Loft insulation - cold vs warm, material recommendations, etc

[DanRobertsB]

Hi

I’m hoping to improve the loft insulation of a cold roof of a large (c. 350m2) wood clad, timber frame bungalow that is currently poorly insulated and struggles with heat retention. It is currently largely single glazed (I’m working on secondary glazing with Magnetglaze Extreme Pro and 4-6mm Perspex). It’s been extended multiple times over several decades. Much of the construction appears to be wood clad single skin brick or breeze block with no insulation. Roof is a lightweight wooden structure with a thin metal roof tile (Decra Classic).

Given the poor thermal performance of the building, I’m inclined towards over-insulationg it / being overly cautious - unless of course that makes no sense.

There are a few options.

1. WARM ROOF
The roof will need replacing at some point, so one option would be to do that, and make it a warm roof (e.g. PIR boards, on the underside of the rafters, with a new metal or EDPM roof). Downsides are the significant cost of the new roof and PIR insulation, and the additional costs of heating the roof space (rather than just the rooms below, as with a cold roof). .

2 COLD ROOF

If going with a cold roof, I'd welcome some tips.

(A) Loft over bedrooms – currently partially insulated with loose fibre.

I imagine the plan would be:

• Add downlight covers over LED lights. Loft Lids seem to be decent, and one of the least expensive options. Other suggestions welcome.
• Add more loose fill mineral wool where there is none between the joists (which are c. 220mm), so c. 200mm. Glass mineral wool? e.g. Knauff 40 or 44? Or Mineral wool? Or actual wool?
• Then add another layer (I assume 100mm – but perhaps 150-200mm worthwhile) everywhere over the joists/existing insulation.
  • The problem with this is you then can't see where the rafters are, which means you can no longer safely move in the loft, which might be needed for access. Any tips on how to address that?
• For where the solar inverters/batteries, etc are, I'd need to create an access “walkway” (well, crawling patch) from the loft hatch to there for occasional access. This could either be done with 175mm Loft Legs, with chipboard on top (loose fill insulation under). Might PIR boards (75mm -100mm thick? E.g. Celotex GA4075 or GA4100) resting directly on the rafters work instead? Though I guess if they also need to be topped with chipboard or similar, there's little point and the Loft Legs solution would probably be just a cheap/effective, if not more so.
• Is it ok to leave the wires and pipes, and simply cover them with loose fill insulation? I think (/hope) the wiring's all low amperage - i.e. for LED ceiling lights. If so, I'm guessing that's not a problem. But who knows what all this wiring's for ... Not sure if there's a way of testing?

Photos:

(B) Skylight
There is a large skylight.

• Pictured from inside the loft:

• The other side is plasterboard into the room:

The (presumably plywood) wooden frame around it is partly insulated. Planning to:

- Remove existing insulation (and reuse elsewhere)
- Add downlight covers over LED lights*
- Add a v1000 gauge polythene vapour control layer over the plywood, studs and downlight covers (so there's a VCL on the warm side).
- Add insulation between the studs. These are c. 45-47mm. So I was thinking of 50mm compression fit Flexi Acoustic Slad Rockwool, or 40-50mm Rockwool RW3 (60kg/m3) for its acoustic performance, as well as thermal. Might be a bit tight compressing it ... 25 or 40mm seems preferable than risking a poor seal using 50mm. However, the Flexi only comes in 50mm, and it's hard to find RW3 in 40mm (unless you order a whole palette!).

- Clad over it all, studs included, with PIR boards. I was thinking 100mm (as part would need to be inside that secondary 100mm frame). Presumably securing, and taping over any gaps, with aluminium foil tape. Should the PIR go all the way to the roof membrane?


(C) Pipe lagging - recommendations welcome.
This may not be necessary - could just put loose fill insulation over?

(D) Acoustic isolation
I have an area that is decently insulated, but I'm keen to bolster the thermal and particular the acoustic isolation. Any tips on what to do here? Another 200mm of loose fill glass fibre on top? Concerned that using something more robust - like ROCKWOOL Twin Roll Insulation 100mm - will compress the glass mineral wool below ...




Comments on how best to approach this and recommendations for suitable materials (sizes, brands, etc), suppliers, etc would be most welcome. Keen to avoid condensation risk/damage to timbers, etc. And I guess a bit concerned that as some of the loft is used for storage, improving in the insulation at ceiling level exposes the roof storage space to greater temperate variations.

I’ve assumed that because I’m essentially topping up existing insulation that there’s little sense in changing horses on material, e.g. mineral wool or actual wool instead of glass mineral wool, but am open to ideas.

 

[blup]

I would concentrate on insulating the loft joists as the space above is not going to need heating, so no to insulate rafters. Just pir and a few crawl boards, loft legs look gimmicky and unstable lMO. You can determine purpose/ cross sectional area of cables by measuring width but shouldn't be a problem with lighting.

Blup

 

[robinbanks]

Any tips on how to address that?

Loft legs as you go and board out

loft legs look gimmicky and unstable lMO

They're suitable for purpose, that purpose being occasional use, storage support.

E.g. Celotex GA4075 or GA4100) resting directly on the rafters work instead?

Yea but they aren't load bearing. Cover with boards

The (presumably plywood) wooden frame around it is partly insulated

What plywood? Not sure I'd bother with VCL if nowhere else has one

Is it ok to leave the wires and pipes, and simply cover them with loose fill insulation?

Depends what the wires serve. 1mm or 1.5mm cross sectional area cores are lighting, 2.5mm are sockets. I'd strive to lift or reroute wires where you can so insulation runs under them, or extend them with junction boxes that remain accessible if needed. Be wary of burying cables; lighting that serves LED lights will be ok, but if you pity cables you should work out how that lowers the current carrying capacity and fit a suitably smaller breaker so that if someone does replace all the led with halogen one day, it trips rather then setting the cable on fire

Lagging pipes is pointless if you're burying them in insulation

So I was thinking of 50mm compression fit Flexi Acoustic Slad Rockwool

do you specifically need acoustic insulation? What noises from the loft or outside are you really looking to quell?

I'd just go for the most cost effective wool you can find, aiming to reduce the thermal- the acoustics seem less important in this application

Wool is easier to work with than PIR. By all means clad over with boards as it'll do a good job of holding the wool in but make sure draughts can't get down the back of it

And I guess a bit concerned that as some of the loft is used for storage, improving in the insulation at ceiling level exposes the roof storage space to greater temperate variations.

It's minimal if the loft is ventilated. If you're using it for storage you should make sure that you store things that can cope with the temperature swings

that there’s little sense in changing horses on materia

Doesn't matter about changing type; this will be an expensive endeavour- put whatever is best value on top of whatever is there already

"How trades insulate"

What a ****show

 

[DanRobertsB]

Thanks for the replies!

Plywood/VCL/skylight

The plywood (or similar) is around the skylight (see below). So on the other side of the verticals here is living space. There's basically no effective insulation, but also no current VCL. I gather a VCL is normally needed on the warm side to help address interstitial condensation risk So, my thinking was (from the warm side (inside) to the cold/loft side), you end up with:

Paint (presumably on plaster)
Wood (assumed to by ply)
A VCL (I assume 1000 gauge) - over the fly/struts, etc - so it's sealed around the skylight
Rigid insulation like RW3 or compression fit Flexi Acoustic Slab Rockwool between the vertical struts
100mm PIR over the whole lot (taping gaps with aluminium foil tape)

• Pictured from inside the loft:

• The other side is plasterboard into the room:

Acoustics / etc
In terms of acoustic performance, there is one section (c. 4x5m) of the loft where acoustically isolating the room below as much as practically possible is desirable. The roofing's (being basically a thin metal sheet) is not quiet (rain sounds, wildlife walking on it, bird song, etc), and in that particular room it needs to be particularly quiet.

But ... the plot thickens! There are two roofs - one more recent construction (220mm joists, as per photos in my original post). Turns out there's basically little to no insulation between the joists, so I'll put 200mm glass mineral wool between the joists, and another 100mm on top. Loft legs and chipboard to provide occasional access.

The other roof - which is perhaps c. 180-200m2 - is more challenging (and this is where the room that wants to be acoustically isolated is) A fair bit of the loft is used for storage. Loads of photos here. It appears to be uninsulated rafters, topped with a slab perhaps 100mm thick which is made of compressed hay/stray (or similar), covered with what looks like bitumen (or similar). This was the old, flat roof. Probably constructed mid-1950s. On top of parts of this there's c. 50-100mm of glass mineral wool insulation (see photos below); other parts there's nothing or boarded storage. On top of this old flat roof, a lightweight timber frame was built, covered with a membrane and the Decra Classic (or similar) lightweight roof. Parts of it are quite low, so access will be challenging to insulate it, and occasional access might be needed e.g. for wiring.

This is giving me serious pause for thought on how best to insulate the flat roof. There appear to be a few options (and doubtless some I've not thought of):

(i) Lift (and possible remove) all the hay slabs. Insulate (as you normally would) under the rafters (PIR? Glass mineral wool?). I'd guess the rafters are max c. 200mm deep. Query whether to keep the hay (which must have some insulating effect - but most of it's bitumen sealed on top); and if so, whether to insulate on top (see B). Or do something else (e.g. remove the hay, and insulate on top e.g. glass mineral wall/loft legs/air gap/chipboard, or PIR boards on the rafters, covered in chipboard). Either way, time consuming and costly, and would need the loft clearing, boading taken up and relaying after, etc. Unsure whether might create condensation issues.

(ii) Insulate on top of the flat roof where practicable - e.g. loose fill. However, I'm unsure whether putting insulation (whether PIR, glass mineral wool or something else) on top of the flat roof going to be effective, and whether it might create condensation issues. You'd have: Ceiling // rafters and air gap // hay slabs // bitumen covering // insulation // cold roof. Upside is this is relatively cheap and easy to do.

(iii) Make it a warm roof, and insulate under the rafters (presumably PIR). If doing that, it might be the time to bite the bullet and re-roof (i.e. replace the Decra Classic) at the same time, either with something like EDPM or a sheet metal roof (other suggestions welcome). Given size, something economical! It gets a lot of leaf (and conker) fall ... Given limited funds, I'd prefer to insulate first, and then replace the roof later if that's really necessary - provided there's a sensible way of doing that. Again, another costly option. (But there may be no escaping that!)

 

[robinbanks]

I gather a VCL is normally needed on the warm side to help address interstitial condensation risk

Could do a WUFI analysis if you want but I'd say that the dewpoint in a PIR+wool+plasterboard buildup is going to be somewhere inside the PIR, and there ain't a right lot of moist air in it.

If doing that, it might be the time to bite the bullet and re-roof (i.e. replace the Decra Classic) at the same time, either with something like EDPM

Were the you poster with the massive long roof lights stuffed with leaves?

I recall the EDPM quote was 10x what we thought it should be..?

 

[DanRobertsB]

Thanks - good to know on the dew point/PIR front. Would definitely be an easier (and cheaper) job without the VCL.

Guilty as charged on the long roof lights. The EDPM estimate was a ball park from a roofer, including PIR insulation underneath, for about 350m2 of a series of roofs (see below), which would also require the remove and reinstallation of 21 solar panels (with bird netting). I've not yet got formal quote, and would very much welcome it being significantly cheaper. Certainly if I could find someone to do a good job on it for 10x less, I'd book it in tomorrow! (And it'd simplify a fair bit of the insulation project too).



For scale, the largest skylight (centre) is c. 2x6m, and the flat roof in the centre is about 6x4.5. Solar panels not shown.