Insulating Steel Box section over Bifolds

[matlob]

Someone may correct me but I was told tile backer board can be used between the steel and plasterboard to prevent cold bridging?

 

[Ivor Windybottom]

I suppose that would be possible if the board was the same thickness as the plaster or plasterboard used on the rest, in which case it wouldn't be thick enough to insulate much.

I'd worry about hairline cracks where it meets whatever is used for the rest of the wall, even with scrim tape and the best plasterer in the world.

I don't think you'd ever get back even the cost of the tile board in reduced heating costs. It's just not worth worrying about.

 

[matlob]

I don't think it was recommended for insulation, just to stop and cold/damp coming through the bottom of the steel and messing up the plaster/paint.

The tile backer was to sit between the steel and the plasterboard. It was only a small part of a lintel that would have touched the plasterboard.

 

[freddiemercurystwin]

Thermablok Aerogel Insulation Strips - Thermablok - Intelligent Insulation - High Performance Thermal Insulation

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[noseall]

I don't think you'd ever get back even the cost of the tile board in reduced heating costs. It's just not worth worrying about.

The main and immediate issue here is condensation (cold bridging), rather than heat loss etc.

As I have stated earlier, the simplest remedy is to to use insulated plasterboard, if you have the space or foam fix plasterboard, if not.

 

[Ivor Windybottom]

It's exactly how any wide opening anywhere is built. There's nothing special about it, so don't see the need for special attention.

If it was likely to result in condensation then I'm sure everyone else would address it, but there's a pane of glass directly below that will be much colder so will do a very good job of removing moisture from the air as condensation long before the dew point for the much warmer plaster surface over the beam is ever reached.

It will all be fine!

 

[Ivor Windybottom]

The tile backer was to sit between the steel and the plasterboard. It was only a small part of a lintel that would have touched the plasterboard.

The joist itself is 100mm wide so would be flush with the surrounding block on its face.

You could add something underneath though, but then you'd have to be careful not to end up lower than the face of the door frame. The door needs to reach the top of the brick opening outside, so you have little space to play with.

 

[noseall]

It's exactly how any wide opening anywhere is built. There's nothing special about it, so don't see the need for special attention.

Not necessarily no. Some are built using thermally broken insulated proprietary lintels, others are built using two pieces of steel (inner and outer leaf).

Were talking about steel, in direct contact with the outside. The nuisance of localised condensation appearing as damp patches or mould or peeling wallpaper, is very real.

 

[Ivor Windybottom]

I agree, I think we're heading towards insulating its underside and not its face?

There's something of a thermal semi-break between the plate and box sections with the hit and miss welding, plus its general thermal mass that's mostly within the room.

It would be done after fitting the doors anyway, perhaps use PIR or tileboard of a thickness selected according to the height of the door frame top?

 

[Ivor Windybottom]

Is 3.15m unusually long? I guess that's the reason for it.

In which case the thermal loss from the glass will be huge anyway, so any heating loss isn't worth worrying about. The room pretty much doesn't have a wall!

 

[noseall]

Is 3.15m unusually long?

Not especially no. It may be that it's supporting a flat roof or a ridge beam above, i.e. vaulted roof etc.

 

[robinbanks]

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

Oh no, not this nonsense again

Question is how to minimise cold bridge?

It would be helpful to see the full proposed detail on plan, rather than just the cross section of the steel

 

[Ivor Windybottom]

Oh no, not this nonsense again

I'll really struggle against such a carefully considered argument.

Rust or not, there's absolutely no point in insulating the inside of a cavity that's fully bridged by 6.3mm steel across its top and bottom between its inner and outer faces. Its outer face is only in the cavity, not outside, and there's only partial welding between it and the outer lintel, deliberately to limit the cold bridging.

The much colder surface will be the underside of the reveal, where the horizontal plate directly protrudes outside. This may be worth cladding, if the height of the top of the door frame would allow it. Which is down to the design of the door and how much gap there is at the top.

 

[robinbanks]

Don't disagree with any of that

Still waiting for the OP to post a full drawing cross section of the detail; someone has hopefully thought about how this is to be constructed..

prevent cold bridging?

Careful with the use of the word "prevent" - there are certainly various degrees of reduction depending on the thickness of board used, but lacking any specifics your post might lead someone to think if they whack a 6mm jackoboard in somewhere and magic; no more condensation risk

 

[STORYTELLER]

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.

ah, thanks for the input on the hit and miss welding...and thermal mass.useful to know. Seems external wall insulation over the steel and blockwork would be the only decent way of insulation the steel to prevent possible condensation & bridging... Obviously, just like all other mistakes on this build... the Lintel arrived with only 150mm bearing at each end... SE specified 300mm (presumably to help react the torsion loads that this type of steel design is supposed to accommodate)