I am wondering how far I need to go with taping the layer of celotex under the rafters? There will be a vapour control layer under the celotex and then plasterboard. Please excuse my ignorance, there is a lot of detailing which has only become apparent now that the renovation is in progress.

There are a few small gaps between the celotex boards - do these need foaming? Also around the velux worries me a little. The picture shows how the insulation butts up against the velux insulating collar and frame.

(the grey patch on the wall is my Diathonite plaster experiment!).

Thanks. And tape all the seams? Even with the VCL going on over the top of it? I have some Tescon vana tape left over. Will this be better than foil tape for the windows at least?

Oh yes, missed the tape! With PIR I generally use the foil surface as VCL. I do not now use foil tape. I have had some very sticky stuff, but I have had some awful stuff too that has started peeling off in days. It is also weak in tension (say on right-angles at edges). At minimum I use Uni-tape (Pro Clima paper-based tape) or similar, and Vana if I am feeling flush.

However you are planning a separate VCL, so strictly you do not need to tape the PIR foil surface. I think the obsessive part of me might still tape the board seams (but in this instance I might 'allow' foil tape since it is only a secondary VCL). Strictly, though, in this instance you do not need to tape the seams (having of course foamed up all the less-than-perfect joints) as you have a VCL over everything.

(By the way, is that blue strip on the Velux the 'insulating collar'? At very least you should infill between the back edge of the PIR and the collar with insulation. I keep a sheet of 12mm Celotex for such eventualities. It is available online in singles, but the carriage is far more than the board cost, so remember to order a sheet when you have a big order going in!

Tape all the perimeters too, so in this example the PIR (or the VCL, or both) would tape down to your 'Diathonite experiment' (which you'd have to 'tweak up' a bit to meet the underside of the sloping soffit). If you are just plastering over the Diathonite you might need to use Pro Clima Contega or Pavatex 'Pavafix Win' (or similar) 'plaster-on ('fluffy') tapes.

Posted By: Nick Parsons

(By the way, is that blue strip on the Velux the 'insulating collar'? At very least you should infill between the back edge of the PIR and the collar with insulation. I keep a sheet of 12mm Celotex for such eventualities. It is available online in singles, but the carriage is far more than the board cost, so remember to order a sheet when you have a big order going in!

It is the insulating collar yes. I can sense a touch of irony! Also from the builder. They need to put in a wooden batten here to mount the plasterboard so I don't think there will be any space left unfortunately.

MV wrote: ''Thanks. Just talked this through with the builder (who thinks I'm a bit crazy). He was OK with taping the insulation joints with foil tape but looked blank when I asked about filling any gaps with foam. Basically because the whole surface will/should be airtight in any case with the VCL. I might be a bit confused too - when does a 'joint' become a gap? In other words why is it OK not to tape the joints but not OK leave a small gap'?''

OK: filling gaps is, in my view, entirely about insulation continuity and only peripherally (if at all) about air-tightness. For example a series of 2mm gaps adds up to a goodly uninsulated chunk. You may decide to use air-tight foam, but that's incidental - your air-tightness will/should come from your taping of all joints and perimeters or from your separate VCL.

A 'joint' becomes a 'gap' in my view when you your thinnest feeler-gauge is a loose fit in it.

For clarity it is only OK *not* to tape the joints when you are using a separate VCL/airtightness layer, and in any case no membrane or additional layer should be used as a 'cover-all' for gaps. I have been told, for example, that it is OK to have gaps between boards in the 'between rafters' layer because it will all be covered by the 'under-rafter' layer. Well it will, but the top and bottom of it is that you are paying for 'full' insulation coverage but (because the gaps are there, though hidden) actually getting less-than-full coverage.

As regards the insulating collar, if there was irony in my reply I missed it! Can I clarify? 'Reading from the right' of the Velux I think I see: 50mm PIR, 30-40mm exposed rafter and 30-ish mm 'collar'. Am I right? And am I also right that the Velux frame has a routed groove to take 12mm plasterboard? In which case why can you not use Vana tape over the 'end grain' of the PIR (as VCL, insofar as my subsequent glueing suggestion wouldn't work onto plastic VCL), insert your pieces of 12mm Celotex (I'll send you some if you pay the postage!) and fix the reveal plasterboard (a) into the groove, (b) through the 12mm Celotex into the rafter and (c) onto low-expansion squirty-foam glue such as Insta-Stik or alternatives?

Green Building Store (IIRC) have a presentation showing how much 2mm gaps in insulation reduce the overall U-value. It's significant. Builders seem to be congenitally incapable of caring about this stuff. A quick bit of therm modelling would probably illustrate the point.

All of the above. Constant battle with the builders over the detailing. I did have the option of using a 'green' builder but the quote was 50% more. If the project had been smaller I definitely would have found it less stressful to do more of the work, or the detailing at least, myself.