insulating then boarding an attic - is it worth increasing joist depth first?


Looking to board my attic for storage; it has some scrappy Rockwool and joists are 4"x2". I was wondering whether to plant 2"x2" timbers on joists to allow for greater depth of Rockwool, or maybe the chipboard would provide enough extra insulation? Another thought - better grade insulation material in place of Rockwool but in the same 4" space? Any ideas welcomed.
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If you add 2x2 timbers on top of the existing joists you are still 4 inches short of the recommended 10 inches of loft insulation. Have you considered also putting insulation between the rafters, say 4 inches?
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Insulation value of chipboard is negligible. Best put high performing foam insulation between the existing rafters. Install 4"x2" joists over the existing at 90 degrees to the existing joists, have a 6" screw through to secure the joists, and then put cheaper rockwool between these, then chipboard over. Check if the existing ceiling can hold the weight.
Also seal all holes in the ceiling with silicon to prevent draughts, where cable and pipes run through. Have a sealed and insulated loft hatch. Your fuel bills will then drop.
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snipped-for-privacy@googlemail.com wrote:

If you are looking at better grade insulation, you could simply lay a rigid foam board insulation over the joists and then screw the flooring down through it. No need for extra battens etc, and no additional weight.
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Cheers,

John.

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On 16 Oct, 00:21, snipped-for-privacy@googlemail.com wrote:

Dear Peter You need to address the issue of the strength of the 100 mm joists and their span wrt their storage capacity. If the storage is light and next to the bearings (ie small objects or old boxes) then no problem but there is not likely to be much use to the area with such limitations. If you wish to store heavier stuff then you need to provide mid span support or beef up the strenth somehow. Not to do this will put you at risk of long term deflection of the joists due to "duration of load" effects. I concur with the others that your best route is to put in 3" of Kingspan or the like leaving 25mm gap under the slates in between the rafters and you can always add inside.
If this does not appeal on cost grounds and you wish to do the ceiling joists regardless bear in mind the following you need min 300mm rockwool or the like you need to lay first layer with the joists and overlapping next layer at 90 degrees eaves ventilation facility a vapour check under the insulation To get storage on this you will need blocks on the joists close to the bearings and run bigger joists on top - quite expensive and not a good structural solution and even then limited to loading
to get a good loading you need a good well-engineered structure
Chris
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the rafters. Much easier to do than foam sheets, and your loft would not get as hot/cold.
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It is a pity it is very expensive and doesn't work too well.
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On Oct 16, 12:21 am, snipped-for-privacy@googlemail.com wrote:

Make sure you don't end up with condensation forming in the loft due to the reduced temperature. The previous occupants in my house had extra insulation with no vapour barrier and no ventilation.
MBQ
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On 16 Oct, 00:21, snipped-for-privacy@googlemail.com wrote:

It sounds as if you are asking 2 things: 1) boarding the loft 2) insulating the ceiling against heat loss
The best way to insulate your ceiling is at ceiling joist level not at rafter level, that is for loft conversions. 4x4 joists should be fine for general storage as long as they are at 400mm crs and not overspanned. Heavier loads should be kept over load bearing walls or near the joist ends. If you have plasterboard ceilings you will have virtually 100mm depth available so drop in 90mm Celotex, Xtratherm Ecotherm or similar ridgid boards cut as a good fit between the joists. To seal any slight gaps run aluminium tape across the top of the joists overlapping onto the boards, it's fairly cheap for a 100mm wide roll. The just lay boards over. Don't use MDF as it is heavy, go for building ply, it's light and strong and if screwed to the joist tops will effectively act as a diaphram and provide strength. Buy insulation at ENCON for best rates.
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I agree with AJH who had a better solution than mine Chris
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On Oct 16, 7:42 pm, snipped-for-privacy@atics.co.uk wrote:

A bit of a U-turn from "To get storage on this you will need blocks on the joists close to the bearings and run bigger joists on top - quite expensive and not a good structural solution and even then limited to loading
to get a good loading you need a good well-engineered structure"
MBQ
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