Loft Insulation

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I have just had my loft timbers/joists treated for woodworm, and getting the house rewired soon. After that is all done, I am going to put insulation down. However I have some questions about it.
Is there a noticable difference in loft insulation (rockwool rolls) in insulating greater than a 4" depth? The reason I ask is that my joists are 4" in height, and anything greater will mean a bit more planning to ensure that I can still walk and see the joists up there.
Also, has anyone noticed a difference between rockwool and other types, such as crown wool? I have checked the u-values and such, and rockwool is a better insulator, but it is also going to cost me about 20% more than crown insulation. (b&q special offer).
I really like the idea of using natural insulation such as sheep wool : (http://www.greenbuildingstore.co.uk/ins-thermperf.php ) but the price is unbelievable.
Any comments/help/advice would be greatly received.
Thanks.
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Yes. The law of diminishing returns says that 350mm is the best environmentally speaking. Any more and the environmental cost of manufacturing and transporting the insulant is higher than the energy saved. 100mm is definitely on the low side. 200mm would be better. The difference between 200mm and 350mm isn't that great.
Christian.
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On Wed, 21 Jan 2004 16:29:12 +0000, Christian McArdle wrote:

Some useful info in there. Leads me to even more questions....
- with currently having a 100mm joist height, should i either increase the joist height (cross joists) or just cross-lay the insulation over the top of the 100mm.
- If you insulation up to 350mm, how are you meant to ever reach places in your loft without falling through the ceiling ;P
- You mention the recommended above, based on diminishing returns, but what insulation material is that based on - as I understand it, different manufacturers produce insulation to different values. So my thought was, that maybe 200mm of rockwool, might equal 300mm of crown or something?
Thanks again
Dean
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On Wed, 21 Jan 2004 18:28:07 +0000, Dean Richard Benson

It depends on whether you want to board over the top.

It may not matter, since you won't be putting many other things there anyway :-)
Before you get over-excited by this idea, take a look at how much heat is going out through the walls and windows.......

If you want a better compromise between thickness and U value, then you could use polyisosyanurate board such as Celotex or Kingspan which have about 4x the insulating property of glass fibre.
However, it costs 15-18 for a 2440x1220 sheet........

.andy
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Andy Hall wrote:

Ah, but never underestimate the insulatng properties of a loft full of junk :-)

Indeed.
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The problem is that with a 100mm joist height you aren't meant to store anything, other than insulation in your lost. Its one of those cases where the builders use the absolute minimum they can get away in order to save a few quid and you end up with some valuable storage space that you effectively can't use!
cheers
David
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Nah. I've seen loads of houses with 100mm loft joists. They're all used for storage!
Floor joist sizes are so large to avoid flexing the ceiling as people move about, not to stop the things collapsing. Light or even medium storage is usually OK, as there is little movement. Any bad results are usually a few hairline cracks, not stuff piling into the room below. Obvious, large water tanks or big boxes of books need special consideration.
Christian.
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Christian McArdle wrote:

This is very true. Timber will suffer unacceptable deformation long before it breaks. Especually on longish spans (a couple of meters or more)

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David Moodie wrote:

Yes, and that is where cross laying more joists , maybe herringbone bracing and a few hangers up to teh ridge/radfters makes sense. Total insulation depth comes up to maybe 8 inches with only where the joists cross as 'cold bridges' and boardiong out the loft stops draughts getong in any less than perefectly insulated places.
I have just complete boarding out my loft with 150mm insulation (rockwool) and its made a lot of difference. Its icy up there when the wind blows (silly building regs vents are enough to take every ounce of moist air out in about 1.2 seconds). Once you get to this sort of insulation depth, the real danger is tiny cracks that allow air movement alongside celotex and through the rockwool.
If yu have as I have, hollow rockwool filled walls, you then discover that things like e.g. power sockets, or the odd crack round a window frame, lets in a tiny icy draught...from air moving THROUGH the insulation and getting into the rooms.
Going to IMMlike levels of insulation only works if you can stop up each and every one of these micro draughts: This takes patience, and decorators caulk and frame sealer inside, and attention to detail up in the loft as well.
I sometimes wonde if much more limited ventialtion, and an electrical dehumidifier in the loft might not be a more energy efficient way to keep a cold roof dry, and allow the loft airspace itself to become an insulator...

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Not mine. Experts on the topic conclude this.

I have always said caulk every hole into the loft: around pipes, cables, etc. Also fit a vapour barrier on the loft floor, stapling down, then put insulation over.

I have always though that may be beneficial. Seal up all vents and install one. The BCO would not like at, as if the de-humidifier fails you have problems. An accurate humidistat would need to be fitted to reduce running costs. There again install a vapour barrier on the loft floor to prevent vapour getting in.
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Are you sure on this. It's just I heard the ODPM people who set the building regs were talking about as much as 450mm in the next part L.
Rockwool is on 3 for the price of 2 at Wickes and so matches the B&Q prices well.
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They talk about all sorts of things. This one is pure pandering to be seen to be doing something towards the Kyoto protocol. It has no basis in economics or anything else when put into the context of where domestic energy should be being saved.
So people in an average house can spend about 100-200 and save about 10 a year.
It's a complete nonsense.

.andy
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Andy Hall wrote:

IMM's uni only taught him about straight lines and binary thinking.
Its no good talking to him about curves and maxima/minima.
The only curves he ever takes notice of, are on page 3.
And when he switched to the socialist worker, he lost track of even those.
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So you studied to build bridges and design electronic boards all in one course. My, oh my. Was this skule in Toytown.
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IMM wrote:

Yes. It was in Cambridge.

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A bulldozer should be run through the lot of it.
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Philistine......
.andy
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And through there as well?
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But arm waving with realism. All I read is that energy will become more scarce and more expensive. As it becomes more exp[expensive so will insulation too. best pack it in now while it is cheap enough. You will not regret it in the future.

See above.

Who said focus on this alone?I never. I said pack in as much as you can while it is cheap as it will not be, neither will energy, in the future. probably sooner than later too.

See above.

That is still cheap and easy to do, even for a DIYer.

It does, as you can tackle the loft easily and cheaply. You plug as many holes as you can, even the small ones.

"at the then current fuel prices"

That is clear above.
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This is all qualitative stuff.

At some point it may well be interesting to do this, but at a saving of a tenner a year, when several hundred quids worth of heat is going out through the walls means that the focus should be on that now and situations where there is no roof insulation at all.

Quantitative arguments not qualitative.

That is not a reason. It's the difference between turning a light on or not - in cost terms a very small light.

On the immediate energy cost or with everything taken into account including replacement sandals for the author?

.andy
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