Fri, 17 Apr 2026 11:32:23 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309968#Comment_309968 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309968#Comment_309968 Sat, 03 Jan 2026 00:37:17 +0000 ChrisinYorkshire
What do you recommend for insulating an RSJ when:

- the wall is mostly solid stone build, and being insulated internally with wood fibre board, lime plasters, breathable system, however;
- there is a retrofit cavity section that spans approximately 4m width of the wall, which involves a row of windows and an external door.
- the inner leaf of this part of the wall has the RSJ above it, with a cavity tray extending to the outer leaf.

I'm concerned that if I fill the RSJ with wood fibre batts between the flanges and front this with rigid wood fibre insulation, that vapour might reach and condense on the beam and saturate/overload the insulation more often than it can dry out, particularly in the winter months - is this a valid concern? Equally I'm concerned that if I fill the RSJ and cover with non-breathable insulation, XPS/PIR or similar, that moisture will creep in and be trapped against it. Since the RSJ is already in situ, I cannot fit a perfect DPM around it.


Other factors:

The RSJ has had considerable rust with flaky rust sheets that I've mostly removed and were presumably caused by approx. 30 years of kitchen humidity with no extraction. I am planning on scraping off any remaining flaky rust and treating RSJ with Jenolite rust convertor.

I can quite easily reach into the cavity and insulate the cavity side of the RSJ.

I also need to consider fire boarding, particularly with this being above the main entrance/exit to the house.

Any guidance most greatly received! :-)

Chris]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309974#Comment_309974 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309974#Comment_309974 Sun, 04 Jan 2026 18:01:25 +0000 Nick Parsons
This might be an issue where you choose to use synthetic material and simply 'do the best you can' on the VCL, since you are going to be trying for air-/fire-'tightness' anyway.

How about:

PIR or EPS in and under the RSJ - glued to it with Gripfill or similar. Battens wedged between webs.

VCL taped with air-tightness tape to surrounding plaster and all perimeters.

2x 'pink' plasterboard fixed through to battens with a squirt of silicone in the holes.

Plaster skim over.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309975#Comment_309975 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309975#Comment_309975 Sun, 04 Jan 2026 20:35:52 +0000 Peter_in_Hungary
Will the recommended above PIR or EPS under the load bearing part of the RSJ take the expected load?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309976#Comment_309976 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309976#Comment_309976 Sun, 04 Jan 2026 20:52:17 +0000 Nick Parsons I had not completely thought out my battens, but I was thinking mechanical fix to battens, not adhesive, of course. Something like 50 x 50 wedged in the webs with 25 outside the webs, so the bottom boards could fix through to the end-grain of those. I was not expecting the PIR or EPS to take any load.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309979#Comment_309979 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309979#Comment_309979 Mon, 05 Jan 2026 08:55:05 +0000 GreenPaddy
For example, ChrisinYorkshire could live in a bungalow, in which case the lintel is only holding up a roof, which is not an "element of structure" in terms of fire safety. However if it were supporting a floor above then it would require protection.

I'm very familiar with Scotish Reg's so had a quick look at English reg's, and I doubt there's any requirement for more than 30 mins (really for fire fighting stairs or refuges) for low rise domestic buildings. Or if there is a wall above or around it that has a special higher fire rating, in which case you would need to meet that same rating.

So as I understand this case, just plasterboard over the lintel like the rest of the wall, no need for pink plasterboard or any other special fire rating. If it were say a downstand beam supporting a floor above, then you would need to do more than just regular plasterboard, as it is both the material and the means of fixing that provide the fire rating, and so you ought to use a proprietary fixing system that has been tested to give the rating level required. That would likely include metal framing and special fire rated boarding.

The above doesn't answer CinY's question about insulation and minimising condensation leading to further rust, but we need to be careful giving fire protection advice. Best to say look at Doc B (B3 for this query).]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309980#Comment_309980 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309980#Comment_309980 Mon, 05 Jan 2026 11:47:22 +0000 Nick Parsons https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309981#Comment_309981 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18390&Focus=309981#Comment_309981 Mon, 05 Jan 2026 17:03:16 +0000 fostertom