Insulating new steel ridge beam in loft

[nghakrmyk]

Hi there,

 

I’m converting my Victorian semi loft by adding an L shaped dormer to the rear. We’re trying to get a warm roof on the rear flat section and are just insulating the front existing roof from the inside.
 

To make the most of the available head height, the architect has suggested replacing the existing timber ridge beam with a new steel beam (instead of adding this underneath the existing).

 

I’ve attached a snip of the detail from the structural engineers - the steel beam looks to be underneath the ridge cap ie cold.

 

No one has highlighted whether this should be insulated, and how it could be achieved between the warm roof / cold roof sandwich, so just wanted to see how others have approached this before?

 

The loft work is about to begin so hoping to avoid walking into issues before too late.

[JohnMo]

Your steel needs to be insulated as the only thing between the steel and open air is a ridge tile.

 

The way to look at insulation is it should be a continuous layer that wraps around the house. Take a drawing and a pencil, draw a line through the insulation and without lifting the pencil can you draw a ring around the whole house - you can only draw through insulation (windows and doors are classed as insulation if installed correctly, so should straddle the insulation). If you hit a section where you cannot move on without lifting the pencil you have a cold bridge to rectify.

 

Your steel beam is an issue and also how the new and old sides of the roof join. Would need a bigger section of the drawing really. Your other area to look at I suspect is how does the warm roof join your cavity insulation or wall insulation if different.

[MikeSharp01]

Interesting! You need to pack it round underneath with as much PU as you can, get some wood along the top of the beam + down the side of the flange at the top and foam fill all the voids down the sides. The fixing of the ridge tiles can still be done with cement as you would be fixing to wood.

[nghakrmyk]

Thanks for the quick replies both - I think it’s back to the drawing board.

 

I have even more questions having stared at it for a few hours - eg how would the cold roof actually vent in this design? The 50mm air gap we were going to leave looks to just flow into the steel webbing (and not up/out).

 

I suspect we’ll go back to what was originally planned (see detail now attached) as I’m not actually sure this stops us doing a warm roof having re-looked at it. There still seem to be questions around bridging though it may just be that the detail dimensions aren’t accurate, eg:

 

- On the cold roof side, the steel flange looks very close to what will be the ventilation gap / essentially be behind no insulation

- How to manage the sandwich between cold / warm sides if we can get enough insulation above that cold side edge

- How a VCL will stay above deck on the warm side and below the insulation on the cold side

- Assume the existing timber ridge beam is not conductive enough to make the steel cold

 

This plan seems to be what’s generally done (I think?) so hopefully more standard though…

 

On your final point JohnMo (ie wall / roof joins) - one side is the party wall so just a single skin and then the neighbour’s cold loft. The other is a double skin wall and no insulation. I’ll ask the architect about that one too.

 

 

 

 

 

 

 

[Nickfromwales]

Have you asked if a flitch beam will suffice?

 

You can just top that with XPS or Compacfoam, and the cold bridging will be far less of an issue. 

[nghakrmyk]

I hadn’t, so that’s now with the rest of the questions for the architect / SEs / builders - thanks Nick

[saveasteading]

7 hours ago, nghakrmyk said:

just insulating the front existing roof from the inside.
architect has suggested replacing the existing timber ridge beam with a new steel beam

'Just' isn't the appropriate word. How are you going to support the roof while removing the timber ridge?

 

Insulating any steel will be the easy part.

 

Can you discuss this with the SE and ask for any alternative suggestions?  Often you find that the decisions pass down the line, as serial decisions.

You ask for an extension design, Architect sketches in  a beam, notices a headroom issue and so suggests taking out the timber,  SE is asked to design a beam as drawn, you pay for the work.

 

If asked, the SE may have better ideas, but possibly has not been asked.

We can't suggest anything now, because we don't know the context. eg layout, spans , headroom constraint.

 

 

[nghakrmyk]

Thanks saveasteading.

 

As an update - the plan is to revert and install the steel beam (152x152) under the existing ridge (180mm deep). More straightforward all round it seems.
 

I should have approx 2580mm from the new floor board to the top of the existing ridge beam with about another 60mm to the underside of the existing ridge cap. So it seems we have enough for a warm roof without needing to change the beam, ie 2x 18mm OSB, 140/150mm PIR, 175mm rafters, plus some change is about 365mm. That leaves approx head height of 2215mm before plasterboard and firrings (these need to be about 60/70mm) but also whether we can use any of the 60mm gap to the top of the ridge cap. I would be very happy if we ended up with 2.2m but seems like we’ll be nearer 2.15m. 

 

I’m not actually sure what replacing the beam would achieve as the limiting factor seems to not be the existing beam ie it’s just the ridge height? Both of those would have been measured as part of the initial plans.

 

Thanks again for the replies all - between this and another recent thread, the advice has been invaluable in discussing things with the architects, SEs and builders. 
 

I’ll update once there is a plan on the remaining questions above ie managing the warm and cold sides / VCL.

Edited October 6 by nghakrmyk

[nghakrmyk]

One thing has come to mind actually - the steel beam under the ridge (and another which will support the new floor joists) span the width of the house and sit on pads - so one in the party wall and one in the internal leaf of the existing external double skin wall. A picture is below.

 

It (again) seems the end of the steels sat on the external wall would need insulating as it’s in contact with the outer leaf? 30mm aerogel or a thermal pad would seem to do the trick - but is that what people are really doing?

[nghakrmyk]

For context, this is how the beam supporting the floor joists has been installed - with a small amount flying over the pad.
 

This steel will sit in the eaves space so I’m less concerned as it should be cold - but wondering if we should add some insulation as belt and braces.

[saveasteading]

You have a cavity so could easily fit pir, orceps  or mineral wool over the face of the beam. Forget calculations: just do it and live with some tiny heat loss.

Edit, don't use any absorbent material.

[nghakrmyk]

Great, thanks - is that just in case the mortar fails? I was assuming we’d need it to be breathable so it’s the same profile as the rest of the wall, eg spacetherm. I’ll have some knocking around as we need it elsewhere. I don’t believe that’s considered absorbent? Build up would then be - spacetherm (eg 20mm) patched over the steel end with some overlap. Bricks (trimmed if needed) fitted on top.

I’m only really worried about condensation - the rest of the house is a thermal sieve.

[saveasteading]

No I only meant to put about 20mm stuck over the end of the beam. Glue or screw. The cavity remains, and the small amount of restriction shouldn't matter.

Probably best to fix small pieces into the recesses at either side of the beam then another over the face.  Or carving a shape out of a thicker piece would be tidy.

What thickness is Spacetherm? I think of it being in a roll and about 8mm thick but could be misremembering

I'd paint bitumen over the exposed steel first just because you can and its something else to be certain of.