Is there a pre-print server for building physics papers? I recently submitted a pre-print (about heating control algorithms for buildings) to arXiv.org, under General Physics, but it was rejected as being off-topic for that server.

I tried to find a pre-print server specialising in Building Physics research but couldn't find any. Wikipedia doesn't list anything. 

Is there a pre-print server that is suitable for that kind of publication?
No, there isn't yet a pre-print server for building physics, although some people have successfully smuggled such papers onto arxiv.

Furthermore, there aren't really any heavyweight open-access building-physics journals, and established journals in the field haven't been quick to embrace open-access.

There is the option of producing internal working papers and circulating it without publication, to get some of the benefits of pre-print, without risking a publisher's refusal. However, it's not easy to reach a very broad audience this way, nor a new audience.

Building-physics papers do often span multiple subjects - we often cross-over, for example into health, for example, when we look at condensation and mould, radon, or dust mites; so there might be other pre-print archives that we can use.

And, although it's a completely different thing, there's always the possibility to air early thoughts on the building-physics tags on the Physics and Sustainability Stack Exchanges, which can be a way to get a bit of an airing, and reach a different audience to that reached by circulation of working papers.