I am interested in boarding over part of my loft area, in order to
amount of storage space in our house. My question is concerning
The joists on the floor of the loft are 3x2, thus if I put boarding
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
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..
First comment is to check your floor loadings - 3 * 2 won't carry much
[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
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.
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.
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
Martin Angove: http://www.tridwr.demon.co.uk /
Don't fight technology, live with it: http://www.livtech.co.uk /
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.
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)?!
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
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.
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!
(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
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