G and J

I have looked at Superfoil. I’m guessing you mean the Superfoil laid directly onto the top of the joists, taped to be your airtight layer, then PIR on top, then ply, then furring strips, then ply, then EPDM?

I can move the skylight up to preserve the 150mm upstand around the skylight if I thicken the flat roof, what I can’t do is raise the parapet as that risks us contravening our planning permission. So I guess knowing how big the upstand (watertight layer to parapet) has to be helps me understand what I’ve got to work with.

Just for you, don’t show anyone else…. I think the joists either side of the roof light are 202mm to give some scale.

Not now. We don’t want to go back through planning. There be dragons!

So, we are now looking at our flat roof. Architect has suggested cold roof inside parapet so cross flow is not as easy as it could be. Not got the details yet but drawing suggests insulation is within the joists so presumably the assumption is mineral wool stuffed in and around the 202mm posi joists. I think this means at best a u value of 0.17 ignoring the joists, so in reality equivalent to circa 100mm of PIR on top. If we convert to a warm roof do we still need a 150mm upstand to the inside of the parapet? If not then I’ve room for a layer of PIR above the joists.

I throw a rope over the ridge, secured to something reliable, thence secured to my ancient and rusty scaffold tower. I’ve got climbing kit but that’s not what I mean. A cheap poly rope with loop knots every few feet help enormously and prevent the tower teetering if it’s prone to that.

Just thinking through all this, and I can’t work out how the membranes would be done in practice. Is there a way of sealing the lapped wall membrane to the roof membrane? It could be just laid underneath it but I can’t see a way of that join being taped up.

Bear in mind that trusses at 400mm centres would mean I’d have to attend slimming world like forever! :-0

Goodness, I’m so confused. What are the things you wouldn’t omit? And by airflow do you mean in the loft?

Plasterboard ceilings need to be supported on 400mm centres? Good heavens, I’m so out of date. I’m standing in my kitchen and both upstairs and downstairs ceilings are 12.5mm plasterboard straight onto 600 centres joists or trusses. But then, this house was built a third of a century ago. How times change.

PS we’ve a 40degree roof. And yes, we know that we could achieve the same headroom improvement much cheaper and easier by going to 39degrees…

I was braced for the grimace re PIR in the frame! I do note your concerns…. and I will price up blown cellulose for the loft….. however as for the ceilings….. I think I’ve understood, so I’ve drawn up what will be our version. I know we could easily raise the ceiling more, but I hate chamfered rooms. I did briefly consider raising the tie enough to accommodate everything, wires and ducts. But it feels like more work and materials thus more time and money than plasterboarding straight onto the bottom chord of the trusses.

Noted re the BBA cert, have started that process. The few downlighters would be in the bathrooms, and we could have a boarded service void just in those rooms which would reduce the number of penetrations.

We’re looking favourably at a Reverso unit, so v pleased to hear you say it’s quiet. Which variant did you use?

It will be me and I like the ply/holesaw thing for the ducts. I was intending to seek a proprietary grommet/gland.

And I am erring towards going that way, but trouble is I’ve soooo many other ‘just another few hundred £’ things I’m looking at and they add up. However raised tie trusses will be made from 100mm timber whereas fink will be 72mm so that’s nearly half again as much weight and that will all be manhandled up there.

That would push our ridge height up which means lower ceilings which we are looking at ways to improve. Noted. I think I need to play on paper with this to get my head round it. Upstairs is timber cladding on counterbattens on fire board on frame with 140mm PIR in frame and 25mm PIR layer inboard. Then service void and OSB inside fireline plasterboard. My ceiling VCL will be taped to the wall VCL, so if my tile underlay is taped to my wall breather membrane then the frame will be connected to the loft from a water vapour flow/airflow point of view. I guess that might be a good thing. Plus I won’t need to get a membrane under the trusses.

Simple pitched roof on fink trusses unless we go raised tie trusses for a little extra headroom in the house. No ridge beam, no vaulted ceilings. I see the attraction but it costs us headroom which we’ve not enough of and I think the time I spend doing a service void well will be greater than making it airtight - given that we’ve a handful (maybe 5 in total) downlighters and less than a dozen cable entries. That was my first thought. I still like the fact that it will be easier to get to below the walkway this way but (subject to cost) a PIR/board sandwich felt like the easiest to live with. Inspired! And we’ve a selection of windows to choose from sitting in the bungalow that will be demolished.

If I’ve understood you correctly then, it should work without significant condensation risk and if I seal completely with the right taped up tile underlay, etc.then I will see a tiny thermal improvement. If my periphery sealing isn’t perfect the loft will still be ok but I’ll not get the tiny thermal benefit. I like the sound of that, thank you. I’m surprised, given how often peeps have mentioned ‘air washing’ undermining insulation, that it isn’t more of an issue in the loft. But I also like the idea of a substantially bug free loft too. Our trusses will stop with their outside edge coincident with the outside face of our walls, so I’m thinking that I can do a truss version of a Tony tray on them to provide effectively a continuous air barrier from the VCL under the plasterboard to the tile underlay. I’m also hoping it will only ever rain at night lol

Good call. We’re intending Ergovent vents for aesthetic reasons, so I think the airtight seal will be to them above each room (I know that’s not what I wrote though, so it’s a very valid point). Very good point. Thank you.

Noted. Will keep looking but I like my ‘Thunderbirds’ type Heath-Robinson type mini projects.

I’ve mulled that over sooo many times. We are restricted on ridge height thence headroom (long and not entirely happy story). So to do that would mean raised tie trusses, adding between £500 and £1k to the build. I’ve toyed with the idea of doing that anyway to gain circa 3.5” of head height which can be done without chamfered ceilings (which I dislike), so it might be part of the answer. But I’m put off by the idea that I have a membrane above the ceiling that can billow in the wind. Can you imagine how irritating that might be? So it would need boarding under the membrane I think, and that would make future alterations really difficult and push the cost and effort up too.

I think that’s an inspired idea for warm lofts. Brilliant collection of the rising hot air to recover the heat from. If I knew of a sensible way to airtight a fink truss roof I’d want to replicate that.

Well done you, out of the ground so the biggest risks are now behind you. I’ll be unbelievably relieved when we hit that milestone.

So, thank you for these suggestions. Clearly I’m no further forward on whether my plan will cause me a problem with condensation, but let’s go with the idea of a warm roof and see where that leads…. Some background: Don’t want vaulted ceilings, just a personal style choice. And the increase in build cost and complexity of a non fink truss roof isn’t attractive. There will be a small number of downlighters upstairs, the plan was to put each under it’s own plastic flowerpot (AKA Loftlid). There will be a maximum of 6 no. 90mm flexible ducts up there breaching the airtight membrane. All emerge in one area and then they connect to each ceiling vent. Plan A also has a 4’ boarded PIR walkway down the middle and lots of easy to lay soft insulationy stuff down the sides. We are determined not to store stuff in the loft, like ever, but one does need access from time to time. Theres few rooms upstairs and so not too many switches and light fittings. Ugly but sealed loft access hatches are available and that’s the default, though I’m devising a better looking homemade one. Noted the shag of sealing up each of these but it made everything else so easy that it felt like it was worth it. I have chosen a cold roof because of the following issues: (I’m opened to be corrected/enlightened) If I go warm roof, I think my airtight layer becomes the vapour barrier under the insulation within the fink trusses. That means I’m taping up round many, many truss penetrations. I think this alone would be more time consuming than sealing round me wires and me ducts. I increase the amount of area to be insulated by a factor of circa 1.4. I think that means a lot more than 1.4 times the cost as it will be different insulation, which will require fitting, and it will require a lower u value to achieve the same heat loss as there’s more area to loose the heat. I think again this is more work than my ceiling sealing. I think part of the issue is that we are going for champagne spec on beer money. Airtight and fink trusses aren’t happy bedfellows. But that’s why I need the buildhub mind….