Tapered insulation board magic

[Nick Thomas]

I'm close to being able to pop a roof on my garden room now, so starting to price up options for a 4.8x3.6M flat roof, which I think is the best I can do this year. Currently thinking a cold deck, uninsulated, perhaps adding insulation below the joists later, preserving ventilation.

 

Getting the fall on the roof is irritating me, though. Managed to find a local-ish supplier of firrings, and waiting to hear back from them with a price for 10x3.6M firrings to run along the top of the joists, but the timber bill already has my eyes watering a bit, honestly. Hunting for alternatives I came across this seeming magic: https://ukflatroofing.com/tapered-insulation-boards - anyone used it? Thoughts good or bad? Is it OK to walk on after laying? Cheaper suppliers? I couldn't find a mention of tapered insulation on previous buildhub posts.

 

The price from this lot does feel unattractive - I'd need 12 boards, 4 each of different heights. Say https://ukflatroofing.com/kingspan-tt46/ecotherm-ecofix-tapered-foil-faced-pir - they'd want £308 + £247 + £277 = £832, with a couple of sheets wasted. 100mm regular PIR to cover the same area is a third of that - I can't see any world where adding the firrings pushes it over to being the cheaper option?

[FuerteStu]

 

What's your cost for firrings? I wouldn't have thought that would be more than £5 per metre, ~160 quid 

 

[Nick Thomas]

Still waiting to find out what the locals can do it for, but yeah, I'm getting that sort of price from various online places that won't deliver to me.

[MJNewton]

I would expect the cost of timber firrings + standard insulation boards to be considerably less than tapered insulation boards alone.

Edited November 16, 2023 by MJNewton

[FuerteStu]

38 minutes ago, Nick Thomas said:

Still waiting to find out what the locals can do it for, but yeah, I'm getting that sort of price from various online places that won't deliver to me.

Online quoted me four times the price I paid for a local timber merchants to make

[Nick Thomas]

So what's the market for these tapered boards if they're so uneconomic compared to firrings? I don't get it. If they were half-to-a-quarter of the price, I'd snap them up.

 

In my head, they make them by squirting foam into a mould... it can't be that much more expensive to have the mould be wedge-shaped rather than square, can it? Or do they make giant cubes of the stuff before slicing it?

Edited November 16, 2023 by Nick Thomas

[MJNewton]

Simple economics of supply and demand, ecomies of scale etc. Square/flat(?) insulation is used absolutely everywhere - floors, walls, roofs - and in large amounts. Tapered insulation is, I assume, only found in flat roofs. And even then very rarely given that firrings and standard insulation does the job perfectly fine in 99.9% of cases. So what does the 0.1% represent? Dunno. Maybe the need for minimising height buildup, or minimising installation time?

Edited November 16, 2023 by MJNewton

[FuerteStu]

30 minutes ago, Nick Thomas said:

So what's the market for these tapered boards if they're so uneconomic compared to firrings? I don't get it. If they were half-to-a-quarter of the price, I'd snap them up.

 

In my head, they make them by squirting foam into a mould... it can't be that much more expensive to have the mould be wedge-shaped rather than square, can it? Or do they make giant cubes of the stuff before slicing it?

The market is less skilled jobs. A labourer can fit Wedges of foam. A carpenter fits timber. 

 

50% of the cost of construction is labour. They don't see that skilled trades save money in the long run. Big industry relies upon fancy demonstrations and promises of long term savings. 

 

In my mind there's no flexibility in tapered PIR . Lots more wastage and planning. Whereas excess flat PIR  can be used elsewhere

[Iceverge]

7 hours ago, Nick Thomas said:

Managed to find a local-ish supplier of firrings, and waiting to hear back from them with a price for 10x3.6M firrings to run along the top of the joists,

 

Can you not make your own fittings from a few 95*44 timbers? 5 of them won't be too dear. 

[Nick Thomas]

4 hours ago, Iceverge said:

Can you not make your own fittings from a few 95*44 timbers? 5 of them won't be too dear. 

I'm not really set up to make long straight cuts.

 

Although, since my wallplate is pretty wide (450mm), I did think last night that I could apply firrings to the bottom, rather than the top, of the joists, just where they meet the wallplate. 10x450mm cuts to get 20 bits of wood seems doable enough with the circular saw. The only downside I can see is that the underside of the joists would be sloped as well as the top - a bit unusual, but not the end of the world, unless it allowed water to run intp the building from the high side 🤔

 

If this place gets back to me with cheap delivered firrings, I'll just go with that, though. I've self-imposed a deadline for getting the roof finished, so there is a bit of a time crunch ^^.

Edited November 17, 2023 by Nick Thomas

[Iceverge]

Tilting the rafters would be easy and you'd get a dead consistent fall. 

 

Sloping roof wouldn't be an issue.