Insulating a painted/plastered wooden wall onto a cold loft - VCL needed?

[DanRobertsB]

’m looking to insulate a vertical wall onto a cold roof in London. The wall, from inside to out, is paint / presumably plaster / sheet timber (unsure whether OSB / plywood / something else) and a few supporting battens / cold roof.

I guess the key question is whether a VCL is needed on the warm side. And if so, where - on top of the plaster? Or fixed on the other (loft) side (not entirely straightforward, given battens, etc - see below).

In the winter, I guess you’re looking at temperature differentials of perhaps max 23’C inside (typically lower – 18-20’C) and say -4’C outside / in the cold roof.

I’m unsure how best to do this – particularly given:
(i) dew points / risk of interstitial condensation / whether a VCL is needed; and
(ii) the likelihood that we may need access to this wall again, since we’re eventually likely to either replace the large c. 5.8m x 1.6m single sheet polcarbonate skylight that’s at the top of the wall, or secondary glaze it. It’s possible that may require additional structural support. Of course, in an ideal world, we’d do the glazing work first but that may get held up for various reasons, and the heat loss through this wall in the meantime is bad. Even once the glazing’s done, I’m still not sure how best to approach the insulation.


Space isn’t an issue. Seem to be sequence is :

1. Glue downlight covers over LED lights

2. Somehow (?) add a VCL [?] over the all the plywood, studs and downlight covers (so there's a VCL on the warm side).

a) This may not be necessary, given (i) the painted/plastered wood; (ii) if using PIR, the dew point for interstitial condensation would probably fall inside the PIR, which I expect would be fine as there’s not likely to be much moist air in them. Haven’t done a WUFI calculation though.

b) If a VCL is advisable, what should be used? 1000 gauge polythene? Would any of the SuperFoil products be more appropriate?

c) If it is advisable, how;s it best attached?

3. Cover VCL with insulation.
This could be:

(a) 300mm of mineral wool (quick, cheap, easy to work with, breathable), held in place by chickenwire. I have both already. Easy to remove and replace for access. Since mineral wool is breathable, and the wall is plastered and painted, would a VCL be needed on the inside of the timber before the insulation?

(b) As above, but sheep wool insulation (more expensive but no nasty loose fibres in the cold loft space, which is used for storage; good resistance to any moisture).

(c) PIR. Probably in two layers (40mm between the studs, 100 or more over the top). More expensive, harder and more costly to cut and fit, harder to remove and refit once fitted. But perhaps doesn’t require a VCL…? Struggling to see much advantage.

(d) A hybrid. 40mm a or b – or RW3 rockwool sheets – between the studs, the overclad the lot in PIR.

I suspect 3a or 3b is the way forward – just wondering whether the hassle and expense of VCL (2) is sensible. And if a VCL is fitted, does that create the risk of water gathering between the VCL and the wood, which would presumably cause damp/wood rot, etc?

Thoughts greatly appreciated!


Loft side of wall/skylight support



Wall and skylight from inside

 

[robinbanks]

Easier to put a non breathable paint on the room side, wool between studs and PIR over them, if you're concerned that the room side will generate lots of moisture that will make its way into the fabric

 

[DanRobertsB]

Easier to put a non breathable paint on the room side, wool between studs and PIR over them, if you're concerned that the room side will generate lots of moisture that will make its way into the fabric

Thanks - that's an appealing solution.
Tempted to use mineral wool instead of PIR as it'd be easier to install and cheaper (space isn't an issue), if that's still ok. Or would it be advantageous to use PIR - e.g. because of the foil, and because it's hard to imagine much water getting into, or condensing, within it?

 

[robinbanks]

I don't personally feel wool is easier to install to vertical surfaces with nothing to retain it all round, than PIR is. When comparing costs, be sure you're comparing like for like in terms of cost per amount required for equivalent thermal performance - 100mm of kingspan is typically equivalent to between 150 to 200mm of wool, and is a lot easier to install to get the claimed performance. It's easy to over-compress wool, diminishing its effectiveness

 

[DanRobertsB]

Thanks Robin.

I have a bunch of spare chickenwire type stuff I could use to create cages (probably and inner and and outer) to hold the mineral wool. That seems quite quick and easy to me, comapared to PIR.

My concerns with PIR are:
- Material cost - it works out more for 150mm PIR than 300mm of mineral wool (equivalent u value), even more so once you figure in labour and alu tape/gappo tape.
- Labour cost higher with PIR - e.g. PIR will only fit through the loft hatch if cut down into <590mm strips. It needs cutting down precisely, given the angles, etc; sealing with tape; and generally careful fitting (I gather it needs to be fitted well to be effective)
- It doesn't breathe. That may or may not be an issue, but if there isn't an effective VCL on the warm side, at least mineral wool won't (if my understanding is correct) create damp issues (e.g. most ceilings onto cold roofs are insulated between and above the joists at with mineral wool, without a specific VCL; but an air gap is left above the mineral wool for it to breathe).

However, it's entirely possible I'm wrong about some or all of the above!

 

[noseall]

That seems quite quick and easy to me, comapared to PIR.

It's 10 times easier. Horrendous stuff to fit. Cavity batts perform a wee bit better than regular loft insulation.

I don't personally feel wool is easier to install to vertical surfaces with nothing to retain it all round, than PIR is

Lol.

 

[robinbanks]

I have a bunch of spare chickenwire type stuff I could use to create cages

So you want to use chicken wire to turn wool into something more like rigid insulation?

I think I'd fit the wool and then use a few fasteners through it and the chicken wire on one side only, to retain the wool

 

[DanRobertsB]

So you want to use chicken wire to turn wool into something more like rigid insulation?

I think I'd fit the wool and then use a few fasteners through it and the chicken wire on one side only, to retain the wool

Yup - that's the idea (chicken wire on the outside only). The idea being it'd help retain the wool in its upright positioning without compressing it / it'd be possible to infill/top up the "cage" (i.e. the part between the wall and the chicken wire) with wool.

I guess wood fibre might also be on the menu as well/instead (though it does seem to be quite a lot more expensive).