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 June 12 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 June 12 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 June 12 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.

[Spinny]

Anymore on insulating this - see sketch above ?

[Spinny]

Starting down this route in the sketch - 25mm PIR taped to the top flange of the steel.

The idea being to stop the potentially cold DPM between the steels acting as a possible condensation point.

Insulate the cold top flange of the outermost steel to stop it being a condensation point.

Stop air in the cavity above the steels mixing with warm air in the ceiling void.

 

Is this any good or am I going wrong ?

[JohnMo]

So you have a cold bridge from the outer block work and through the steel. Steel is a great conductor. Your plan does zero to address the bridge.

 

I would do something like this

 

 

 

Red 25mm PIR, yellow mineral wool

[Spinny]

Thanks @JohnMo I really don't want to put 25mm PIR under the steels because (a) it will further reduce the already lowered ceiling height and (b) ceiling has already been battened into the steel level with the bottom of the steel ready to board up. I could put some insulating plasterboard under the steel and across the whole ceiling but it will have downlight holes and has to be fireboard under the steels.

 

I guess the question is should the steels be treated as in the warm zone or the cold zone ?

They are really in the internal warm zone of the ceiling void but just happen to have something of a cold bridge acting on one face.

 

Whatever I do the steels are a cold bridge because they are bolted together. I either put them mostly in the warm zone, or insulate all around them putting them mostly in the cold zone.

 

In this video the steel is on the inner leaf but has a cold bridge plate to the outside, but the answer is just to insulate the steel close to the cold bridge...

https://youtu.be/1-qJc2BaXo0?feature=shared

 

Is there some way to estimate the temperature the steel would reach ?

 

(PS Does warm moist air percolate through rockwool and would it still condense on the steel - unless adding a vapour barrier too?)

 

[JohnMo]

Cold bridges.

 

If you can draw a line without lifting pen off the paper through the insulation around the whole building you are thermal bridge free. Now apply that to your sketches. Do you have continuity of insulation?

[Spinny]

OK. However in this case it is impossible to get continuity of insulation. Because the steels are bolted together and support outer leaf and inner leaf they will always be a thermal bridge (save removing them and inserting insulation above them - which isnt happening).

 

Insulating the steels moves the bridging point but cannot eliminate it.

 

Taking the case of say a steel which penetrates the insulation layer - what do you do ? You cannot separate the steel internally from the rest of the structure across its entire perimeter and length. You can insulate the steel close to the penetration, but at what point can you stop ? What is the temperature gradient in a steel beam ? If I put a bag of frozen peas on top of a steel in a warm room how much of the steel gets significantly colder than the room ?

 

(I agree the thermal bridge is wicking heat out of the building. However given that can't be completely stopped, the other question is how can condensation be avoided ? If the room is 21C what temperature surface will it condense onto, and will the steel reach that temperature ?)

 

 

Edited June 16 by Spinny

[JohnMo]

12 minutes ago, Spinny said:

What is the temperature gradient in a steel beam

Almost zero, it's a conductor.

[Spinny]

12 minutes ago, JohnMo said:

Almost zero, it's a conductor.

OK. So If I have a 10M beam, and 9.5m of that beam are within a heated building at 21C and 0.5m of that beam is outside at 0C, what is the temperature of the steel beam ?

 

Won't the internal innermost  end of the beam be at 21C and immune to condensation.

Meanwhile the outside end of the steel is at some temperature below 21C and above 0C and is radiating heat into the outside air.

Bad for heat loss, but...

 

How much of the internal steel must be insulated to protect it from condensation ? 10mm, 100mm, 1000mm 10000mm ?

(The more length of the beam you insulate internally the colder the beam will get.)

[JohnMo]

Have a read on here and the download section 

 

https://www.thermablok.co.uk/our-products/thermablok-aerogel-insulation-strips/

 

[Spinny]

I am aware of aerogel as had it specified by the architect in the form of aerogel wallboard. Ironically for the same type of issue in the vary same room. Alongside the steels over the wall opening I also have a 1m piece of wall. This is a continuation of the outside wall of the house where the warm roof on the side extension abuts this wall. So again above the roof the wall is external, but below the roof it is internal. The architect said this wall must therefore be insulated and as I wanted the thinnest possible option came up with aerogel wallboard which has been installed.

 

I have come across this one https://steelconstruction.info/images/5/53/SCI_P380.pdf which includes section 1.7 on condensation.

 

This type of detail - steels to create an opening in an outer wall, must be so so common. So I am a bit dismayed there isn't a clear practical solution in common use.

 

At this rate I am going to have to pay the architect exorbitant amounts to give me an answer. What the hell does everyone do ? (I am sure they are not commonly buying aerogel to install in every knock through extension).