Loft insulation...

I would. Loft "floors" are rarely built to the same strength as a real floor, all it has to do is hold up a bit of plaster board and insulation...

As for the OP it depends what is in those banana boxes. Books weigh a lot, general loose household "junk" much less.

Reply to
Dave Liquorice
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Some of them are full of books, some are not. Mostly junk though. I have those placed in the centre where the beams run over a stud wall.

Reply to
John Smith

Once things have settled down, a quick brush on one will not dislodge much. It's when it's being handled in vengeance that things get messy. For normal access, if you are not touching the stuff on purpose, it would be fine without.

Reply to
John Whitworth

I was just repeating a previous answer to be honest, which just seemed to make sense to me.

I'm not too sure how you could really measure it like that. If the C.H. was off, then surely there are always going to be cold spots, with or without insulation. And if the C.H. was on, the ceiling would surely have the same temperature throughout, but the heat would be escaping faster, and hence the boiler would be working harder to maintain the same heat?

But you are probably right here. Ours is plastic boxes full of cuddly toys, suitcases, loads of boxes etc...

My roof pitch isn't too low - but even so, I can't imagine prannying around the loft another 20cm higher. I whack my head getting into some areas as it is. That said, if I can get something sorted in the garage loft first, as a trial, maybe I'll consider doing some of the main loft.

Reply to
John Whitworth

Its not as bad as you think.

Also, its already been proved to be adequate with existing joists, so why would it need strengthening anyway?

Straw man argument. Next?

The

Grow up.

Reply to
The Natural Philosopher

Or you could spend a couple of hundred on a structural survey and ignore these two perennial 'there is only one correct way' farts.

Its your loft adequate strengthwise? Yes. Yiou already had stuff up there.

So igonore all the bullshit .

What is teh purpose of this work?

To improve insulation and still get back to a usable storage space.

Ok, whats teh best way of doing that?

More insulation. BUT that means there aint no stricture to lay a floor ron

SO add structure..

Right. Now it can be done two ways - along existing beams or across them.

What's the difference? Along existing adds stiffness (but not much local strength as the load at a given point is still taken by a joist end

It also leads to a large direcet wood path bypassing the insulation, a cold bridge.

So run the new joists transverse. No cold bridge, better insulation, and the overall strength certainly no worse, and actually quite a lot better than it was, The analysis is of course quite complex, and not to be found in the 'muppets guide to engineering structures' handbook, but my crude maths says its slightly better than twice as stiff and twice as strong as against the other case which is slightly worse than 4 times as stiff and twice as strong.

If you are bothered.

If you want more stiffness its easy to herringbone brace, but frankly, why bother?

As usual the usual suspect has tried to divert you with horrors stories about lime plaster. Yes lime plaster is shit, but its his SPECIAL SUBJECT so he has to tell everyone how much he knows about it.

But hey, don't listen to me. I only designed and built a house, and only have a degree in engineering. WTF do I know about it?

Reply to
The Natural Philosopher

Not if plated over with floor material. That I can tell you works a bloody treat. Stops draughts dead. And gaps between celotex is worse than leaky rockwool.

Reply to
The Natural Philosopher

It isn't. Its just not quite as stiff that's all.

Its sound advice structurally, if you intend a loft conversion. Its totally irrelevant if its a lightweight storage area and insulation is your target, not structural rigidity.

Usual case of someone who has picked up a little bit of knowledge without picking up the context, and now touts it as The One True Way.

There are more ways to skin a cat... you could indeed put tables up there legs screwed to joists and stuff yer junk on that...

Reply to
The Natural Philosopher

Oh FFS.

Reply to
The Natural Philosopher

Doubling joist depth reduces deflection under load by a factor of 8.

70mm is a very shallow joist, under load it will sag and bend, and the ceilng crack and move.

that's useful.

The choice is between a sound structure or a little more insulation.

NT

Reply to
NT

Neither weight as much as several human beings having sex, or a bath full of water, or a grand piano, which are the typical peak loads a normal floor has to cope with.

I've bent a 5 ton pulley suspended off a 2x4 beam trying to get a car engine out of a car. We raise the car instead..so that's where things BREAK. i.e the typical 6x3 joist will over a short span easily take a couple of tons. It will bow like shit of course.

But it wont break.

So you don't want more than about a cubic meter of books (probably 1/2 ton)on a single joist.

I don't own more than a cubic meter of books., that's about 500 LARGE paperbacks I'd say. Maybe more.

Reply to
The Natural Philosopher

Heh, my joists look more like 2x3s

Reply to
John Smith

The central heating was cycling, so the house was being heated, but temperature had stabilised.

Thinking about the physics, and simplifying slightly, if the air space in room and loft both have uniform temperatures, which is practically true, then in any areas where there is more heat flow, due to lower insulation values, the surface temperature of the ceiling will be lower.

Isn't this simply the reverse of examining walls with an infra-red camera? Room and outside world temperatures are constant, but in areas of poor insulation, the wall is warmer.

Basic physics: greater heat flow requires greater temperature gradient.

Chris

Reply to
Chris J Dixon

No, its the choice between a floor designed for stomping on, and a ceiling designed to keep you warm.

Oddly enough, they are not the same thing.

Prat.

Reply to
The Natural Philosopher

What for? We already know what the OP has and what the options are.

I think you imagined that bit.

adequate in that it didnt collapse, yes. Adequate to prevent ceiling problems, it depends how much weight is put where, and on the joist span. I think its fair to say though that 70mm is at best borderline. And at worst dangerous if loaded. We don't currently know the span involved, nor the joist spacing or width, so personally I wouldnt want to propose a system that in some cases would be unsafe.

Given the choice between a sound structure and a bit more insulation, I know what I'd choose.

shear strength at the ends isnt a problem. Bending sagging and cracking typically is with such small joists, and if the span is long then safety might be an issue too. Parallel timber achieves the same end result /and/ also solves the joist size issues.

If solid timber joists were used, say 2" every 16", then 12.5% of the heat path is timber, 87.5% of it is fibreglass.

If you use 2x4 atop blocks of 2x4 rather than continuous 2x8, lets say

4" long blocks every 2 feet, then: 88% of heat path is 11" fibreglass 9% of heat path is 8" timber and 4" fibreglass 3% of heat path is 11" timber

That insulates about as well as what you propose, but is a lot better structurally. Its also cheaper in timber.

Compare to your proposed option, again if 16" joist spacing:

88% of heat path is 11" fibreglass 4.5% of heat path is 3" fibreglass 8" timber 4.5% of heat path is 8" fibreglass 3" timber 3% of heat path is 11" timber

The only insulation difference is in that 4.5% of the area, in which your option has 8" fibreglass versus mine 4". The insulation gain is very close to zero, at the cost of a weedy 70mm floor structure.

The resulting strength is little better, as the load still all rests on the same old timbers. The only improvement is in load distribution.

Re stiffness, again cross timbers wont make much difference unless theyre attached to the walls at each end. Doing that is more work and cost, and we dont know whether those walls are rated to take almost the full load of the new loft floor, which they would. In many cases is would also invoke the party wall act, which is more work, delay and cost.

How did you conclude your proposed structure was twice as stiff and strong as the original? Call me doubtful.

thick plaster will crack up if flexed, its simple enough. Or are you saying that's not so?

If you're convinced you know what youre doing then you can justify your claim that your proposed structure is twice as stiff and twice as strong as the original. Should be interesting.

NT

Reply to
NT

when all else fails, just make stuff up

Reply to
NT

ge

IME a trip to a fibreglass insulated loft isnt a great experience, but nothing terrible. They do get everywhere, but only in tiny amounts. Boarding over it will pretty much solve that though.

NT

Reply to
NT

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The OP has 60+ large (40 litre) boxes. He won't rescue many of those in the event of a fire. The prospect of rummaging through that many boxes in a dusty loft even in normal circumstances is enough to warrant renting an affordable unit for anything other than the usual loft junk.

Cic.

Reply to
Cicero

Hmmm...OK - that argument is compelling. You got me there! :-)

But the compressed insulation argument still seems compelling too. I think I'll stick with my crappy 1997-level of insulation loft. Unless we can dump some of the stuff that came from the other half's mother's when she passed away.

JW

Reply to
John Whitworth

Cripes. Ive got better things to worry about in the event of a fire than boxes of old magazines.

If its that valuable it should be in a safety deposit box not in a public storage facility.

Reply to
The Natural Philosopher

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