Iceverge

Hi @Otter and welcome. The first mm of insulation is the most important. Can you afford to loose any head height from the floor? If you could loose as little as 50mm you could put down: 25mm PIR. 11mm OSB floating on top. 11mm OSB offset from the first. It wouldn't be the last word in heat loss of course but it would make the room much more comfortable and much better than nothing. For the walls. There's many takes on internal insulation. Badly done it can be a recipe for disaster with condensation and mold behind the insulation and frost damage on the existing structure as the insulation will keep the wall cold. If you go down this route you must ensure that the wall doesn't take on excessive external moisture from wind driven rain leaky gutters etc and also that moist air cannot get to the wall in the first place. Most of all though it must be able to dry out again if it does get wet.

I suppose there is situation where you could just put the Siga majrex /Intello plus etc membrane inside your current ceiling, ensure the RH in the house is low, let the roof slowly dry to the inside then pump something moisture resistant and permeable like EPS beads behind the membrane. I think it theoretically should work, again if you again have a totally sealed membrane, but I very much doubt anyone is going to offer you a warranty. I know funds are tight but is there any possibility of borrowing the money to do the job properly now? Even with interest payments it'll be cheaper than doing the job twice. I can't imagine a bank not being receptive to fixing a leaking roof as it's sheltering their money as much as yours if you're mortgaged.

With insulation in between the rafters the dew point will be much nearer the inside of the house in cold weather, the roof timbers will get damp and especially when they are almost completely surrounded by impermeable PIR they will stay wet. The DPM is really a mute point in the case above as the moisture comes from the house. How to build a moisture-safe flat roof | SIGA Have a scan of their hybird/compact roof here. It may give you options. I wouldn't do it without ensuring the workmanship was very good. The internal variable membrane needs to be done really well.

How about that if you can make it out. With a warm roof and such a thin layer for insulation you'll be at poor U value. That's why I think a metal panel might be better.

Mineral wool batts pushed up against the OSB. https://www.ubakus.de/en/r-value-calculator/index.php?is the one I play with. Have a look at that calculator, you may need to create an account but it's free and it'll give you a good feel for it. In principle if you had a 100% solid vapour barrier you could have all the insulation below the roof structure. The issue here however is that the roof will never be warm enough to dry if any vapour gets in there. That's why there's so many issues with hybrid and cold roof designs and people are much more comfortable with a fully warm roof.

Why not this but omit the screed and powerfloat the concrete?

https://forum.buildhub.org.uk/topic/23724-diy-mvhr-acoustic-silencer/#comment-366744

It's been a while but I seem to remember much more dilute that that. Safer to start weak 90:10 maybe and up the dosage from there. If it's just green algee a brush and a garden hose will shift it. Otherwise a cheap karcher or similar will get the job done.

The safest method is the pure warm roof, with all the insulation above the Rafters. A hybrid option can be done with between and over. Roughly 50mm of PIR at 0.022w/m2K over the top and 50mm of batt insulation below the OSB would be ok. Play with Ubakus.com for ideas. Any more and you'll risk pushing the dew point too far outboard. This will mean that water vapour will condense in the roof and you're back in the same place. Making it a cold roof with insulation only below/between the rafters would work but I would want a bulletproof vapour membrane below the insulation and some really clear ventilation above it. This may eat too much into your head height. As a curved ball could you opt for pre insulated metal sheeting as it's not a long term solution. https://www.steelroofsheets.co.uk/products/kingspan-ks1000lp-lo-pitch-roof-panel/ 80mm panel foam jointed and max 80mm rockwool batt insulation below should be about a U value of 0.2W/m2K and small labour costs.

Buy an extra thermostatic shower bar mixer and plumb it in to the wall. A poo on every other non shataff equipped lol will be like a dirty protest in comparison.

Something like this is what I'd look at if going below a floor level of 150mm. The door would still need to be above the DPM termination in my mind so you'd be stepping up to step down. I would tank the floor to the walls with a waterproofing kit and put in a drainage sump for when the floor floods too.

Here's a quick sketch of a robust detail. Note the 65mm upstand which joins the insulation between 150mm studs to mitigate the thermal bridge. I would consider tanking/rendering the outside of the foundation blocks to stop rain water getting into them as over time the freeze thaw action may crack them.

Here's the reason that any timber products are kept well out of the rain splash zone. 150mm is recommended as a minimum. It can even be a problem for single skin masonry too. The splashed water can find its way to the inside via mortar joints etc. The problem is made worse by having a surface that will allow water to pool or bounce easily. Gravel is a good way to stop this. Make it deeper and put some drainage pipe there and you have a French drain that will prevent the local water table ever rising up and pushing up water inside the floor, even if the DPM fails. ( Assuming the French drain can always flow to a lower plain)

Some thermal simulations of what you've drawn. Think of keeping heat in like keeping sheep in a field. The will get out eventually but you have to try to make their progress as slow as possible. Higher R value materials like Mineral wool and polystyrene are like denser bushes in the fence that will really slow the sheep ( heat!). You need less thickness to slow them to an appropriate level. Lower R value materials like concrete and steel are like really non dense bushes that the sheep can move through easily. When you have a low R value material like brick/concrete connecting the inside and outside it's a thermal bridge. It's so significant that it renders all the other insulation almost ineffective. For my analogy it's equivalent to letting the gate of the field open, the sheep are going to run straight out there and the thickness of bushes ( insulation ) elsewhere is irrelevant. Demonstrated by the Flux vector ( arrow) diagram lowest above.

+1. Spray or brush on a dilute solution. Be careful with your skin and eyes obviously. Doing it in slightly damp/misty weather seems to help it soak in I find. Quite often just a rince with a bucket and brush once the chemicals have worked will do the trick but a pressure washer will make it a simple job.

Will do a model in an hour or so if I can for you ....

Like this one? Had one, it wasn't great. Some for sure but not all. The best analogy I can think of is squirting cordial into a glass with a straw and sucking out beside it. Almost all of it will still be diluted in the water, very little pure cordial will be left.

Consider how best to jamb the PIR upstands in place. You can put them in place first and then wedge them with the floor boards or stick them later with some expanding foam . I think I prefer the former. I would go for more than 25mm if I could. It's just a bit fragile at that depth. Wouldn't bother with the blue foam TBH. Just more PIR.

I reckon if you ran an MVHR unit without any ducting in a central room in a compact house and left all the doors open then you'd have acceptable air quality. Otherwise just make sure you have a good unit, large enough ducting and an appropriate silencer. You can DIY one if needed.

Floor level 150mm above ground. Otherwise you're into a basement type detail really.

@Ticky You must decide if you are willing to accept a 150mm floor level or build as you would for a basement with all the associated detailing and drainage works. Trying to get away with detailing a ground level floor the same as a raised one won't end well I'm afraid.

There's no saving in materials as you'd need to extract from every room so no reduction in ductwork. Good systems are inaudible. Extracts can make plenty noise too if done wrong. With all your ducting running off one of the MVHR fans you'd need an MVHR unit roughly twice the size as the job of shifting air is normally split 50/50 amountg the 2 fans, not 90/10 or 95/5.

Much stronger. Blockwork and brickwork isn't that strong on its own and benefits from being tied into itself (cavity wall) or floors and roofs. The wall plate is a structural element. 75mm gives lots of hold for nails and screed etc from the roofing timbers. At 47mm they would pull out much easier.

Assuming a cavity wall with strip foundations that perimeter strip has a noticeable effect on the thermal performance of the wall floor junction. Thermally I'd put as wide a strip of PIR in there as I could fit with the caveat that too wide and fitting carpets will be a pain.

No, cavity wall with strip foundations.