Boarding a loft & insulation

Hi,
I am interested in boarding over part of my loft area, in order to increase the amount of storage space in our house. My question is concerning insulation. The joists on the floor of the loft are 3x2, thus if I put boarding directly on to them, I can only have a maximum of 3" of insulation (I think I should have a lot more ?!) Should I screw some lengths of 3x2 or 4x2 at right angles to the joists, place extra insulation in the spaces. and then board on top of these ? I know I will lose a small amount of height, and it will cost a bit more, but the house is a 30's semi (with no cavity walls) so I would like to get the loft fairly well insulated.
Would I also need to put in some form of damp membrane ? The roof is not felted, so there is quite a bit (!) of air flow through the roof space. I was not going to board over the complete area, so would it be ok to not put down a membrane on the warm side of the insualtion ?
Any comments appreciated..
Paul..
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First comment is to check your floor loadings - 3 * 2 won't carry much weight. [By that I mean that it wouldn't carry the kind of load you would put on an average floor - so if you plan to stack heavy things and walk on it as well you could be in for a surprise] The building regs. carry tables which show maximum load for cross section and span and spacing for beams.
I don't know if additional beams at right angles will increase the load bearing capacity by spreading any load, or just count as additional weight on your original 3*2 beams/joists and reduce the amount you can store. I assume that of you screw at every joist this may provide a more rigid structure.
If you screw additional joists along the top of your existing ones (e.g. add another 3*2 to make a nominal 6*2 joist) that may help but it won't be as strong as a single 6*2 joist.
For loft conversions they put in a new load bearing structure - but this is way OTT for a bit of extra storage :-)
Alternatives are to use more efficient insulation e.g. Celotex between your 3*2 joists but this is likely to be expensive.
All very imprecise, I am afraid.
Cheers Dave R
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Consider insulating the roof instead. Then you get a warm loft situation, which is better for your stored items and any water pipes passing through. If you do so, ensure you pay adequete attention to the ventilation of the rafters, or you might get rot.
Christian.
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This is something that has got me thinking - what with moving house and everything. I *could* insulate in the plane of the roof, but with ventillated eaves this isn't going to help the warmth of the house much. However, if I block the eaves, how can I keep the roof structure in good nick?
Hwyl!
M.
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Martin Angove wrote:

Standard trick is to use celotex between rafters and allow the draught to go over the top of it: You then seal between celotex and loft floor at eave level.

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The open eaves are good. You make sure they enter the ventilation space between the roof covering and the insulation. You may also need ridge ventilation, especially if you have ridge boards or a half roof.
Christian.
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Wordy wrote:

Beware of such joists. I've seen a case where long cracks appeared in upstairs ceiling due to loading of stored items in boarded loft. The cracks were exactly underneath the joists ...
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I'm also interested in doing this, though I have 6" joists. I'd be interested to know when / if a damp-proof membrane is required and if anyone knows a cheap / reliable supplier of celotex insulation? What thickness of celotex would be reasonable (has anyone else done the calculations)?!
Thanks!
Leigh
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With 6 inch joists, you are laughing. If you can afford celotex, just use the complete thickness and board over if you wish. Alternatively, a 25mm section cross layed over the top will reduce cold bridging through the joists. The damp proof membrane goes under the insulation. If you are doing it from scratch, it goes between the plasterboard and joists. If the ceiling is already in place, use plastic sheet over the joists, but under the insulation.

www.seconds.co.uk
Christian.
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Christian McArdle wrote:

You should not do that - the regs insist on an airgap to allow air to circulate over the joitsts etc.
In my case they insited on eave (soffit) vents and where I had impervious sarking, ridge vents as well.

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How can you have a floor, if you must leave an air gap between the joists and the boards?
Can I presume you thought I was still talking about insulating the roof, not the floor?
Christian.
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Christian McArdle wrote:

Yes. Sorry.

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Thanks for all replies. I fancy using Celotex as I also need to do some angled sections which won't be the easiest to get into. Might try and take a couple of photos at the weekend to give people a better idea of what I'm after, but the replies so far are what I was after!
Leigh
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L Reid wrote:

(i) Celotex incorporates its own foil damp proof coating. Use foil tape to seal it to joists etc.
(ii) There are no cheap suppliers of Celotex.
(iii) 50mm is a minimum thickness, 75mm is equivalent to 6" of rockwool. Well into top regulatrory requrements. However the joists form a 'cold bridge and ANY tiny draughts will reduce effectiveness, so its not all roses.

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