Redbeard

Hello. I think we need to know more about 'where these studs are' before we can comment much. What is on the other side? The outside world? Another heated room? An unheated room? You mention 'bridging' but not 'thermal bridging'. Is sound-proofing more the issue than 'thermals'? More details, please!

Sadly not, if you really need it to do exactly what ply would do. Even if you could find a rigid board claiming insulation qualities at that thickness it would be likely to have all the insulation value of a thin thing. I am aware that anything is better than nothing but I feel that no 10mm board is going to give you an exciting effect in the lay-up you describe.

OK. Got it! So your proposal is 100mm total PIR, as per your original description: *not* as per the drawing. Confusion over. Thanks!

@thetdog666, noted the OSB and membrane missing, but apart from that is that *the* dwg of what you plan to build, with a total of 145mm PIR, as against your description, in your original post, of 100mm? Edit: Cross-posted with @sgt_woulds

@sgt_woulds, I think it's down to the difference (see my post above) between the dwg and the description. The OP's description (above) has OSB over the studs.

I am very confused! Thanks, OP, for your response to my last post. The annotations (notably the dimensions) on your drawing do not seem to match up with the words in your description. The dwg suggests a total of 145 PIR (25 and 120). Your description below suggests a total of 100 (80 and 20). Can you clarify? @Russell griffiths above referred to terminology. Am I right in assuming that the reference to 'battens' below actually refers to the (vertical) structural Studs?

Not sure what you mean by 'on the face of the battens'. Do you mean between them? Have you found 20mm PIR? On the initial assumption that you were not having PIR between the battens I was going to suggest the VCL goes on the face of the 80mm, with the battens over, but if you have PIR between the battens that changes.

Unless something has passed me by *all* PIR boards are made of foam. I have done a number of floating floors on PIR,on XPS and on rigid wood-fibre, and in none of those cases has the floor felt anything other than solid. No discernible bounce whatever. I have never done one with rigid rockwool, though, so unfortunately I cannot comment, except to say that if I were trying it as an experiment I would start with the external wall insulation grade of rockwool. BTW the web says this about thermal conductivity of the RW boards you mention: "Thermal performance. ROCKWOOL Thermal RockFloor has a thermal conductivity of 0.038 W/mK. ROCKWOOL Acoustic RockFloor has a thermal conductivity of 0.040 W/mK". So just over half as good as PIR for a given thickness, but it has the acoustic properties which PIR neither has nor claims.

Have you thought of EWI? You could consider building the extension solid and EWI-ing it, carrying the EWI round, at the same or lesser thickness, to the main house. Given the choice between EWI and IWI I would usually choose EWI.

What's the finish on the house? If it is rendered watch out for staining from run-off from the lead. We had similar with EWI to gr floor only which we capped with lead. Happily the staining is confined to a relatively small area but it is noticeable on the light-coloured render.

'Mineral Wool' is usually used as a 'cover-all' term for either Rockwool (or rockwool!) or glass-fibre. I don't like working with either of them so tend to avoid them, but if they don't bother you, use what you can find cheapest. (Note that rockwool is available with a range of lambda values. 0.044W/mK is 'standard'-ish, but I think you can get it down to 0.035 or even lower for rigid or semi-rigid batts (the EWI stuff for example tends to be around 0.034, but you would not use (nor want to pay for) that in your roof void.

If it's 'blown' (the seals on the separator bars between the panes have failed) there will be condensation inside the units, between the panes. If not, they're OK. It is very hard (without a special detector) to establish whether a unit has low emissivity glass to at least one pane, but if you can find the date of the windows you may be able to 'reverse engineer' with ref to the Bldg Regs at the time.

Crudely, as long as you can see through it the insulation value is not that good. A really good triple-glazed window may get you a whole-unit U value (i.e. everything; not just the glazed unit) of as low as 0.7W/m2K. Assuming your cavity has no fill and is 'typical' for 1975 (circa 50mm) you'd get 0.55W/m2K or a little below with blown-in insulation. (I'd recommend graphite expanded polystyrene, not mineral wool). Your area of wall will (unless your house is very atypical) be far greater than your glazed area, so the right order is walls 1st, windows and doors later (unless your roof insulation is the original (?circa 50mm) in which case carefully lay 300mm on top: U value around 0.14-0.15W/m2K, and do that before everything else). What you do with doors and windows also depends on the state of the whole units. If the frames are not original they may have a reasonable standard of draught-stripping, in which case your main 'let-down' is the glazing spec, and when it comes to glazing compared to other fabric elements it's a question of 'less bad' more than 'better' (The 'holes in the fabric' will always fare worse than most other thermal elements even with 'bells and whistles').

How about rigid EWI which you can fix to the concrete and then render over?

The lead under the tiles is (are) the soakers, AFAICS, laid as you'd expect as per the slates (I think). There is then a sort of 'mini-skirt' flashing over the upturned soakers, again, as far as I can see. Although the abutment is different that's how the soakers work on my extension abutting the main house wall with ridge at 90 degrees to the main house wall. My 'cloaking' (over the soaker upstand) is different in that it's a thinned-out layer of EWI, and comes down further, but the principle is the same. The practice is different in that mine does not leak... Looking at it again we used to allow a 150 upstand for soakers and stepped flashings and bottom and top 'aprons'. Do we think rain could be 'bouncing' over the lead-line, soaking into the sand/cement 'fillet' and running down? (Not certain I buy that even though it's my suggestion, but 'discuss'!).

Is the stack within the thickness of the party wall? I can't see a 'chimney breast'. Can we have external pics? How does the purlin socket line up with the edge of the stack, for example? Can we assume it is about on the bottom side of the stack? If so, in the absence of any other 'hypotheses' so far, can I throw in porous stone effectively 'by-passing' the flashings and soakers? Have *you* had the roof re-done or was it like that when you bought the house? Did you have exactly this same problem before you had the pointing, flashings and soakers done? A bit unlikely, but could water be getting (in some volume) *inside* the stack, soaking the wall and transferring through. I assume you had the flaunching done at the same time as the other works?

+1 Can I guess that you have already been quoted an approximate 'price point'?

Phew!

How am I going to listen to The Archers then??!! 😉

What did your plans say about the Warm Roof build-up and about insulation and fire-proofing of the steels? Got any pics so we get more 'context'?

Is the house newly-completed? I knew someone who was very worried about their 'thermally-broken' alu windows in their new-build suffering from considerable condensation. However, once the plasterwork had dried out the condensation stopped occurring. I was not there at the time so I cannot comment on the actual frame temperature. I am not convinced that your humidity is excessive. I too am concerned that there is no isolation between frame and masonry, but tell us more about your house. Is it new or old? Is the frame connection to the inner or outer skin if it's a cavity wall? If it's not a new house but *is* a cavity wall, fixing to an un-insulated cavity closer (probably a brick) could, I think/wonder, effectively 'by-pass' the thermal break.

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I took 'bare walls' as meaning that the walls were stripped back, whereas in fact 25mm is pretty thin, but as your experience shows it is on the 'steep bit of the curve' in terms of insulation efficiency. Nevertheless, since you do not have what I thought was a 'blank canvas' (to 'engineer' a lay-up with both insulation and support) I suspect the answer is to fix battens for the units through the insulated plasterboard, doing your best to make good the VCL which you puncture (a 'liquid grommet' of silicone is the best I can suggest, or small 'tabs' of EPDM or butyl tape) and then make sure your screws through the back of the units go no further than batten depth + plasterboard depth. If your insulation were much thicker I would be worried about torsional stress on the screws but 25mm is pretty minimal. Edit: Just realised this is still in the 'introduce yourself' section. OP, you may want to see if the moderators can move it to the correct section, where it will be more accessible for others searching re similar issues in future.

Is the kitchen below? Can you drop the shower waste down and take it out at GF ceiling level, or even GF level?

I think the only 'perk' would be that if he did go self-employed in future there is a small income tax offset allowed for use of home as office. I think the top rate (it's based on how many hours you use it) is £312 p.a.