Loft insulation

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Now that is asking for trouble. :-)) I said, only if the other boards around it were fitted into the tongue and groove of the other boards. That way, any overhangs are left out at the edges which you can't stand on. But the design of these boards is meant to take the weight of normal use on the tongue and groove sections.
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spamguard@_spam_guard.com says...

position as David. My house has 4x2 joists (not measured the spacing yet) and I'd like to board at least the centre section but probably a bit into the eaves as well. My original plan was the same as David's - Put cross joist across the existong one to give extra height to allow for more insulation and ventilation but I take your point about extra loading.
I'm a bit confused about your previous advice here though - If I lay 4 inches of insulation between the joists and then board over am I likely to run into condensation problems due to lack of ventilation to the insulation? Is it generally OK if I were to not board over the bathroom but to add extra insulation here instead?
I take your point about the boarding preventing air circulation and acting as insulation but how does this compare with more fibre? The current suggested minimum is 200mm (and I note that some people have gone upto 450mm - Surely there's a law of diminishing returns here!) The suggested solution would only have 100mm insulation, and boards?
Thanks,
Peter
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wrote:

There certainly is a law of diminishing returns. If you look at the U value of a roof with 100mm of insulation and work out the heat loss for the area of a typical house it is generally a great deal less than that which is lost through the walls and windows.
Doubling it to 200mm reduces that heat loss of course, but generally to the tune of tens of watts. One can argue that the insulating material is cheap enough so that the ROI on doing this is short but it is rather like changing a 100W bulb for a 40W bulb while all the time there is a 10kW heater running.
If the job can be done easily, then fine, but otherwise, if there is a bunch of cost to install it such as timber etc. and that is not otherwise useful, then going mad with roof insulation is not the most cost effective way to deal with energy cost reduction.
.andy
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says...

As Andy Hall says, there is not a huge difference in the effect of laying extra insulation when the same can be done by trapping the air between the existing material with boards on top. The insulation works by stopping air movement through the space between the boards and the ceiling, hence not allowing colder air to travel through and cool the space down. The trapped air is then heated by the very slow convection through from the room below and has to stay where it is.
So by increasing the thickness of the insulation your actually just slowing the air lose a very minuscule bit more than one layer does. But then your giving yourself more work to try and increase the height of the space you need before you can lay boards as a storage platform. Doing the math shows that it is really not worth that kind of expense to gain so very little more.
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spamguard@_spam_guard.com says...

curently is fairly sealed so I'm intending to fit ventilated soffits (in the future) and not insulate to the very edge etc. Is boarding laid directly over insulation with very little air gap likely to lead to problems (condensation etc) later? If I lay less insulation to give an air gap then I will lose any insulation properties that the boards give as the air will no longer be trapped?
I don't really want to go the whole hog of laying plastic sheet (as a vapour barrier) if I can avoid it hence my thought that the greatest amount of 'damp' air is in the bathroom so I won't boasrd over that...
Thanks for the advice so far...
Peter
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says...

If your bathroom has a window or a fan extraction system, then why worry about damp air escaping through the plasterboard ceiling ? You can insulate and board over the whole area in the loft of you want, unless that is, you have holes up through the bathroom ceiling into the loft space.
No one is stopping the ventilation into the loft, you're only lagging the space directly above the room ceilings to trap air so it doesn't escape as quickly as it otherwise would. If that makes sense. :-))
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Air is the insulator used in loft insulation. The thicker it is the more air is trapped and the better insulation.
By boarding the loft, you automatically trap air under the boards, and provided that air doesn't circulate, I would think that there is no advantage in providing a thicker layer below the boards. In fact if you sealed the gap between ceiling and board completely, you wouldn't need any fibreglass, the air would act as the insulator as in double glazing.
The fibreglass you have there will impede the air circulating and so I would just go ahead and board over, paying attention to general loft ventilation, especially over the bathroom, as discussed.
Colin
PS Don't forget to box in any halogen downlighters if fitted
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a poor assumption, not only do I not remember you other posts, I didn't even read them - I maybe read 10% of the thread in uk-d-i-y at the moment.

insulation below, and then put thicker insulation over the -parts.
--
Chris French, Leeds

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