Loft insulation advice - unusual construction

[DanRobertsB]

Hi

I'm keen to insulate a loft with unusual construction (see below). I'm hoping that can be the usual "roll out some mineral wool" but am weary of condensation risks (all the more so as this is a timber frame, timber clad house). 

ROOF:

There's about 180m2 cold pitched roofs, seemingly built over an older, pre-existing flat roof. 

•  Construction appears to be uninsulated joists, topped with a flat roof made of a slab perhaps 75mm thick of compressed straw, covered with what looks like bitumen (or similar). 

• I think this may have been the original flat roof, probably constructed somewhere between c. 1955-1967. 

• On top of parts of this flat straw/bitumen roof, there's c. 50-100mm of glass mineral wool insulation (see photos); other parts there's nothing or boarded storage (there are also parts where some of the straw's been removed). 

• On top of this old flat roof, a lightweight timber frame was built, covered with a membrane and then sheets of Decra Classic (or similar) lightweight metal/composite roofing “tiles” (with what are I think GRP gulleys). Rafters are generally (a wopping) 1200mm. 

• The whole structure appears to have very poor thermal performance, and so need better insulation. However, given the unusual construction, it’s not obvious to me how best to economically deal with it (and to do so in a way that doesn’t create the risk of condensation issues).

Parts of the Decra are in poor condition - cracked / banana’ing / patch repaired. See attached report from Langley. However, we’re not aware of any obvious damp / water ingress over the past 18 months or so (may have been a problem before that). Probably warrants at least some patch repairs. 

Note bending of Decra around chimney (suspect due to weight of leaves which we found there - now cleared)

Loft hatch area showing straw - between ceiling and cold roof

Inside of roof - section of strawboard lifted to show joists (uninsulated between joists)

Showing 1200mm (uninsulated) rafters etc. The thin Decra is then on top of this. 

Some parts have up to 100mm mineral wool insulation above the old straw flat roof

 

Would mineral wool (up to 300mm) - or possibly sheep wool ( (more expensive but no nasty loose fibres in the cold loft space, which is used for storage; good resistance to any moisture) laid out over the old flat roof (i.e. on top of the bitumen felt) work? I'm hoping/guessing a VCL isn't necessary (or the ceiling and//or bitumen already acts as one).

 

The (considerably more expensive) alternatives would seem to be:

- Open cell spray foam of the entire inside of the loft. But concerns about impact on insurability/mortgagability, etc - seems some insurers/banks won't touch homes insulated with the stuff.

- PIR warm roof - but probably only sensible if re-roofing (which I doubt we can afford). But the structure may not support the weight of PIR plus new roofing materials. Ultimately, however, finding a solution to the roof may be necessary (perhaps just patch repairs/replacing the Decra). 

[Timedout]

Has someone had a rave on your Decra?

[ProDave]

I see two completely separate issues.

 

The poor insulation

 

The poor condition of the decra roof.

 

Both need tackling separately.

 

I see no reason why the insulation cannot be fixed by adding more on top of the existing straw.  The devil is in the detail, you do not want ANY of the original roof uncovered, e.g. the bit you removed to have a look will just let cold air get under the rest completely negating any insulation.  You want to ensure the whole roof is covered with no gaps and only then is it worth adding more.

 

How has the decra roof been fitted?  It is a lightweight covering often used on mobile homes, and I have always understood it is lad onto a flat board.  Your sagging decra tiles suggest it has been laid like traditional tiles on battens and has sagged between the battens.  You probably want to lift some tiles to have a look.

 

 

[DanRobertsB]

Thanks. Yes, definitely two issues (well, probably more but that's another story!)

 

INSULATION

 

I can put that chunk of strawboard down that someone cut out, tape it down, and roll out layers of mineral wool over most of the strawboard area (hopefully there's no sense in a VCL). There are a few potential exceptions / complications:

 

- Some of the loft (maybe 50m2) is boarded - raised on wooden battens about 10cm off the straw board. This would need lifting, raising (perhaps on large loft lefts) and relaying.   Other parts are not boarded, with things stored directly on the strawboard, etc (maybe 40m2). So a similar approach would be needed there. 

 

- There are some areas where the straw has been cut out and there's wiring/piping in it. Fortunately most or all of that area is over the utility room and/or pantry, and I'm less fussed about the insulation properties there.  If it's not ok just to put 300mm mineral wool directly over this, it may be possible to fit some 150mm PIR board over it to seal it off - but I suspect for much of it that won't work (not level), so mineral wool would be preferable if that won't create any issues.

 

Photos to follow below, showing only about 1/5 of the 200m2 section, excluding the boarded area. 

 

The priority (not least given outrageous heating bill s) is really the insulation. It's also a cheaper and easier item to tick off the list. 

 

DECRA

 

The Decra is in poor condition. I've yet to lift the tiles, but (strongly) suspect it's not laid on any flat board, just onto the battens (with that black - presumably waterproof - membrane underneath). I think it's also borne the weight of a fair bit of leaves and been walked on (not following the proper instructions) for maintenance etc a fair bit over the years (I suspect decades). Seems to be reasonably water tight but at the very least needs patch repairs. I'm not sure the structure will take anything heavier, which I'm guessing limits the options to Decra and perhaps sheet metal. TBH I'm struggling to find anyone willing to work on it - almost everyone (save for one who wanted to do the sheet metal) wants, e.g. with EDPM over PIR. The whole section of this roof is about 200+m2, and then there's a further c. 100m2 (some with solar panels), so that's a massive job. And it's unclear the structure would take the weight. No ones provided a finalised quote, but to do the whole 300 odd m2, the suggestion has been it might be c. £100k+ (yikes). Which seems kinda crazy to me (and beyond the budget) ... Patch repairing with some new Decra therefore seems rather more appealing, if I can find someone to do it. 

 

[DanRobertsB]

[Nick Thomas]

One thing I discovered about decra roofing is that it's quite easy to bend the panels back into shape if they're a bit deformed. Ours had been walked on a lot - including by a younger, less-informed me - and the ¿asphalt? granules were coming off some bits. We had trouble finding replacements of the right size for patch repairs, so they got taken off, bent back into shape, put back on and sprayed. Labour-intensive and a few cuts here and there, but a decent result overall.

 

That wasn't 180M², though - just a few panels concentrated around the chimney.

 

Loving the straw by the way. You don't feel like adding more?

[DanRobertsB]

Hi Nick - thanks for the tip! May give that a go if we can't find replacements / for the worse areas. What did you spray it with?

 

TBH, I'd not thought about adding straw. I suspect what's there (other than the panels) is a compressed straw panel that's been pulled apart for access/to put in pipes. Property has been built and rebuilt several times (before our time - history of all this is rather vague!), and working on it throws up the "Chesterton's fence" dilemma all the time - never quite sure it's safe to undo what's already there (e.g. wiring, pipework etc)!

I suspect straw is a bit of a nightmare in terms of fire risk, but then I've not looked into fire ratings of other insulation materials either. 

 

There's space to play with, so I'm really just keen to get some inexpensive but effective insulation in asap, in a way that doesn't create unnecessary risk (particularly moisture/condensation/wood rot, etc). 

 

Cheers, 

Dan

Edited August 30, 2023 by DanRobertsB

[Nick Thomas]

Compressed straw has the air forced out of it and has good fire resistance ('B' rated IIRC) as a result - the surface chars, the body doesn't burn - but loose straw is a tinderbox. All the wires mixed in do make me feel a bit queasy, honestly; it was mostly a joke.

 

We sprayed the whole roof with corksol in-situ, which is a mix of cork granules and elastomeric paint. I'd imagine anything that keeps the metal from rusting would do the trick, though. We experimented first with a detached decra sheet, bending and rectifying the bend after the spray had dried and it didn't flake off, it all looked pretty sound and a good way to extend the life of a 50-year roof. It needed a good wash with a pressure washer first to get the moss and lichen and dirt off it.

[Nick Thomas]

Here's a video of the decra: https://files.mutual.email/s/3XdHEPk6a5w3WcW - can't figure out how to link to it from its place on facebook, and it's too big to fit as an upload to this site. It shows the panels being deformed and un-deformed again, too.

 

I did a sheepswool insulation with a friend a few weeks ago and it seemed fine, nicer to work with than fibreglass, of course. No special expertise though. Good luck with it!

Edited August 30, 2023 by Nick Thomas
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