between the rafters insulation?

14 Nov 2010
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United Kingdom
Our loft is blooming freezing and the cold air comes down into the house.

There is some insulation at floor level and it has been fully boarded. The roof is fairly new good thick slate tiles and has a breathable membrane on the back. The rafters are only about 70mm deep.

We were thinking about putting insulation between the rafters to make it a bit warmer and stop the draughts a bit.

We have had a quote for the spray foam insulation but are also considering using boards between the rafters such as kingspan or celotex.

Because our rafters are quite shallow and you are supposed to leave a 50mm gap before even putting the boards up we would only have about 20mm space for the insulation board. Is this enough? Is there anything else anyone can recommend?

Does anyone have any information on the spray type of insulation.

Sorry for being a bit vague, it's not something either of us have any knowledge of and would appreciate some real advice on.

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hi it is just for storage at present but may use as a room in the future so were thinking of plasterboarding it around the rafters too.

our roof is always the first to clear after snow so there is obviously something lacking but the temperature upstairs is our main concern. maybe it is something as simple as upgrading the loft hatch?

Will check out the spray foam forum, thanks :)
i would not spray it. it wold be easy to use a multi foil insulation fixed to the underside of your rafters. fast ,clean and works..
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I can't see how cold air can come down from the loft when warm air is held by the ceiling

If you mean a draught, then change the loft hatch
Changing the loft hatch is a good idea, it's a bit flimsy anyway! Where we live is very exposed and fairly windy so I guess the wind pushes in through the slates and the membrane and down into the house. Upstairs is generally cold, not sure if it's got worse or just I notice it more since our son came along.

The multifoil insulation looks ok and easy to install, are there any drawbacks to it and and do you have to leave a gap at the eaves to stop condensation like other insulation products?
If you insulate at rafter level with any foil insulation, then you will be most likely creating a condensation problem as the warmer air in the loft hits rafter level

The only way to stop this would be to ventilate the loft, which will defeat the object of insulating it at rafter level
foil is fine fitted as i said with no gaps at eave level but dont block any air flow between the out side of the foil and the breathable underlay.
warm air will be retained inside the house .
that's great, thanks, would we be able to plasterboard over it when the time comes?
have you worked out yet how cold air from the loft gets into the house? there must be holes in the ceilings. Have you got downlighters, or pipes, or a badly fitting loft hatch?

presumably you have already got a thick layer of loft insulation.

if you are thinking of converting the loft into a habitable room in future, I would advise against plasterboarding it now, or doing anything on the rafters. You will only have to rip it all off when you rebuild to comply with building regs.
JohnD, I think it is mainly the loft hatch, apparently we have the original lathe and plaster ceilings so that may also be a culprit.

To be honest the whole house is draughty, it comes up through the skirtings in the front bedroom too, (that's our next mission) but it's a very noticeable blast of cold air coming down the stairs and as I said before, our roof is always the first to clear when it snows, suggesting the heat is getting up into the loft.

We haven't pulled up the boards in the loft to check how much insulation is under there, the guy before us did it and he did ok with everything else in the house so wrongly or rightly we have just presumed it is sufficient.
modern standard is 250mm (ten inches) in the loft which is a lot. Many years ago 50mm was the norm, then 100mm, then 150. If the loft is boarded it probably has insufficient. If you want to keep it boarded, a very neat modern solution is to lay the wooly rolls between the timbers as usual, then lay rigid foam insulating boards on top of the timbers, with your flooring laid on top. You will find a display in your local DIY shed. It will be sponsored by one of the superior brands of insulation board, such as "space board", which is rather expensive, but I can't see why 100mm expanded polystyrene shouldn't work just as well.

If you have L&P partition walls, cold air can blow down them. You can probably stuff the tops with loft insulation from the loft.

A blast of cold air coming down the stairs might not be coming through a gap, it could just be cold air descending having been chilled by contact with an inadequately-insulated ceiling.

BTW I saw in today's paper that Focus and B&Q have both got a short-term giveaway offer on loft insulation at about £1 a roll, where it has been subsidised by an energy company to meet their green commitment. Fill your boots...
It is a sheer waste of time and money to insulate both ceiling and under the rafters. do one or the other and insulating under the rafters only has logic if you want to make it a habitable space.

Until then stick with doing the ceiling as adding anything under the rafters will bring you nil, zero , nada gain.

How much insulation under the boards ?

Are the rafters really only 70 mm deep? If so , I doubt I'd be putting anything up there or walking on them at all.
Just an update, we put the foil up and in the process one of the loft boards split and so we had a look under there, there was what seemed like just rubble with bits of old newspaper and a few bits of polystyrene, we called hillserve, who came the next day to do a survey and within 3 days we had our loft insulated for free.
Only drawback is that as the standard for insulation is so thick now, we are having to raise the joists up to be able to put the boards back down, house is a lot warmer but still in chaos with everything from the loft stacked everywhere, but there is light at the end of the tunnel!!
Thanks to everyone for your advice.

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