Loft Insulation - 270mm?

The message from "DiddyS" contains these words:

You do what I'm going to have to do tomorrow - put some timbers along the tops of the exisiting joists so that when the insulation's down they're flush or above the top of the fill - then you can put boards back or tight-rope walk as you prefer.

Don't be tempted to put the timbers across the existing joists, it'll make it impossible to lay the fill!

Actually you SHOULD do it that way, lay one layer between joist, then tack in new 'joists' at right angles, and lay a new layer again at right angles between THOSE, then board.

Yep. It reduces cold bridging greatly.

"The Natural Philosopher wrote

That'll near double the weight of ceiling timber!

The message from The Natural Philosopher contains these words:

Why?

The message from "Doctor Drivel" contains these words:

Timber really doesn't conduct /that/ well and provided the insulation it tucked well up against the wood I can't see it making that much difference.

It makes a big difference. The cross joists only touch the lower joists at a small point. Extending the thickness of joists leaves a large surface area in contact with cold air in the loft which could be around zero C, conducting heat out of the house. Also if the loft is 50C plus this conducts heat down to the house beneath.

The difference in surface area conducting heat is massive from one method to the other.

The message from "Doctor Drivel" contains these words:

Yes, but the thermal conductivity of 8" of wood is small.

You will have about 10% of the loft area a massive cold bridge, conducting heat out or in 24/7. That is not small. The thermal conductance is quite high, just not as high as the likes of steel.

For the benefit of our regular readers, it might be worth removing the effects of the dribble reality distortion vortex from some of this hyperbole...

(in fact this dribble impersonator is worse than the real one was prior to his timely demise)

Dribble speak Reality

Massive Tiny Just not as high as steel Order of magnitude less

Thermal conductivity for wood varies with moisture content, but for most constructional softwood (at around 12% moister) will be in the order of

So in absolute terms wood is a very good insulator, but only about a 1/4 to 1/3 as good as loft insulation. So you could guestimate that a 4" deep joist is only as good as about an inch of insulation. Needless to say that the proportionately much smaller area mitigates the effect of this quit a bit.

Wood also has a higher thermal mass (i.e. specific heat capacity) than insulation - but this is not necessarily a bad thing.

So what?

No doubt the Chav has had a night out on the town Probably throwing plastic chairs around outside pubs. Sad isn't it.

It does in comparison to 130mm of rockwool. Do the calcs.

Its surprising how much difference it makes..once you get to 'superinsulation' levels..and we are being pushed that way by regulations.

Timber conducts 5 times more heat than rock wool and 7 times more than Celotex.

A typical joist is 75mm wide at 400mm spacing..

So the conductivity ratio of pure rock wool versus rock wool with timer in it is 325/400+75*5/400.. or in fact 75% worse than the rock wool alone. The timbers account or nearly half the total heat loss.

Now if a second layer is applied at right angles, the calculation gets very tricky indeed, because the temperature across the centre is not uniform..but it is definitely BETTER than simply doubling the beams up. And takes just as much timber.

It can take less timber as well, since the original roof timbers satisfy the structural requirements, and the cross ones need not be as stiff.

Not compared with rockwool it ain't.

The regulations are full of examples of this type of 'cold bridging' and how to avoid it.

Because then you have no large areas of continuous wood to conduct heat outwards.

See previous post - the timber nearly doubles the heat loss - by crossing the timbers MOST of the roof is ALL rock wool, a large fraction is SOME rock wool and SOME timber, and very small part is ALL timber. As far as heat conduction goes.

Otherwise you need another (extra) 4" of insulation due to the official calculations for the losses through the timber.

Hi folks,

I do seem to have sparked a debate don't I? All very interesting if a little technical for me.

If I've followed it correctly, then the answer to my question is to put a bit more wood on top of what's there already and then walk on that.

Thanks guys,

Derek.

Yep. put the wood at 90 degrees to the existing joists and cover the new joists with boards.