybottom]

My guesstimate is that its U-value will vastly exceed the insulation of the sealed glazed unit, even if allowing for the fact that there will be a degree of cold bridging via the metal lip to the outside. It has hit and miss welding to minimise this effect, its thermal mass will mean that it remains much closer to the room temperature than outside temperature.

I'd suggest that if you built a sauna within the room while making endless cups of tea and using a wallpaper steamer then the glazing would be running with condensation long before any dampness appeared on the wall around the steel joist.

I really don't see the issue, and can't see how it could be done any other way - at least not without having a visible steel or concrete joist in the outer wall that nobody wants.