Loft in our house is a bit of a mess – part insulated with itchy stuff, part with cat litter stuff, part not at all. I want to re-insulate to modern standard but ideally without raising the floor (headroom is already limited). Current ceiling beams are a combination of 6×2 and 8×2 but I am being told I need 27cm of standard insulation and I want to board over the top. Are there any materials that are still easy to fit but can be laid in a thinner layer to get the same insulation value? Could use celotex but chappie I am using says it will take an age as each panel needs to be individually cut and is pretty expensive (estimate I have about 100m2 to do). Suggestions appreciated.

Seconds and Co are good for imperfect celotex-type insulation, but even that gets expensive. 140mm board would be about £1400 supply only for 100 m2, and then you’ve got to pay your bloke to fit it – an easy enough job TBH, but more work than rolling out some rockwool.

The issue with using Celotex between the joists is it really does need to be fitted well – if you’ve got any gaps you’ve got a channel to bypass the insulation. If you’re paying someone else to do it you won’t really know it’s been done well unless you get an infra red camera on it afterwards, and even then you’ve got to do something about it.

How about a mixed approach? It’s what we did for our last place. Take out all the existing insulation. New Rockwool between the joists so that layer is well insulated without gaps (it’s easy to cut and fit rockwool well). Then 70 or 100mm of Celotex over the top. Loft boards on top of the celotex.

Depending on how you’re going to use the loft will determine whether you need to add secondary joists to support the loftboards – if it’s for storage, ie not being walked on on a regular basis, you could just lay the loftboards on top of the celotex (with some bracing to make sure it doesn’t slide. Worked well in our loft and ours was a walk in room we *were* going into regularly.

If its a fllored loft youare using surely the insulation should go above the loft room?

TJ – only if it’s being used as a habitable room. No need to insulate if it’s just for storage.

Celotex with spray foam around the edges to seal will give you more insulation per mm of depth than most others. It’s expensive but if depth is your limiting factor it is the way to go. Or have the whole thing sprayed with foam – even better sealing etc

The other option to insulate the roof and turn the whole space into a conditioned one. Saves headroom.

Individual fitting of celotex panels is a ball ache, i’ve done it and i wouldn’t fancy it again tbh. It was in a good working space too.

I’d be thinking along the lines of simon above^^^. I’d probably be putting in additional joist, easy enough

Fitting celotex is not that tricky. I recently used it to insulate the lath and plaster walls in my attic flat. Measure, cut, fit then fill all the gaps if any with bodgers foam. and that was in a horrible working space ie the eaves of the flat. I had to crawl in and out with every board

Is it feasible to provide part of the insulation between the joist and part between the rafters? You’d need to do some U value and dewpoint calculations, I think. I might have read somewhere that it wasn’t considered compliant with Building Regs to split it, but I suspect that meant not compliant with the ADs. That isn’t legally required provided you satisfy the Regs another way but some Council officers can be awkward. I have been thinking about it myself but haven’t fully researched it yet; does anyone else know?

Do you really need 100sqm of storage? Our loft is a touch bigger at 150sqm but we only boarded about 30sqm. Raised it up to get 250mm underneath. 400-500mm everywhere else.

I’ve just done a whole house (280m2 roof) in superfoil, ti’s bloody brilliant, if a little fiddly at times, house is proper toasty.

You can lay straight over joists letting it sit down into them and then board straight over as 1 option.

Sui, looks great. Looks expensive! How much did it cost, and which version did you opt for?

if you only want head height from a low floor in the middle of the loft, how about using denser\more expensive\less insulation there, then thicker wool stuff everywhere else, with higher ‘floor’ over it for actual storage? I only have full access to the middle ~2m strip of my loft, everything else is fitted out with shelving

UrbanHiker

Member
Sui, looks great. Looks expensive! How much did it cost, and which version did you opt for?

The pitch part of the roof, was supposed to be SF40bb outer and just SF19 inner (i’ve gone duet system), but the SB40bb was out of stock, so i went with SF19bb out and SF19 inner and back filled with acoustic insulation, this gave me a 0.13U thermal thing which frankly i didnt need but have it now.

SF19 on it’s own gives about 0.28U, which for a loft floor conversion is fine, it’s also on 10 meter rolls so you get very little air passage as you just roll it over and let it drap in between, even then you get a load of tape and tape the seems up. It costs about £80-90 per roll if you look around, and as it’s effectively 15m2, that works out as £6m2, whereas you will pay ~£20m2 for the equivalent U values.

Roofing superstore is a good place, but so is Ainsworth Acoustics Ltd (work out roughly the same price)

Cutting up PIR board is horrible, and even work with Rockwool stuff is not very nice plus you’ll never get rid of airgaps. The other alternative is to have a company come in and spray – i’m sure it works well, but hvae my doubts how healthy that is for a building.

I’m very dubious about multi-foil insulation – there’s a good argument that it actually only really works because you’re getting a high level of airtightness. I know people who’ve used it in a loft conversion that they complained was never warm.

YMMV.

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