Insulating steels that support 2nd storey.

[Blowfelt]

Hi there, I am looking for advice on the amount of inslation I need to cover the I beams that run through my living room. We built a second storey on an existing first floor extension, but because of the 45 degree rule we could not build on the outside wall. Therefore the wall had to come in 900mm.The flat roof is a warm roof and the cavity is also insulated to building regs. I obviously want to stop condensation and cold bridging on the outer most beam and meet regs. Could someone please help me.

Thanks in advance, Blowfelt.

[Spinny]

Anyone ?

I am interested in this one too as I have a similar situation where the outer wall of the original house was knocked through and then a pair of steels used to support the house wall above the opening. I too have a warm roof in the adjacent single storey side extension and this sits at a level just above the position of the steels. Must be a very common scenario.

 

Because it is supporting the original house outside wall above which is 1930's that cavity wall above has no insulation, just a 50mm ish cavity. And there is a cavity tray formed using DPM over the steel.

 

So in theory - outside leaf of original house gets cold - this conducts down the wall beyond the 150mm PIR warm roof, then hits the steel making it colder, then through the steel to the plasterboard underneath. Total distance 150mm (PIR thickness) + 175mm (joist depth under) = 325mm stone blockwork, then steel 152mm, then plasterboard/plaster 15mm - total distance to room around 500mm.

 

In theory the steel is in the warm zone and above the ceiling, but it has a semi cold outer block resting on it ?

[Temp]

The usual recommendation for insulation is as much as you can fit in or afford. Perhaps 250-300mm in this case

 

I think I would fit a vapour barrier (plastic sheet) below the insulation and then plasterboard.

 

Edited yesterday at 17:09 by Temp

[Spinny]

Question is, does putting insulation around the steel actually do any good, or does it make it worse ?

 

If the steel is in the ceiling void then the ceiling void is also likely to be semi-warm given holes in the plasterboard for downlights, and/or ordinary plasterboard.

If you insulate all around the steel (except the part in contact with the blocks above) then arguably you are stopping the steel warming from the surrounding air in the ceiling void. All you may do is create a nice cold steel insulated conductor down to the bottom face against the ceiling plasterboard ?

 

 

[Spinny]

Just wondering if there is some rough calculation that can be done. The outside leaf steel is bolted to the inside one in 3 or 4 locations so given it is a good conductor it represents a single thermal element which will reach a single common temperature throughout.

 

Estimating the surface area of a 152mm by 102mm I-beam would give roughly 300L+400L = 700L where L is the length, so for 2 steels it is 1400L, but only 100L of that surface area is in contact with blockwork some 300mm away from the outside air.

 

So if the inside temperature is 20C and the temperature of the blockwork is 0C, then the steel is 13/14 exposed to 20C and 1/14 exposed to 0C.

So what is the temperature of the steel - maybe 18C ?

So there is no reason to worry about an internal cold spot, although it is a source of thermal loss ?

 

Am I talking horse manure ? Probably.

Edited yesterday at 19:10 by Spinny

[ETC]

Are the beams within the thermal envelope?

[Spinny]

2 hours ago, ETC said:

Are the beams within the thermal envelope?

Well yes in that they are below the level of the warm roof. However as the beams support the two leafs of the original house wall, the outer beam is supporting the outer leaf which as it runs up to the main house roof above the single storey side extension is therefore exposed to the outside air.

The plasterboard for the ceiling will run directly below the beams but will have downlight penetrations and will not itself have insulation directly above. The ceiling void the steels sit in is therefore within the thermal envelope.

Edited 23 hours ago by Spinny

[ETC]

And is the cavity wall it supports insulated?

[Spinny]

No the cavity wall above is a 1930's house wall, outer leaf stone blockwork, inner leaf brickwork, variable size air cavity of around 50mm due to variable block depths.

 

See sketch.

[Spinny]

 

Google's AI search response tells me:

 

"...stone's R-value is around 0.108 to 0.114. "

 

"While stone has some insulating properties, its walls are more about thermal mass than insulation. They absorb and release heat slowly, which can provide a thermal buffer, but they do not significantly reduce heat transfer through the wall. "

 

 

[Russell griffiths]

Shouldn’t there be a cavity tray above the flat roof area, not on top of the steel 

any water penetrating the cavity wall will drip down and sit on the steels. 
 

very slim chance of water getting in here, but I thought that was standard when you change a cavity wall. 
 

I would insulate that cavity up to the height of the top of the warm roof. 

[Spinny]

Steels were put at that level across the opening because to put the lintels higher in the wall would have meant disrupting the upstairs room and floor above and the existing ceiling joists.

So the steels will reduce the ceiling level in that one room from 2.63m down to about 2.48m.

I guess the builders put the cavity tray in as that was all that was feasible.

 

To insulate the cavity area you suggest would mean cutting out the DPM cavity tray between the steels, and would still be very tricky given the variable block sizes and cavity width. Or else drilling holes in the wall upstairs to pump in XPS beads or something. Not sure I see it as worthwhile given the air itself has insulation value, and it doesn't stop the outer blockwork penetrating the roof.

 

I am thinking I could try to do something under the DPM between the top flanges of the steel. (Anyone know if ordinary DPM roll is a vapour barrier ?)

For example put some 25mm PIR directly under the DPM by taping onto the steel upper flanges with tescon vana air tight tape ? Or push 50mm rockwool in ?

[Spinny]

Utility room below has boiler, cylinder, washer, dryer, sink in it and wall extractor fan.

 

Not had condensation internally before except on a single glazed window and in the old 1980's single skin kitchen extension. Not sure how much impact a rear and half side single storey insulated extension will have. The architect did say put insulation onto top section of the walls where they continue up to the outside, but not feasible (or considered) when the steel was put in. Thousands of openings must be made like this in the UK every week.