G and J

Just for you. Frame insulation: Then VCL and more timber battens and an extra 60mm of mineral wool.:

Thank you. We start the new year with our chilly having a birthday at Christmas. With him and me on site the average site age has just risen to just over 70. You’ve heard of Jerry built. Well ours is geriatric built. lol

I’d strongly agreee, using a piece of wood knelt on as a guide for me really sped it up. Both polystyrene and PIR made mess, but for me only PIR was unpleasant.

We ended up with 335mm to 345mm to fill to get to FFL which includes screed with UFH. I am keen on a good thickness screed so wanted circa 100mm. So for us the insulation thickness was given. We want the biggest living space we can fit in to our narrow plot so our walls haven’t the most fantastic U values so to an extent the floor and ceiling can sort of compensate a bit, so I targeted 0.1 for the floor. Going from the architects suggested 0.13 U value to that felt ok, cost wise. Beyond that felt silly.

Because I’m not on site today I just calculated that had I gone for all PIR instead of 120mm each of polystyrene and PIR the payback (assuming underground temp of 8C, COP of 4, peak time electric cost, etc.) would be in the region of 117 years. Hmmmm. Perhaps a good illustration of the diminishing returns of insulation.

I’ve heard of ash felt floors being laid in old places which as well as being water resistant, apparently made the places warmer than they were (probably was bricks direct on damp earth or the like). It would be interesting to see how homes are built in 50 years time given the pace of change.

How long before some clever wotsit comes up with a self levelling compound, made in a lorry on site and pumped in, that also functions as a DPM. Until then we’re still talking sheets. I accepted a slightly higher wastage (~4%) as I didn’t want small bits and when I planned it out that required a couple of near half sheets spare. Some of that had been used elsewhere like the former for the shower tray to exclude the screed. Unusually for me, the sheets went down almost exactly to plan. I think the only change was to make the PIR layer slightly easier to get as tight a fit as I could.

In the excitement of moving soil from a heap to a hole I forgot. Sorry. Will try harder!

I think you need a certain number of air changes per hour to keep all fresh. With trickle vents it’s a blow through. But with MVHR one still (as I understand it) get a good flow through if fresh air. It at first appears confusing that we have a really low air change score to make MVHR worth having, but that’s a test of the leakiness of the house, not the number of air changes that a good ventilation system will give.

I honestly couldn’t disagree more. But as we’ve not built the house yet my opinion is not based experience, just theory. However I do know from experience that moving air takes a lot less energy than heating it.

We’re self builders. Faffing is what we do. Why do the obvious when one can avoid doing anything for weeks while researching? 😕

I’m not sure I agree that it’s impossible to calculate a payback for the cost of MVHR because using Jeremy’s spreadsheet our heating requirement is a lot less than half of what it would be with sub one air changes per hour compared to say five air changes per hour that you’d need with a non-heat recovery system. However, the extra spend to get airtight would be quite difficult to calculate and indeed we’ve not done a cost benefit calculation in payback terms, we’ve gone for it just as an act of faith for comfort. So despite it being theoretically possible, I think calculating payback for MVHR for us at least is rather pointless.

Me too. Big time. I’ll take and post some pics tomorrow but you really should get out more lol She has served, and hopefully will continue to serve, with distinction. Bless her. I’ve become quite attached.

PS. Regardless of economy polystyrene is so nice and easy to work with - the spring in it means you can easily get it really tight - for me that makes it worth serious consideration. PIR May have a better lambda but the better fit of polystyrene must compensate a bit methinks.

I was dead set on 250mm of polystyrene with the DPM effectively sandwiched in the middle. Our slab however came up higher than planned so we ended up with 240mm of space so we converted to a PIR layer atop the polystyrene. I sort of priced various options using online prices and almost ended up designing by spreadsheet. Then I talked to my favourite building supplies bunch. A few mildly confused emails led to a clarifying phone call and it being pointed out that if I used 120mm of both then stocks are held locally and a good price could be had. Suddenly it got easier. We got prices matching or beating the best I could find online. So given my experience I’d have a coffee with the most helpful local dudes and see what they suggest. I would not contemplate a step in the DPM. I’d put a flat layer of polystyrene down then the DPM then vary the above later. I found to get good straight cuts in PIR I had to saw and 120mm is not much more nasty than 50mm. Both I found horrible. But I did notice that when the foil face became detached the sheets were much weaker and had to be handled more carefully. I suspect one gets less horrid dust with foil faces too.