I don't doubt your figures on the relative heat loss between steel and timber. But if I've read the OP right, he's putting a thin (3mm) sheet across the top of the cavity, presumably to spread the load over both skins. If so, only the edge of the steel willWhat do you doubt Tony? I may have made a mistake somewhere but I think the basic nunbers are right.
I was going to add a bit about the geometry but decided to keep it simple.
show internally and externally (ie a surface 3mm x 100mm). Even though there will be a high heat loss through that little bit,
it's only over a very small area relative to the beam and wall as a whole.
Just my two-cents worth
PS just noticed JDs post re condensation; you may both be right as it will tend to chill the blockwork around it.
Maybe I've been talking out of my ar$e, which isn't unknown (except when it comes to Brexit).
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