Insulating a big steel beam

[SBMS]

Hi all

Will be speaking to our architect to get advice, but he's away at the moment and keen to understand what's possible.  We've designed something which requires a fairly large steel to carry the rear of the house where a single story flat roof intersects at the rear.

 

The smallest height steel we can get is a UC 356x406x235 - spanning about 10meters.  To minimise any boxing in, in the kitchen, we are looking at moving this steel as far up as we can - ideally level with the bottom of the first floor joists.  I'm trying to understand how we insulate the steel and prevent as much thermal bridging. I've drawn a crude diagram of our ideal profile of this beam.  Our cavity is 200mm and the beam is just under 400mm wide so we can presumably pack some insulation into the face of the beam as shown in A and B below.  I assume that we won't be able to do this where the first floor joists run into the beam, and also the top and bottom flange marked in red:

 

 

Are there any other options?

Rear elevation of the house, floorplan, and section drawing with the beam marked in red:

 

[SBMS]

Having re-read my question... is my steel beam entirely within the thermal envelope? Am I worrying over nothing?

[DevilDamo]

I can’t see any issue in raising it, providing it doesn’t raise above the level of insulation.

 

It looks like you’re going for a cold roof and if so, what are ventilation details at the parapet, rooflight upstands and wall abutment?

 

What is that beam being supported on as I assume you have or require some kind of goalpost frame?

[SBMS]

4 minutes ago, DevilDamo said:

I can’t see any issue in raising it, providing it doesn’t raise above the level of insulation.

 

It looks like you’re going for a cold roof and if so, what are ventilation details at the parapet, rooflight upstands and wall abutment?

 

What is that beam being supported on as I assume you have or require some kind of goalpost frame?

We are doing a cellulose full fill so we shouldn’t need a ventilation cavity as long as use a breathable membrane. 
 

You’re correct, the beam is on steel columns down to foundations. 

[ADLIan]

Flat roof needs redesigning, cannot fully fill the joist void with insulation!! Correct insulation to flat roof and packing void between 1st joist and steel insulation marked ‘A’ should minimise thermal bridge.

[SBMS]

6 minutes ago, ADLIan said:

Flat roof needs redesigning, cannot fully fill the joist void with insulation!! Correct insulation to flat roof and packing void between 1st joist and steel insulation marked ‘A’ should minimise thermal bridge.

Why can’t the flat roof joist void be fully filled with insulation?

[ADLIan]

Won’t comply with relevant BSs or Building Regs. Architect should know this. In a cold roof 50mm cross ventilated void is required above the insulation, can’t see how this can done here. Warm roof is the best (only) option.

[SBMS]

2 minutes ago, ADLIan said:

Won’t comply with relevant BSs or Building Regs. Architect should know this. In a cold roof 50mm cross ventilated void is required above the insulation, can’t see how this can done here. Warm roof is the best (only) option.

Sorry, I see what you’re saying. Architect has specified 50mm ventilated cavity above the insulation and below the final roof deck. 

[ADLIan]

Cold flat roofs are frowned upon now. Not sure how you can cross vent your roof. Warm roof is safer option

[SBMS]

19 minutes ago, ADLIan said:

Cold flat roofs are frowned upon now. Not sure how you can cross vent your roof. Warm roof is safer option

Thanks @ADLIan  I can see your point especially with the wall junction. What would one typically recommmend as a makeup to achieve a circa 0.12 u value? Can you insulate between the joists with rockwool

and then above with PIR for a warm roof?

Edited August 5, 2024 by SBMS

[DevilDamo]

35 minutes ago, SBMS said:

Architect has specified 50mm ventilated cavity

And a ventilation path at the parapet and wall abutment? A warm roof would seem to be a lot more logical in this situation.

[SBMS]

14 minutes ago, DevilDamo said:

And a ventilation path at the parapet and wall abutment? A warm roof would seem to be a lot more logical in this situation.

Good question, we are only at draft stage and I know the architect has a detailing for it. However I am inclined to agree that a warm

roof is looking like a better option. 
 

am I right in thinking there is no ventilation requirement for a warm roof? Is there anything stopping us putting Rockwool between the flat roof joists as well to further lower the u value?

 

I am wondering about whether to abandon the idea of cellulose on the pitcher vaulted roof as well if so and go for a warm roof there too..

[DevilDamo]

23 minutes ago, SBMS said:

am I right in thinking there is no ventilation requirement for a warm roof? Is there anything stopping us putting Rockwool between the flat roof joists as well to further lower the u value?

Correct, warm roofs do not require venting. You’d have to carry out a u-value calculation but as an example, 150mm of Celotex on top achieves 0.15.

[SBMS]

5 minutes ago, DevilDamo said:

Correct, warm roofs do not require venting. You’d have to carry out a u-value calculation but as an example, 150mm of Celotex on top achieves 0.15.

Can we also fill between the rafters with Rockwool or PIR? 

[DevilDamo]

Yes but I would stick with the same manufacturer or at least PIR boards.

 

Edit: And btw, I am initially referring to the flat roof.

Edited August 5, 2024 by DevilDamo

[SBMS]

3 minutes ago, DevilDamo said:

Yes but I would stick with the same manufacturer or at least PIR boards.

 

Edit: And btw, I am initially referring to the flat roof.

Thanks. Why would you not advise mixing PIR on top and Rockwool in between?

 

also - what are your thoughts on warm roof for pitched roof?

[Mr Punter]

Warm roof works well for flat roof.  Easy to fix PIR down and no heavy tiles to drag the whole thing down no ventilation needed and they often abut walls.  Pitched roof is easier cold / intermediate.  Need ventilation above insulation.  Warm pitched roof is a pain.  Very long fixings needed and adds lots of height.

[SBMS]

Just now, Mr Punter said:

Warm roof works well for flat roof.  Easy to fix PIR down and no heavy tiles to drag the whole thing down no ventilation needed and they often abut walls.  Pitched roof is easier cold / intermediate.  Need ventilation above insulation.  Warm pitched roof is a pain.  Very long fixings needed and adds lots of height.

Thanks @Mr Punter  what’s an intermediate pitched roof? We were looking at blown cellulose - what would you recommend?

[DevilDamo]

On 05/08/2024 at 22:35, SBMS said:

Why would you not advise mixing PIR on top and Rockwool in between?

I’m thinking more along the lines of the different thermal characteristics, warranty, etc… PIR is more common for thermal resistance while quilt is more common for sound resistance. Unless you’re insulating at ceiling level and Rockwool (or similar) is usually the common “go to” product.