Is 200mm loft insulation + boarding equivalent to 270mm just loft insulation?

[John D v2.0]

Hang on bit-all, are you sure about that. I've never used them, but I was under the impression that you put them on each joist as you go along, so the span has got to be the adjacent leg. Your thinking suggest the boards would be at 45 degrees to the joists, rather than cross boarding.

Go and have another coffee, and think about it a bit further.

He meant it's point supported rather than lineally supported, so the distance to the nearest support can be further then if it were beams

 

[Doggit]

Nope, still think you're as wrong as he is John. I know what he's saying, but I still can't agree with it. The supports go on each joist, and the boards then go on top, so the shortest distance, has to be the width of the joists, not the diagonal.

But I'll happily stand corrected, as I'm always interested to learn new things.

 

[John D v2.0]

I am not saying whether he's right or wrong, all I'm saying is normally you have tramline support ie 400 centres or whatever, so wherever you point at the board the max distance to any supporting timber is 200mm.
However if you have point supports at 400 centres (like the number 4 on a games dice/die) then if you point at the spot in the middle, equidistant from all 4 points, then the distance will be √(2x200²) so 270mm or so which is significantly above the 200.

That's Not to mention the extra shear forces on the chip board.

 

[r896neo]

I Simply used 100mm kingspan board and floated chipboard over it. No thermal bridging and plenty strong enough for storage.

 

[John D v2.0]

I Simply used 100mm kingspan board and floated chipboard over it. No thermal bridging and plenty strong enough for storage.

Yes that's a great idea and exactly what I've been doing only with 70mm.
The cavity wall sized kingspan is really convenient size to get up into the loft

 

[^woody^]

Really? Here we have to notify of any "Renovation of a thermal element'. Admittedly nobody would if they were just insulating the loft, but if other works were involved they would likely find out.

Insulating a loft is distinct, and is not renovating a thermal element.

 

[John D v2.0]

Insulating a loft is distinct, and is not renovating a thermal element.

however, change the tiles 1+m above, and you have to insulate!

 

[^woody^]

however, change the tiles 1+m above, and you have to insulate!

Not if you insulate first.

 

[phatboy]

Insulating a loft is distinct, and is not renovating a thermal element.

Ah different rules for us then. If a "Pitched roof - insulation at ceiling level" has a U-value greater than 0.35W/m2k, then it must be upgraded to be lower than 0.16W/m2k.

 

[Ian H]

I wonder if I would be better off cutting my floor back a bit? I boarded right to the edge but have 200mm between 8x2 joists.

 

[big-all]

I am not saying whether he's right or wrong, all I'm saying is normally you have tramline support ie 400 centres or whatever, so wherever you point at the board the max distance to any supporting timber is 200mm.
However if you have point supports at 400 centres (like the number 4 on a games dice/die) then if you point at the spot in the middle, equidistant from all 4 points, then the distance will be √(2x200²) so 270mm or so which is significantly above the 200.

That's Not to mention the extra shear forces on the chip board.

yes exactly the point i was making
if you take parallel timbers that is the maximum span
but where you have point support its the diagonal assuming in a grid pattern
you also have the lack off weight distribution that timbers at 90% will give

 

[Doggit]

Thanks for the better explanation guys; it makes much more sense now.

 

[Doggit]

You're fine Ian; there's enough of a gap to allow the airflow up, so nothing to worry about.

 

[John D v2.0]

I wonder if I would be better off cutting my floor back a bit? I boarded right to the edge but have 200mm between 8x2 joists.

You'll be fine, if an SE was calculating floor loads due to ceiling height the live loading would be small there.
Also there are plenty of joists to support by the looks. Maybe a 500mm cantilever would be risky, but not 200 there.
You should be more worried about losing things out the back of the cupboards, but presubmitly you'll be plaster boarding or something.
Remember that acute corners are high risk for condensation mould so make sure plenty of insulation as the warmth won't permeate there well. Maybe that's why the basic detail is cold eaves.