Redbeard

I would normally advise not to remove plaster (though that applies particularly to lime, and I would normally remove Gypsum) and NOT to use 'pure' dot-and-dab. Preferably full adhesion of the boards, or at very least full perimeter beads with cross-hatchings. If you get air movement behind with D&D you may thermally separate the original wall and the insulation ('thermal by-pass'). With D&D that movement is almost completely unfettered. Removing the plaster (what was advised 40 years ago -along with D&D - when I started IWI) loses you a valuable air-tightness layer. Slim solutions as suggested are unlikely to meet the 0.3W/m2K wall U value target for Bldg Regs. Many would say simply do not apply for B. Regs, but it can potentially come back to bite you when a future purchaser feigns horror at your failure to get approval but agrees to be less horror-struck if you drop the price by £10k. If it is impracticable to meet the target waivers are available.

"As you can see from the photo the wall between and just below the window sill is damp." And above, surely, unless my eyes are faulty. I cannot see below the windows but above seems to be 'blooming' (?moist?). So the chimney ran between the windows and out at the ridge, yes? We need external pics. Faulty verge-work? Faulty/porous brickwork?? More 'base data' please! Then the 'hive mind' will start buzzing. Thanks.

Checking I have got my facts right: This tiny roof void is open all the way from one side to the other (past the porch in the middle, yes? We have already noted that it has ventilation 'in' (at the soffits) but it has no 'out' above that, as it stops on the wall. The only way, then, for that 'in air' to get out is via a 'plenum chamber' at the top of the roof slope against the wall which is open all the way across the whole roof and then exits via soffit vents on the small hips at each end. That way you might actually get adequate cross-ventilation (although I suspect that 'top chamber' might be a little 'under-fed' by the small soffit vents on the hips). You can check cross-sectional areas and I cannot. IIRC it's a C/S area equal to a (some say 15, some say 25) mm slot along the whole length of the eaves. Usual caveats re really tight fitting of the PIR, and v close attention to sealing at jts and perimeters. Air-tight tape and ait-tight foam. (I use Illbruck FM330 as many have mentioned. Expensive but good. (Come to think of it I could maybe apply that to the majority of the materials I use!) Have fun!

I was about to say forget the PIR and go for quilt alone, but that would limit the insulation value at the 'point of the triangle'. You want either a U value of 0.16W/m2K at all points or an average U value of 0.16 for the whole area. You are talking basically 150 PIR and 300 quilt even not allowing for the 'timber fraction' (the proportion of the timber-and-insulation total which is not insulation).

Yes, I heard that too, and have certainly never heard anything to suggest that it's true. Anyone else?

I think there are recent references to Heb Homes on this site.

"The order: 4200x150x50 C24. The delivery: 4800x150x50 C16." A similar thing happened to me. 'Never mind; bring it back and we'll sort it'. Got it all off the roof-rack. Bloke comes round with a stamp, blanks out C16 and stamps C24: Sorted. The explanation, which one has to believe, of course, is that sometimes supply prices are so close that they buy graded (but not stamped) C24 and stamp as C16 or C24 according to the buyer's request. Alternatively they could of course stamp it all C24 and sell it to you for the C16 price!

Switching on my brain I can correct myself here! I must only have had half an eye open not to notice that the porch roof is a hipped roof. So you do have 'ins' and 'out', on the LH and RH sides. Good-sized soffit vents and you are done! I stress'more vents than gut-feeling might suggest' as many builders seem to put c70mm round vents at 1000-1200mm centres. Compare the cross-sectional area of those vents to that of the ideal 'slot' over the entire lgth of eaves and you may be unpleasantly surprised.

Any lean-to roof has a problem in that, ventilation-wise, you have an 'in' and no 'out'. Good cross-ventilation is not ensured, and sometimes cannot be. I am not sure I understand your reference to the 'canopy'. Do you mean the roof to the bay windows. I cannot see from the pic provided whether there is going to be a continuous roof void above the bays from extreme L to extreme R. If there is, and the porch roof connects to it, then there is your 'out'. 'In' via soffit vents (more vents than gut-feeling might suggest) and out via the vents which either already are, or will be provided in the bay windows' roof.

Reading the 'recent posts' panel it reads: Oh Dear Oh Dear Bulb Confusion Oh Dear Oh Dear .... Pocster, what have you done?!

You say the tiles on the front are identical to those on the rear, but the ones on the front don't have a crack straight across, like this (copied from above): Would be really good to see a 'top shot' of that area. Camera, big stick...

+1 to @bassanclan. OK, having seen @Flybywyre's latest pics my presumption ("Can I check what I appear to be seeing? I seem to see a split straight across a slate (slate?), which is mighty unusual.") was wrong. They are obviously concrete tiles (with a different lap pattern) of some sorts. Utterly intrigued by that horizontal gap... Next time you hear a drone flying in the park ask them to take your phone camera up for a spin!

Suggest you follow up ST's point before spending any money to 'fix the leak'. Can you tell us more about the roof? Is it over a habitable space? Is it insulated? If so was the insulation done really well? If it is a Cold Roof (all the insulation below the deck) there should be a clear cross-ventilation path (ideally 50mm minimum) between the top of the insulation and the underside of the deck and (sorry, this is stating the blindingly obvious, but it very often does not happen - you see lots of 'air gaps' without 'ins' and 'outs') it must have 'air in' and 'air out' at opposite ends, unobstructed.

Can I check what I appear to be seeing? I seem to see a split straight across a slate (slate?), which is mighty unusual. I also seem to see a lap pattern which is very unlike the one I laid last year. Yes, we need 'top pictures'. The crack looks quite uniform, suggesting (?? - we need pics!) that it might run all the way across - ??? Would strongly recommend that you do not 'gob it up'. Find the real problem and fix that. Look fwd to seeing the pics - I am intrigued!

Well that sounds a bit positive. Is that the floor dpc I see in one pic? If so, take the opportunity to tape the dpc to the plasterboard (ideally it would be to the VCL -foil - behind the plasterboard but you cannot win them all. Gaps can be air-leaks and since (esp. if you are having MVHR) your ideal is to have the house lined with an air-tight 'bag', those junctions are important. When you deal with one plane think of the joints with all the others, and of continuity - insulation and air-tightness. For example is the kitchen single-storey? If not, my future plans would include taking up floorboards as req'd in the room above, sealing (I would use air-tightness primer and air-tightness tape) around the joist ends (potential source of air-leakage) and then insulate from the kitchen below up to the ceiling (or to the underside of the floor if you intend to 'come back to it later'. I suspect you may have missed a trick in that I assume the insulated plasterboard was fixed up to, and touching, the exg ceiling. If I do a GF room my ideal is to cut back the ceiling and run the insulation all the way up to the floor above, with air-tight works as described. (Yes, on the wall with the jsts parallel you might find a tight space (or no space at all) but cross that bridge when you come to it. It's all tedious, but think of the feeling when you know you've cracked it! Good luck.

"Perhaps we're simply better off double glazing (prioritising on the poor windows) and just living with the weak roof insulation? " Noooooooooo! Please don't think that. You are in the relatively rare position (in that most of us inherit part-insulated buildings) of potentially going from 'nothing at all' to 'something' in terms of insulation, and that is the steepest part of the curve. It's all diminishing returns after that. If you think you can add 100 mm (50 between, 50 under, resulting in a net loss of 50mm 'head-space') then, with PIR, you should get a U value of around 0.25 - 0.3W/m2K (includes a very crude adjustment for the 'timber fraction' - the amount of the insuation depth which is not actually insulation, but timber instead). The target is 0.16W/m2K, but the un-insulated 'base case' which you have is 2.00! Do it, do it, do it, please! Windows can be subjected to 'quick fixes', particularly with regard to air leakage. If you do not have to open them why not tape them up for the winter? And don't discount the 'cling film' 'secondary glazing' - remarkably good. Both short-term measures (and make sure you retain enough 'intentional ventilation') but can yield good results while you concentrate your major interventions on the roof.

Forgot to say, where do you think the air is leaking from? Is there a ventilated suspended floor? If you are 100% satisfied that there is no air-space between board and wall then maybe you don't have to worry re that leakage cooling the whole wall. If you are that satisfied then maybe just seal the bottom of the board with FM330 (though I prefer belt, braces and other measures!)

So was there plaster on the wall to which the insulation boards were attached, or bare block? If the former, maybe OK (although personally I don't like trapping gypsum plaster in a 'sandwich' on the cold side). If the latter, there may be leakage. My preference is always for a parge coat (lime for me) as an air-tightness layer before insulating.

How were the boards fixed? I think I'd be asking a lot more Qs. Perhaps worst case is that they are on adhesive dabs without a full perimeter bead (meaning a risk of - cold - air movement between wall and insulation). If so the best you can hope to achieve (without taking it off again) is perhaps to drill a series of 10mm holes at say 40 - 50mm centres and gun in Illbruck FM330. all round the perimeters. Make good with jpint filler and scrim (or ask the plasterer to do it).

Agree with @ProDave that it won't, as you describe it, have been done under Bldg Regs. Agree that the 'joists' may be spindly, but until you have looked further I would not automatically say don't spend £15k insulating it. A significant percentage of the 19th Century terraces where I live (many of which were built with attic rooms - and some which weren't) have approx 75 x 55 joists. Yes, things get a bit 'springy' if you push the stud walls further into the void, and I am not saying that a SE would be deliriously happy, but delve a bit first. In the terraced house example above there is a spine wall mid-way through the (?9m) depth of the house and stud walls which (albeit with too few nails) to some extent 'hang' the floor from the purlins reducing the front-to-back floor measurement to around 4m. If you can do the insulation work without significantly increasing the loads (and if there are no signs thus far of 'distress')then do consider it, after the aforementioned 'diligent investigation'. What are the joist and rafter sections? It's worth noting that although since 2010 common practice has been to put 50mm PIR between 75mm rafters leaving a 25mm vent path, the 'new' (and poorly-publicised) Gov't guidance (search room-in-roof best practice) suggests min 50mm vent gap, leaving you only 25mm of PIR. Of course we don't have dimensions but it looks like there's little room to spare. In your exploration I would include checking that cross-ventilation is not blocked at the ridge. Even with PIR it is highly unlikely you will achieve something 'Regs compliant' - that would need abt 150mm. Wood-fibre (undoubtedly 'nicer' in my view) would need nearly double that. Not much room left after that. Interesting Q - although the 'conversion' may have been unauthorised it is now part of the house, and the Regs (Part L) say that if you 'add or replace a layer' (add to or take down the exg ceiling) of a thermal element you must either meet the target U value or agree with BCO a lesser std if accepted by them. I would say that you are within the remit of part L in what you do. Other views may be held. Plenty more thoughts to come if required. Some sort of structral opinion once stripped out would be desirable. (IANASE*) * I am not a structural engineer

I note this cheaper one is 145g/sq. m rather than 160-165 for my previous purchases, but seems fine to me.

I have previously paid between £38 and £50 + carriage + VAT per 50 x 1m roll of render/plaster mesh. I ran a bit short and needed just one roll. I was expecting to be charged at the top end of the price range plus carriage. Almost by accident, yesterday, I found E-Roofing, who have 50 x 1m pink mesh at £28.99 including next-day delivery and VAT. It arrived this morning. Excellent service at a very good price. Can't fault it. (I have no prior experience of this company nor of their performance and prices w.r.t. other products.)

"This sounds like the stuff i have found in other other wall, i imagine it was done a long time back (can't really tell, only been in the house a couple of years) crumbles to nothing when you hold it. I was able to take it out in fairly large chunks. You can see some of it hiding in this small cavity, it's the white stuff protruding, comes out it largish chunks, but will crumble quite easily." Yours looks to have fared a great deal better than my former client's, then. If it is cohesive, as it seems to be, leave it, in my view. Do I need to worry if it is formaldehyde?" As I said, quite possibly not really. Most UF foam was done in '60's and '70's AFAIK and is very likely to have done all the off-gassing it wants to by now. Don't take my word for it; do an internet search.

EWI does not need a void. It is generally a 'straight-on-the-wall' system. Agree with @ProDave re the blockwork - may well be single skin. I worry about the piers as a thermal bridge. The foam in your cavities - was it done a long time ago.? A lot of 1960s (and older houses 'done' in the '60s) have urea-formaldehyde foam CWI. Do not, at this stage, worry too much re formaldehyde, but do bear in mind that it can degrade. I took apart parts of a house to replace spalling brickwork and the UF foam was mainly crumbled to dust in the bottom of the cavity. Not everywhere - oddly - but in places. You will know if it is UF. If you find any 'overblow', say in the loft, it will disintegrate as you pick it up.

Just noticed @SteamyTea's ref to "industrial minting wasteland". Do they have that in Kendal too?