Rishard

I’m doing a cut roof so I was going to bring vcl from ceiling down to meet internal plaster. At the ridge I’m using glue lam which are on show. I was going to drape a metre strip of vcl over the top of the glue lam sandwiched with a 6x2 timber plate which the rafters would fly over. This could then be taped into the ceiling vcl when roofing done. Im using 175mm rafters. My latest plan was to fill them with 180mm of mineral wool at 0.034 with 100mm pir boarded directly below the rafters taped, an additional vcl and then a 25mm service void. I’ve looked at the above make up and wonder if I could flip them, pir on the outside as you say. My concern is that if the vcl isn’t 100% then I may run into issues… thoughts?

Yes it’ll work well with posi joists. I saw the green building store had to rip PIR to flush up the web on their posi rafters which would be a bit of a pain.

Would there be much of an issue with the vcl moving to below the mineral wool? It would make joining with my airtightness layer internally much easier.

Just worked out for mine, the same thickness of Knauf Omni fit slabs 0.034 come in around 2k cheaper than the blown cellulose to the same thickness. Cellulose - 0.038. Bit more time installing but not drastic work if rafters set out to suit the slabs.

I love a vague plan. Thanks for the geography lesson, must have needed more sleep.

Is that a little leaded oak lintel? Lovely stuff

You can’t hang slates off a 22mm woodfibre sarking board. Depending where this is, Scotland slate roofs differently to the uk. Uk on to tile batons which is what this plan refers to I believe.

Looks great! The stone here is very soft sand stone and would degrade even with an NHL. What is the brick you’re using?

Ah yes of course. Are you having a service void? I had a calculation done similar to your build up with a 45mm mineral wool filled service void and using a 40mm woodfibre externally and got to 0.10W/m2K

I’m the joiner fitting so shouldn’t cost much 🥴 Which option roof build up are you favouring then? Any wood fibre on yours?

Would you use something like propassive osb for that osb layer or another vcl laid over the osb? I’ve cut too much PIR. I’d love to not do it for my house.

Lime is another world. My guesses are, if your Bricky isn’t familiar with it then you would need to find another. I’ve no doubt he could work with lime but it does require a bit of guidance. It would be a bit of a learning curve, if you’re happy for him to learn on your job, that would be up to you. What you really want is a hot mixed lime mortar, gauged appropriately with a local pozzalan to fit in with the local vernacular. I’ve rebuilt a barn with hot lime, some solid walls and some cavity wall with insulation. In this case it was stone with a 150mm bed with wall ties. We laid stones roughly 7-9” inches height up to 30” long. Compressive strength is very different with hot lime mortar. It firms up very quick unlike putty mixed lime which means you can lay as quick as you could with sand and cement.

Maybe not your first choice but one of our recent grade 2 listed projects ended up with solar slates. Not cheap but certainly look the business from street level. To the untrained eye they look a bit like Spanish slate. All arranged through official planning applications with listed building consent ect through our architect specialising on listed buildings. As others have mentioned, a local architect is priceless in knowing what the local preference of the planning department is.

I have a similar calc done with pir with 120mm above and 100mm between on 120mm rafters. I would need to use purlins to get my rafters down to 120mm which I’d rather not. I reality I need 175mm rafters. Does the 18mm osb between the pir and wool need to be there? Or is that for a bit of racking strength? I’ve seen roofs with pir over rafters without a deck below on pitched roofs. Are you using the PIR as the air tightness layer in this lower drawing? Or is the osb part of the airtightness layer?

For the JJI joists which are supplied to me in full lengths ready to cut to length with very little waste, mostly the angled cuts for the ridge and wall plate come in at £3200. For the same lengths in eco/posi joists they came in at £7320. The posi’s would need more designing that this though as I have a valley which I would be hand cutting with the JJI joists. I haven’t done a posi valley so if they can’t be supplied with enough waste on the end for the cut no doubt there would be a greater cost for supplied/cut to lengths. Who did you approach for blown mineral wool? I see Knauf do a supafil product. What was the product you were quoted on?

While researching for my roof build up I came across these. I’m in a similar situation, considering heights and insulation / timber build up. Can I ask, how does it work with the pir option with mineral below? I like the idea of full PIR sheets and mineral between the rafters due to it being easy to install. I also like the larger service cavity for some ‘slim’ mvhr ducts in a couple of rooms, which both these examples have.

Jumping in on this as I’m in the process of planning a similar buildup for my roof. I’ve managed to get a price from a warmcell installer which is a good start. His material price plus install is very similar to a mineral wool alternative laid to the same thickness, if not a little cheaper if you consider the 0%vat. I’m looking at a few other insulation options currently and if I were to go with warmcell or a deep mineral wool I’d be using JJI rafters due to being easier to cut and work with onsite. They’re also half the price of eco/posi’s for my job. Are you having vaulted ceilings? @SBMS @Nickfromwales could I get Gordon’s number also?

What uvalue dis you achieve with this insulation set up? Did you stick with the 150mm pir + the mineral wool between? I like the concept as it’s a pain fitting pir in between. Mineral wool is much more forgiving but doesn’t pack the same punch and requires greater thickness. Full sheets of Pir externally over the top sounds dreamy, minimal cuts and good performance. I’ve looked at this type of warm roof with wood fibre as the external insulator but the price per m2 is twice as much as Pir and half as effective (in a warm roof) it’s hard to justify. Any other iterations of warm roofs which don’t require 300+mm rafters?

I guess the further down the fixings the more stability? Once the noggins are all in between the rafters and the blockwork the strap just locks it in place. If the fixings were only in the top block then it had a higher chance of that block becoming lose? If the fixings ran further down the wall into bonded blockwork I guess this would help a bit?

In hindsight maybe a warm roof would have been best in this situation. I’m a bit limited with head height on some areas of this build as 2 rooms have low eaves (1.8m) if I was to move over to a warm pitched roof I’d have to lose the height internally. Not entirely out of the question. I may look into my options regarding this. Thinking off the top of my head, I have 200mm rafters which i planed to fill with insulation. If I introduced a purlin mid span I could reduce my rafters down to claw back potentially 75mm from the rafter thickness. i wonder if I could use a really long straight restraint strap (2m) and bend it down 800m into the cavity for more fixing points and have 1200mm across the underside of my rafters.

That all sounds pretty sensible and practical. A much easier approach then trying to get the plaster in to skim a meter strip around the whole joist band area. Will look into airtightness paints next. My eco joist supplier is able to provide my joists with an amount of ‘trimmable’ ends in order to get the best fit. In reality it is much more difficult building up all the masonry 2 storeys without the first floor to work off. We’re about done now so looking forward to putting a roof on and installing a floor.

The inner skin of blockwork is often cut to just below the rafter top. Is there any reason it couldn’t be left a little short and cut to the underside of the rafter? In this situation it would allow my cavity insulation to connect to my roof insulation (insulation between rafters) I could still mount restraint straps to the underside of the rafters however the straps wouldn’t be ‘built in’ and they would need a masonry fixing to the blockwork. Most of my ceilings are vaulted / cathedral ceilings.

I hear what you mean regarding the cantilever. I will have a word with the sparky and see if a pre planned conduit run or chase could work behind the ledger. In regards to airtight paint. Do you mean seal the top and bottom edge of the ledger to the blockwork? Would an airtight silicone type mastic be good for this if the timber shrinks? Never used airtight paint direct to blockwork/timber. Am I right in thinking you’re talking about applying it at this junction, prior to plastering?

In relation to this topic. I’ve installed ledger boards in the past with resin bolts and had good success. I haven’t ever wet plastered behind the ledger boards. Would you just wet plastered the area behind the board before installing it or the whole room? Obviously difficult if there is no floor in yet. I plan on using ledgers with my design and eco joists on 400mm centres. I remember once spacing the ledger off by 25mm behind the each resin bolts and had so the electrician had an easy route down for his cables. Do we think this is advisable or should be avoided? I have a concrete ground floor so imagine most of the socket ring main and lighting coming down from the first floor void.

We often over size the ‘height’ which allows for some adjustment up the roof to work with your roof tiling/flashing kit. Once the window is fitted the opening should have a plumb piece of plasterboard running into the low side of the velux and a horizontal plasterboard to the head. This allows air to flow in and then out of the velux reveal. All standard practice and outlined in the fitting instructions. The rough framing is always better left taller on height to allow for this. Hope this helps. Width is usually 40mm oversized for perimeter insulation depending on supplier.