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

r_c

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We have had a loft person in to quote to insulate and board our loft. He said that they will quote to install the following:
  • 100mm loft insulation between existing joists
  • 100mm loft insulation between new 4x2 lengths (perpendicular to joists)
  • Chipboard resting on the new 4x2s
I questioned him about only laying 200mm of insulation, as I thought 270mm was the recommended depth, and he said that the combination of 200mm loft insulation plus the chipboard is equivalent to 270mm of just loft insulation.

Could you please let me know whether you think this is right. (Experience has taught me to greatly value the opinions expressed on this forum.) Or do you think I may have mis-understood something?
 
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no not really so dont trust them
buuut the laws off diminishing returns says
lets keep it simple say 100mm off insulation will save 80% off the heat loss through the ceiling and to keep it simple we assume you are loosing £100 a year you will save perhaps 80% or £80 a year
the next 100mm will again save you 80% but now off £20 so now £4
the next 75mm will save a further14% but now off £4 so now around .50p a year
 
He's clearly got no idea what he's on about, 70mm mineral wool is way better than 15mm or whatever chipboard.
Otherwise we'd all be using chipboard to insulate our houses as it's really thin and cheap
 
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We've had some interesting debates on this issue recently, and someone had squashed his insulation, and the unlagged pipes in the loft still froze, so not much heat was being let through. They'll be fitting 18mm chipboard, and whilst someone on here could calculate the heat retention of the chipboard, I suspect that whilst what he's suggesting may not be exactly correct, but like big-all suggests, diminishing returns on the last 70mm may not be enough to worry about.
 
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Clearly not true; mineral wool insulation is about 3-4 times better than wood IIRC. And you also have more thermal bridges due to the extra battens to support the floor.

Having said that, this is still probably the right thing to do if you want to use your loft for storage.
 
Strictly speaking, if you are improving the insulation I believe Building Control should be involved, and then you will be required to insulate up to 270mm of Rockwool to meet current regs.

That's how it works in Jersey at least!
 
Thank you all for your pragmatic comments.

If I were to be the stubborn sort and insist on wanting 270mm insulation, how do you create boarding above the 170mm of insulation? Because I cannot seem to find timbers on Wickes or B&Q's websites with timbers of that are 170mm wide. It does not seem to be the easiest timber to find, where as 4"x2" are easily available.
 
if you are improving the insulation I believe Building Control should be involved
We have not yet got our completion certificate from building control, so I'd better look into this. Laying insulation is bad enough, lifting it must be even worse.
 
You'd go for a 150x50, and then squash the 270mm of insulation ever so slightly. But if you start getting building control involved, they may insist on loft legs rather than adding wood to the joists.
 
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You don't need building control involved for loft insulation. Nor are they interested in loft boarding.

To board over insulation that is not a standard timber depth, you use the next timber size up, or two timber sizes to make up the depth. Or those loft legs.
 
You don't need building control involved for loft insulation. Nor are they interested in loft boarding.

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.
 
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If I were to be the stubborn sort and insist on wanting 270mm insulation, how do you create boarding above the 170mm of insulation? Because I cannot seem to find timbers on Wickes or B&Q's websites with timbers of that are 170mm wide. It does not seem to be the easiest timber to find, where as 4"x2" are easily available.

I would use 8x2, thus leaving a small space between the insulation and the boarding. Good fun manoeuvring it around the loft though!
 
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And you might find that doubling 4 x 2 is a lot cheaper than 8 x 2- good plan using cross battens, not used those loft legs but they look very weak.
 
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And you might find that doubling 4 x 2 is a lot cheaper than 8 x 2- good plan using cross battens, not used those loft legs but they look very weak.

dont like loft legs because the span is the diagonal rather than the distance between adjacent parallel timber
 
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.
 

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