I have no direct experience - strikes me the difficulty is the steel column carries quite concentrated loads over its small cross section area, and depending on wind loads it might have shear as well as compression. There aren't really any insulation materials that can carry such concentrated loads and so be 'grafted' into the steel at the right height, or placed directly under it. The connection would also have to be bolted through, which would thermally bridge across the insulation layer.

It's easy to say 'redesign it so the steel is all outside the envelope' but you already thought about that!

Can you clad all round the steel with insulation, over its whole height through the heated area? Aerogel perhaps?

Or can you build a masonry pier dwarf wall on top of a course of foam glass blocks, stand the columns on top of that, and also support the floor beams on it?

(Just inserting standard comment: steel and b&b have much worse embodied carbon emissions than TF and susp timber floors do, this might drown out any benefit of chasing thermal bridges !)

I would insulate around the column from the insulation layer down to the base, and at the base I would use something like Armatherm 500 or Farrat Struktra thermal breaks to stand the base on. Use stainless mounting bolts/rods. But I am not an expert, so please ask one!

There's a potentially useful guide at https://passivehouseplus.co.uk/magazine/guides/the-ph-guide-to-thermal-breaks

My regular work with structural engineers gives me the clear impression that they are by nature very conservative individuals (for lots of very good reasons), who follow tried and tested, and therefore not recently updated, techniques. On a small project, you're more likely to have a SE who will be witness to fewer innovative methodologies, compared to those working on larger projects, where there are drivers for novel/green detailing.

I usually follow the "insul down the column" detail, as echoed above. There are so many other things to watch out for on a build, getting a detail buttoned down is a win. If you manage to work out the actual heat loss due to thermal travel up the 1m of steel from base plate to slab, then add a bit more insul somewhere easy to compensate.

Sweating is, for me, the bigger concern. If you've some way of modelling the steel surface temp, with the base plate at say 10oC (usual solum temp) and room temp and hum as you choose, you might get an idea if sweating is an actual issue. I'd think around 12oC or below is a likely condensation temp. If you think that's regularly likely then, as DJH suggests, add a bit of insul around the steel through and above the slab as well.

I haven't seen steel post bearing insulative materials, but with only ever a a couple of posts on most builds, my design time is better spent looking for other thermal wins, and herding the contractor cats to be on site, and actually look at the drawings.

Posted By: lineweightSo, just wrap the column in rigid board insulation, stuck together with expanding foam,

Sounds like a plan, I would use EPS at what ever thickness you feel serves the purpose and I would treat it like EWI with a coating of EWI adhesive and mesh to give some mechanical protection and keep out the vermin
(EPS I have found works out the cheapest / u value even accounting for the extra thickness over the more exotic board insulation)

I came across a couple of photos of an insulated column base here:

https://passivehouseplus.ie/magazine/upgrade/enerphit-upgrade-breathes-new-life-into-dublin-home

Posted By: GreenPaddyAs PiH mentions, protect the exposed bit which will be in the solum void, up to the u/side of the beam/block floor.

And any below ground because the critters burrow as well.

Marmox thermoblocls have a higher compressive strength than 7N dense concrete blocks. How about foundation, thermoblock, pad stone, steel, as a standing buildup.. and then insulate the whole lot. Marmox sized suitably for load being borne

Red-steel
Grey-concrete/bnb
Yellow-insulation
Orange-Marmox
Brown-dirt

Posted By: cjardMarmox thermoblocls have a higher compressive strength than 7N dense concrete blocks. How about foundation, thermoblock, pad stone, steel, as a standing buildup.. and then insulate the whole lot. Marmox sized suitably for load being borne

Red-steel
Grey-concrete/bnb
Yellow-insulation
Orange-Marmox
Brown-dirt

http:///newforum/extensions/InlineImages/image.php?AttachmentID=8619" alt="IMG_3798.jpeg" >

I'd have to argue that this would provide a significant benefit vs simply insulating around the column, and without access to fairly detailed thermal modelling, I don't think that I could.

I'm fairly sure something like this would result in raised eyebrows from structural engineers and/or contractors. Unless anyone can tell me that there's some version of this that's become "standard practice" to some extent.

Also... it strikes me that if you are going to embark on a solution along these lines, why not build up that footing for the column so that the Marmox course(s) are higher up and closer to being inline with the floor insulation.

Posted By: djh

Posted By: lineweightIn the case of extensions

Who mentioned extensions? Nobody that I can see.

I think it would have been clearer if you'd started by explaining your situation. I won't try to do it to avoid misinterpretation.

What it now seems like to me is that you're asking somebody on here to run a fairly simple thermal bridge calculation, to avoid doing the work yourself. I hope that's wrong?

I thought I had tried to make my question reasonably clear in my OP. The question is about a commonly occurring situation - steel structure passing through floor insulation layer. And I am asking in broad terms about a detailing approach to this situation that might not be seen as overly fussy on site.

I explained that this is not in the context of a big budget or targeting something like Passivhaus standard. I'm looking for pragmatic approaches to make buildings less energy-inefficient than the default that happens if you let everyone follow the path of least resistance; that is the world I operate in most of the time.

Perhaps this type of question is not best addressed to this forum because I know that a lot of people come at things from a self build perspective where you aren't always having to try and design something that has a reasonable chance of being built right by a regular builder without intensive supervision and negotiation.

Perhaps I should have mentioned that this particular situation often crops up in extension or renovation projects - I suppose I didn't think that a necessary bit of detail because it hadn't occurred to me that in other scenarios the SAP process might offer a strategy for arguing for better details.

No, I am not asking anyone to run a calculation for me. Why suspect that I am here with that intention? This is a discussion forum for those of us interested in talking about sustainable building. I'm asking if anyone can offer perspectives on the issue I outlined in my OP and subsequent posts. I've been posting on this forum for over ten years and it's a bit unpleasant to suddenly be accused of looking other people to do my work for me.

Posted By: Peter_in_HungaryI can understand the problem and the desire to mitigate it but without some thermal bridge numbers it is difficult to quantify how much mitigation is justified. E.g if the steel is 150mm from the ground then a fairly short path to a cold bridge which might warrant insulation under the steel OTOH if there was 1000mm then perhaps external insulation of the steel is all that would be justified.

Yes, of course.

I guess my thinking though, is that there is probably some mitigation that can be done for relatively low effort/expense - and that might include wrapping the column (whatever length). And then there is some (like insulating between column base and foundation) that is inevitably somewhat expensive/tricky, either because most contractors aren't used to doing it, or because it requires special products. Or both.

So if, for example, wrapping the column is cheap & easy to do, it would make sense just to do it, regardless of column length. Calculations become a little academic if insulating the baseplate is basically out of scope regardless. The calculations might tell us, yes it would be beneficial to insulate the baseplate if the column length is over a certain length and if we are making the decision based on certain pay-back times for energy savings, or if we are targeting some specific standard, or just doing the minimum necessary to satisfy building regs, or whatever. But there's no point going through these calculations if they just tell us whether it's worth doing something that we can't do anyway.

I don't know if that makes sense. I don't like it that we have to design in this kind of very woolly way; I'd like it if it were the case that a clear best course of action could simply be arrived at via objective calculations but unfortunately that's just not the way a lot of building design can happen. A fact that is (I think) baffling/frustrating to many people coming from other engineering/design disciplines.

I'm trying to think of other examples where preconceptions of "reasonable" spec have changed with time. Triple glazing perhaps. Not that long ago, specifying triple glazing was seen as kind of extreme or only appropriate for a few very high performance buildings. Now, it doesn't feel like that. It's more widely available from mainstream glazing suppliers. What's changed is not that there have been big step changes in glazing performance or building regulations. It's that a bunch of things have gradually moved on to the point where, for whatever reason, asking for triple glazing isn't now seen too much as unusual, or something that will necessarily frighten away a regular domestic building contractor. In my observation.

600mm of steel has the same thermal resistance as 0.3mm of PIR - so for practical purposes, no resistance at all. Steel is so massively more conductive than other building materials that it's difficult to get heads round.

So, treat any surface in unrestricted contact with the steel as effectively being in contact with the inside of the heated envelope. If it's a big enough patch of surface that, if it were in the heated envelope, you would insulate it, then the same benefit comes from insulating the steel.

If there was a patch of floor say 200x200 in direct contact with the ground (analogous to the bit underneath the foot of the steel) then you'd perhaps insulate it for a PH but not elsewhere.

If there was a patch of uninsulated wall 600hx900w (analogous to all the exposed sides of 600mm length of a 150x150 "I" section) then you'd be pretty keen to insulate it.

The bolts are also highly conductive and so will bridge through any pad-style insulation layer placed directly under the foot of the steel. Best to put an unbroken course of Marmox/foamglas near the ground, then several courses of masonry on top to receive those bolts, and to spread out the concentrated loads from the steel onto the Marmox.

The combined thickness x insulation value of those solutions is a little better than nothing, but not what many would consider 'insulated'!

Posted By: WillInAberdeen600mm of steel has the same thermal resistance as 0.3mm of PIR - so for practical purposes, no resistance at all. Steel is so massively more conductive than other building materials that it's difficult to get heads round.

So, treat any surface in unrestricted contact with the steel as effectively being in contact with the inside of the heated envelope. If it's a big enough patch of surface that, if it were in the heated envelope, you would insulate it, then the same benefit comes from insulating the steel.

If there was a patch of floor say 200x200 in direct contact with the ground (analogous to the bit underneath the foot of the steel) then you'd perhaps insulate it for a PH but not elsewhere.

If there was a patch of uninsulated wall 600hx900w (analogous to all the exposed sides of 600mm length of a 150x150 "I" section) then you'd be pretty keen to insulate it.

The bolts are also highly conductive and so will bridge through any pad-style insulation layer placed directly under the foot of the steel. Best to put an unbroken course of Marmox/foamglas near the ground, then several courses of masonry on top to receive those bolts, and to spread out the concentrated loads from the steel onto the Marmox.

The combined thickness x insulation value of those solutions is a little better than nothing, but not what many would consider 'insulated'!

I'm not sure I completely follow the bit about analogy with patches of uninsulated wall.

If there's a large area of uninsulated wall then at any point on that patch there's a certain heat flow from inside to out.

With the total exposed surface area of (per your example) a 600mm length of H-column, then the length of the heat path to the inside is going to be different, according to which part of the column you are looking at.

I think you are sort of saying that the conductivity of steel is so high that you can ignore this.

But does this mean that a scenario with 1200mm of exposed column under the floor insulation layer, rather than 600mm, loses twice as much heat?

The way I'd tend to visualise this would be more focussed on the cross-sectional area of the steel column. I'd imagine the steel column sliced off in line with the underside of the floor insulation. And it's that "face" that would be losing the heat directly to the ground and outside air (in reality via the continuation of the steel member). If the cross sectional area of that face increases then of course the heat loss increases, but I'd regarding the length of column below this level as relatively independent.

I think that what you are saying is that the surface area of the column exposed to outside air is relevant because it's acting like a radiator.

This means that the way I'm visualising the effect of insulating around the column is not really right. I am thinking of it as simply extending the length of the heat path from "cold" to "warm" but actually it's not as simple as that?

Cheers, that helps clarify a few things in my mind.

Presumably the relationship between exposed surface area of steel, and heat loss, is not actually linear though?

I mean if the exposed sections of column were 1km long above and below the floor then the heat loss rate would not be 1000 times that of a scenario with 1m lengths?

Well, I've now had a chance to discuss all this with the structural engineer.

He's said that their normal detail is to encase the column in concrete, up to FFL to protect from corrosion.

Not keen on the idea of building up from the foundation with some kind of insulative blockwork and then sitting the steel on that - wants the column base plates to sit onto the foundation so they can be bolted down.

He's very familiar with using thermal breaks like these Farrat ones

https://farrat.com/category/structural-thermal-breaks/

but usually in commercial builds for things like balcony connections to steelwork. Says he's never seen them used in a domestic project. He's not against us trying to use them though.

The question is how much benefit they will offer, when used at the base of a column, especially if the whole thing then gets encased in concrete. My next step I think will be to see if Farrat themselves can give any input.

In some cases, the steel columns must make a direct connection with a steel ground beam. This ground beam sits below ground level, and is also encased in concrete. Following WillinAberdeen's way of thinking about things, if the columns are directly bolted to this ground beam then all of the surface area of that ground beam becomes essentially directly connected to the interior of the building. I think a heat path to the ground itself under a building is less bad than a heat path to the cold air in an underfloor void, but it's a bit beyond my means to quantify this.

If the column bases must be encased in concrete I will argue for this to go up to the underside of the floor insulation, not to FFL, and then I will insulate around the outside of the concrete where it's exposed to the underfloor void.

Posted By: lineweightThe conductivity of steel is such that we can pretty much ignore the length of any heat path through the steel, or the cross-sectional area perpendicular to the heat paths.

I don't quite get what you are driving at in this statement but what you are describing generally is the difference between what are called linear (psi value, W/m.K) and point (chi value, W/K) thermal bridges.

There is a distinction in units, but also in the way that they are remedied. A linear thermal bridge can usually be resolved through adding other layers in the wall build-up, whereas a point thermal bridge will usually, by necessity penetrate through all these layers (unless thermally broken with a structural pad as described in the posts above) and is thus often harder to design out.

Others can probably comment on their thermal equivalence or otherwise (I'm not convinced), but I'm struggling to see why you'd want to describe them in the same way when they aren't likely to be used for the same thing in reality.