Redbeard

So, we've got (?) sub-75mm rafters and some Recticel. The current gov't standards recommend a 50mm ventilation gap with sufficient vents on each side. What thickness of Recticel did they use? I fear that problems may have been built in under that nice-looking roof. (and of course its failure to run off into gutters is at least one). I agree wholeheartedly with @Nickfromwales. They need to be invited back to tell you how they will make what they have done actually work in practice. (Of course we don't know the specifics of the 'contract'. It would certainly be useful to know, for example, is there was any 'you do this and I'll do that' agreement, which could alter the facts of what we appear to see).

Has the roof been stuffed full of insulation? If so you could end up with condensation on the underside of the roof 'deck'. What did the installers tell you?

Apparently it only works when you sit it on a 230V hot-plate.

Are you doing this from the top or from below, with the boards in situ? I assume from the top as you refer to taping as VCL (on the warm side). Why not carefully cut joist-width slivers of 25mm PIR and glue to underside of joists, then when you slide the 125 PIR down it should meet at the bottom of that 'fillet'. Yes, the joint won't be tight as a duck's whatever, but an extra piece of softwood won't be very insulative either.

I forgot to mention that. Maybe it's a basic H & S measure - glue your left glove to the stack with silicone while you use more of the same stuff to make a right mess just below...? (Remember not to take hand out of glove).

Oh yes, poor insulation and a cold void will do it! (a) The supplier should come back and remedy that (unless insulation of ducting was specifically 'by others') and (b) What did they say/promise? I'm trying to establish whether they in effect gave you a 'performance specification'. If so, the 'actual' has not met that spec. You say that you did not expect the MVHR to provide for your full heat load, but not too far off. What did they tell you it *would* provide?

Surely some sort of balancing issue? I would be asking the supplier exactly why they believe that a 10 degree difference in supply air temp is to be 'expected'. I would then expect them to point to design specifics if they are to explain why.

Hmmm. I'd like them to show me it was roofing mastic designed for 'pointing' lead. I have never seen that in white. Even if it is not it may 'behave' OK but it is the wrong horse for the course. Is the scaff still there? Sad if not. It would take 30 mins max including climbing to sort it now. 99% of passers-by will not notice it but I suspect you will, and it will annoy you. Given that they appear to have used 'Leadmate' below I guess it may just be a case of having the wrong stuff in the gun and using it anyway. I am not a roofer, BTW.

Welcome. If you have not already done so, please start a new topic under the relevant heading so that the replies go to that rather than in this intro section.

Looking back on previous posts I cannot be certain that your addition of timbers has actually increased structural strength. Have you got a designer? If your pic 2 went a bit higher then maybe you could avoid a sketch and simply annotate the pic. Critical issues are: How much will the floor come UP (assuming existing additions are not SE-specified) and How much does the roof come DOWN (assuming exg rafters 75mm and the need for an absolute min of 150 PIR, plus a min 50mm air-path above, and a 'plenum chamber' below the ridge). I have some concern re height under the dormer ceiling too. Let's see what it would look like with the same 150 insulation/50 air path and, perhaps (not mentioned for sloping roof, but needed) counter-battens for a Service Void.

Don't think pics show enough. Draw a quick floor plan so we can see where stairs might go. and a section to see if any bits of structure will 'impinge'. At 7'6" (just under 2.3m) its sounds like it is going to be really tight once you 'come down' with the requisite insulation and ventilation above that (assuming 'cold roof' constr'n). Again, lets see a section dwg with dimensions on. It can be as 'sketchy ' as you like, as long as dim'ns are there.

Could have wished for your fully-installed prices, @JohnMo!

I think it never makes sense. Keep the full one.😉

In another thread @saveasteading said: "Hijacking the post. Apologies. Wouldn't it be useful if members could publish the cost of their windows divided by the area?" I'll start: Green Building Store 3G timber, U value appr 0.75W/m2K - factory glazed and finished- 1 bay, 1 opening casement. Appr £635/m2 installed: 2014 Local Yorkshire window manuf'r, 2020. 3G TF to similar spec to GBS: Appr £690/m2 installed Green Building Store 3G timber U value appr 0.85W/m2K, *supply only*, 4 windows and one fully-glazed door: £734/m2 2022

Several years of experience tells me that this may not be 100% accurate! Yes, if you buy a £5 (or whatever that is in Euros) gun it may clog up or bend whatever type of foam you use. No gun which I have used with FM330 has ever failed and they have all been in the £10 - £20 range.

Probably insulation, primarily. Then of course if you want UFH you are all ready to go.

A whole bucketful of them, I suspect.

Yeah, this is head-scrambling! So the original plan was to take off the roof-covering, insulate with PIR leaving the T&G below the rafters and install further PIR below, yes? Insulation-to-insulation contact is, in my view, always best, but yes, if it was only the T&G there and you could get both layers 100% tight to the T&G there should be no risk of thermal by-pass (cold air in the 'layers of the sandwich'). However with the L&P there you are really going to struggle because of the 'snots'. HOWEVER... Since the L&P is 'under-boarded' and no longer needs the 'snots' to hold plaster to lath you could rake off and hoover out the snots and insulate tight to the lath. If it were me in these (less-than-ideal) circumstances I would primes the sides of the rafters and, after foam sealing any gaps, air-tightness-tape the PIR to the rafter edges. It also took me a while to 'orientate' myself re the 25 x 38 battens but I now see they form the 50mm vent path as per BEIS (now DESNZ) best practice guidance. As @ProDave says, this is not a Warm Roof. You will have a Warm Room below it but the roof structure is a(n insulated) Cold roof.

Please be reassured; 'freehand' in my previous post refers to the guiding of the plane(r). The architrave in question was clamped to the bench! There was simply no width-guide on the planer, but a line I worked to! All fingers intact!

Relative piece of cake. Yes, what you suggest is what I was suggesting. If you have a guide on your planer it helps, but I have done it freehand before.

If it is not too thin at that point how about running a planer over part of the width? It has worked for me in the past.

What does the builder's 'padstone' consist of? Assuming that your Building Regs application is Full Plans they could reasonably refuse to approve any lesser alternative, unless the builder has proof that his chosen solution is equally satisfactory. Better to ask for it to be changed now than when there's a whole wall built off the beam.

@TerryELink does not work for me...

I imagine @Gus Potter or others may be along soon but a first stab from someone who definitely isn't a SE. I take your point about spread and the 'job of the truss'. I wonder if something can be done with steel (or, perhaps better, timber with 'properly engineered' connections in terms of trimming for the opening but still keeping the strength of the truss in terms of its role in preventing spread. My initial thought was a specially-fabricated 'strap' from the 2 neighbouring trusses to 'give back' the strength to that cut one. On the other hand the steel probably gives you a complicated insulation detail at what is generally a weak point (the hatch and opening) anyway. Timber - properly 'engineered' - seems the best bet. Insulation at that point will still have to be thought out carefully.

Good? Well, it depends on your view. Like a garage door? Yes, we can do that. So as not to lose too much space the logical idea is just to leave the up-and-over (I assume) door 'for show'. If it is to open you will need a store-room equal in depth to the part of the door which projects in to the garage. I would build a timber stud wall which is finished externally as if it were 'proper external', including excellent air-tightness - and on the internal side well insulated (with a VCL too) of course. Where will your extractor fan to the front go? Might I suggest a trickle vent above the garage door (and a corresponding vent in the stud wall and have the fan (which does not look particularly pretty) at the rear?