Does anyone know the regulatory requirement for insulating the stud partition walls of an attic room?
I'd sorta assume the same for roof spaces - 300mm rockwool equivalent? But rockwool would be difficult to fix for vertical walls - meaning c.150mm PIR? That'd be expensive, as well as resulting in uneven insulation where it meets the pitched section, with only 50mm PIR.
Anyhoo, about to board out a loft space so any advice welcome.
https://assets.publish "58. Currently, in England, Wales and Northern Ireland a vaulted or flat roof should achieve an ‘improved’ thermal transmittance (U-value) of 0.18 W/m 2K, and a roof insulated at ceiling level should achieve an ‘improved’ thermal transmittance (U-value) of 0.16 W/m 2K. Where this is not technically or functionally feasible, then the roofs should be upgraded to the best U-value possible. In all circumstances, the U-value must be calculated in accordance with the conventions in the current version of BR443 conventions for calculating U-values.
It doesn't say anything for room-in-roof walls as such. Meanwhile:
Roofs: 0.15 W/m²K Walls: 0.18 W/m²K Floors: 0.18 W/m²K
For existing elements in existing dwellings, the limiting U values will be:
Roofs: 0.16 W/m²K Walls: 0.30 W/m²K Floors: 0.25 W/m²K"
so the Part L U-value for new (exterior) walls is roughly the same as a roof, with more leeway for existing walls. I think 0.18 sounds like a reasonable target, although there's wiggle room if not achieveable.
I like this site for doing U-value calculations (use the demo version):
I need 80+80mm of PIR to get to U=0.18. If I switch PIR for 'Rockwool, stone wool' I need 80+150mm to get to U=0.18.
Theo
Also, about fixing Rockwool to vertical surfaces: in my loft conversion there are fibreglass batts between studs, they're held in with string which is stapled to the studs, a bit like a wire fence. I suppose you could do a second layer if you had something to attach a new set of wires to.
For what I've got I'm not super happy about the arrangement, because the existing batts are cut too small and there's a gap between the batt and the stud, ie a chimney where air can convect. Also, I'd like to have a foil barrier to reflect heat. However the current thinking is to add foiled PIR between the rafters to make the loft a conditioned space, making it more useful for storage etc.
Theo
Many thanks for that, appreciated, plenty to go on.
At the moment, at least on the bits I can see, there's 75mm rockwool between the studs, held in place with bits of plasterboard.
Yes, it could be done, but not as neat as PIR. I've just ordered a load of PIR seconds - I'll see what that's like when it arrives before making a decision.
I've seen the PIR between rafters in the roof void on a few re-roofs I've watched. But I really don't get the point - surely heat will be lost around the soffits etc? Added to the fact roofers seem to use a rough interference fit, which must leave gaps.
I take it you mean exterior stud walls rather than partition?
If so 0.18 W/m^2K
PIR in the space between studs and then a thinner layer over the lot.
Here is how I did it...
Yep.
Thanks.
Yes, exactly that. PIR has pretty much doubled in price since. I got some grade B at £10 per 25mm sheet locally. Not sure yet what that entails, but the whole thing is nuts. The payback at retail prices would be decades. For such a simple retrofit fix it should, at the very least, be VAT-free.
Some may wonder why PIR boards are expensive. The answer is quite simple. The extraction of basic raw materials for PIR board production is costly, resulting in a higher price for the final product."
"the best time to buy PIR boards appears to be in November and December." <=== Then you check what month, the web page was updated :-)
But of course, material cost is not the only factor. If a market isn't competitive, you don't get the best price either. There are lots of ways for suppliers to fix prices. They fixed the price of bread here, having previously succeeded at it, one hundred years ago. Price fixing is nothing new.
Imagine for example, if BEV cars came without a battery pack, and you shopped the open market to get one. Your car would be "damn cheap" then. But in the long run, if you arranged things that way, you would be very very sorry. The illusion of success would be transitory (one company would end up making all the batteries, and it would be a monopoly just as the battery industry has achieved on previous occasions).
You would look for monkey business then, among that set of companies. Did someone cut off supply, restrict exports ? Etc. It's a chemical, and I don't think you "mine it".
Paul
I'd have thought the raw materials come from oil, so the extraction isn't that costly. I don't know how difficult the chemical processes used to make them are, but the boards are lightweight so there isn't a lot of material in each one.
Also transport costs. You don't get too many boards to a truck and they don't exactly pack down like Ikea flatpacks. There are only a few plants so there's a lot of transporting involved from them.
An alternative option would be spray foam, where you make the insulation foam on site from the liquids. But that has quality control issues, and the fact that rigid insulation boards are nicer to work with than the sticky spray foam.
Theo
Depends... rockwool is easier to shove into gaps, OTOH PIR gaps can be foamed to hold it in. In the US they insulate stud walls with rockwool all the time.
Another option would be rockwool between the studs (easier to fill out the gaps) and then PIR boarded over the top. The challenge there is avoid gaps that form chimneys behind the PIR.
You'd insulate the soffits too. To allow the rafters to breathe you'd need to leave an air gap between the PIR and the tiles, and that gap is vented out the soffits and ideally ridge ventilation too.
The problem with an interference fit is that I doubt the gaps between joists or rafters are square or even, so cutting the foam to interference fit is awkward. The alternative is leaving a gap and expanding foaming it, but that makes it harder to remove if you ever need access afterwards.
Theo
OOI I looked at what I paid in 2003, it was £670 (today that would be £1,171).
(note that I got mine at a significant discount from the builder's merchant price from a local supplier of seconds - the BM wanted ~£2,200
- I expect there is still a similar variation depending on what you get and form where)
Payback will depend a lot on the starting point. With any insulation each doubling in thickness (and hence cost) only gives half as much again in savings.
Yes, I'd do that - if only to reuse the rockwool.
Still don't get it! The rafters I understand. But what's the point of insulating an under-roof area open to the outside? The under-roof void, beneath the rafters, is always going to need fresh air passing through. In the properties I'm talking about (Victorian terraces) there aren't soffits as I understand the term - it's just a wall plate; the rafters rest on that and ventilation works by entering the roof void between the roof and the wall plate (onto which the gutter is fixed).
And another thing(!) - what's the point of the permeable membrane the roofers use over the PIR? There's no way I can see that moist air is going to get through that. Anyhoo. I'm sure I'm missing something as I've seen this method used on 3 roofs.
Yes, been there, with some very irregular joists/rafters - so plenty of foam filling. The roofers I watched seemed to simply chuck in the PIR - and the ineffectiveness of it all was demonstrated every winter as their new roofs were the first to thaw snow - albeit not knowing the internal temperatures.
On 05/12/2023 00:29, RJH wrote: <snip>
Sounds like a "warm roof" with insulation above the rafters and no air flow below them.
A breathable membrane allows any moisture on the PIR (including condensation) to escape out rather than in.
There are a number of factors at play here. If the tiled roof has a breathable sarking, then you can full fill the depth of the rafters with insulation. If it is an older impermeable sarking then you normally leave a ventilation gap between the insulation the the tiles. Then have vents at the ridge and the soffit. So you have an envelope of insulation that isolates the inside of the building, but there is an air passage on the potentially damp side of it.
The modern solution are products like gappotape. They fit to the edges of the boards and provide enough "wiggle room" to make a proper full depth air tight seal even of the board cutting accuracy is less than perfect. They also mean in many cases you can precut all the boards without needing to adjust the fit to cope with slight variations in rafter gaps etc, since they can take up 20mm of variation.
Here's a picture:
My proposal is the right side. You insulate between the rafters so the part behind the kneewalls becomes conditioned (heated) space. That means you can store stuff in there without it getting damp/rusty/mildewy/etc.
To do so, you create a ventilation path behind the insulation so the rafters are exposed to outside air and moisture/condensation in the timber can escape. For example a 50mm gap is traditional. This gap is ventilated top (ridge ventilation) and bottom (soffit vent). You join the rafter insulation with the wall insulation so you have a continuous blanket.
How to do the joint is a question that depends on your wall construction, but if it's a cavity wall then presumably you want to join your rafter insulation with your cavity fill so there's no air gap. There is a complication that the cavity needs some way to breathe to allow moisture out, which may affect the choice of insulation in that spot (rockwool, sheeps wool and wood fibre insulation are porous to some degree, PIR isn't).
A permeable membrane allows the roof timbers to breathe while preventing wind and driven rain water getting at them.
In short, a building enclosure design needs 4 layers:
That's in order of precedence - if water can get in, don't bother trying to control anything else.
In such a roof we have: Tiles - rain control Permeable membrane - air control PIR - thermal control Foil backing - vapour control
For more info on the theory see:
If the roof is a habitable space, the roof surface is say 150mm of stuff away from the heated space. If the roof isn't habitable, most of the structure is several metres of ventilated air away from the heated room below - the heat will be lost out of the vents, not conducted through the tiles. So it isn't necessarily a good guide to look at the snow melt - it's only if you're comparing equivalently constructed roofs.
Theo
Indeed, the key here is the ventilation is for the roof timbers. If there is condensation it needs to be able to get out, otherwise the timbers will stay damp and rot. Traditionally the roof space was ventilated so the timbers were exposed to outside air and rot wasn't a problem (unless it leaked, when you had bigger problems). If you start insulating you need to make sure that moisture can still get out. Wood is porous, so as long as some part of the timber has a breatheable contact with the outside air then that's enough.
Gapotape helps to an extent, but more of a problem is that older hand-built roofs may be much more uneven than that - eg you could have 350-450mm joist/rafter spacing, which could vary from one side to the other (joists not square to each other). This makes it a PITA to cut insulation because you can't just cut a stack of 400mm, you have to measure every single gap and cut accordingly.
Theo
Yup it can be a PITA job. If you have the head room, then less insulation between the studs/rafters, and a thicker layer on the warm side is easier to fit.
Ah yes - thanks - and for the remaining explanation snipped here. I'm actually talking about directly below the apex - where Gable Vent is shown. That roofspace was membraned/PIRd.
For the top, they have it with an open space that's ventilated at the gable, ie outside air can flow from the gable through to all the truss bays. That obviously only works if you have a gable, so is no good in a terrace.
The alternative is not having airflow between bays, but having ridge vents instead. Then you get airflow from soffit through to ridge.
As also mentioned, if you have a permeable sarking membrane then the timbers can breathe through the membrane, so you don't need a separate air space.
Theo
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