Loft floor boarding

Bendy_House

Could anyone speculate wisely on what the lesser equivalent loft insulation thickness would be if the insulation is boarded over?

I mean, with a loose-laid 'open' layer of loft insulation, the current requirement is what, 270mm? But loft spaces are windy places, so surely a good few 10s of mms, perhaps, of that will be thoroughly ventilated, so won't be anywhere near fully effective? Whereas, boarded over, the through-draught will be greatly reduced, making a lesser thickness equally effective.

And, the boarding layer itself is a good insulator.

So, 270mm of loose-lay, open to the elements, loft insulation will be no better than, ooooh, ~220mm? of a snugly over-boarded layer?

In theory. Mine.

FreeBear

Bendy_House said: And, the boarding layer itself is a good insulator.

So, 270mm of loose-lay, open to the elements, loft insulation will be no better than, ooooh, ~220mm? of a snugly over-boarded layer?

In theory. Mine.

OSB or chipboard has a u-value of 0.13-0.14 W/m²K.

Fibreglass/mineral wool is around 0.04 W/m²K

Celotex/Kingspan for comparison - 0.022W/m²K

OSB/chipboard is a relatively poor insulator. Where it wins is its ability to support weight and block draughts.

Your theory is not supported by real world numbers. If you want to run some approximations, https://www.changeplan.co.uk/u_value_calculator.php will do the basic calculations for you.

Bendy_House

FreeBear said:

Bendy_House said: And, the boarding layer itself is a good insulator.

So, 270mm of loose-lay, open to the elements, loft insulation will be no better than, ooooh, ~220mm? of a snugly over-boarded layer?

In theory. Mine.

OSB or chipboard has a u-value of 0.13-0.14 W/m²K.

Fibreglass/mineral wool is around 0.04 W/m²K

Celotex/Kingspan for comparison - 0.022W/m²K

OSB/chipboard is a relatively poor insulator. Where it wins is its ability to support weight and block draughts.

Your theory is not supported by real world numbers. If you want to run some approximations, https://www.changeplan.co.uk/u_value_calculator.php will do the basic calculations for you.

Yes, but...

The added insulation value of the flooring is just a wee extra.

My main contention - and I do contend it* - is that 300mm of loosely-laid loft insulation - and loosely is how it's done - will not be much - if any - better than - oooooh - 200mm of snugly-fitted loft insulation pushed neatly in between the joists and topped with a sheet of flooring.

(a) it'll be like a sealed insulation unit with little or no convection, and (b) I'd lay odds that at least 50mm of the top layer of exposed loft insulation does relatively little as the loft's air flow will be passing through it like a dose of stuff. Then there's the almost certain fact that, in practice, the layers of loft insulation laid down by the average bod will have gaps between each layer, and voids below it, and between the insulation and the bottom corners of the joist/ceiling. Ie, in most cases I have seen - and that includes my own - a glance tells you it's perfunctory at best.

Give me 200mm of boarded-over any time :-)

* with no evidence at all.

Lastonestanding

Bendy_House 

Give me 200mm of boarded-over any time :-)

* with no evidence at all.

And maybe some carpet up there too

Bendy_House

Now you're being silly...

Chickereeeee

There is supposed to be a 5cm space between the insulation and the loft boards (according to most sources  anyhow) to prevent moisture build up. So you would would still get a draught, albeit reduced. I have read that the best solution is to cover the insulation with a vapour permeable membrane, to eliminate the draught effect, but I have yet to see it done.

Bendy_House

In that case, forget everything I said...