Under rafter loft insulation advice

Jeepers_Creepers

You could spray as much foam under these rafters as you like, staple a layer of quilt over it and then nail up whole sheets of Celotex to fully skin the underside of the rafters - and it wouldn't help the warmth level in the house.

That's because the loft would still need to be fully ventilated. So when it's cold outside, the loft will be equally cold inside. Ergo all that insulation has been for now't.

If you stop the draughts - block off the ventilation - then, yes, the loft will become a little warmer as it's heated by the house and this heat is no longer ventilated away, but it'll also become damp, and condensation will form on the coldest surfaces just like it does on the corners of your window panes in the morning. The loft might be a little warmer than it would otherwise be, but it'll still be cold relative to the house = it's where moist air will condense out.

You can wipe up the cond on your window panes, but your timbers upstairs will be vulnerable to rot.

goldfinches

wpp34 said:

Thanks  for the replies, very helpful. A few answers:
1 house is 3 storey Victorian semi so poorly insulated. Loft partially boarded over and the rest has around 200 mm of insulation. Want to reduce heat loss.
2 difficult to lay more insulation because of boards so cannot add more layers over the floor space.
3. I am worried about moisture build up so have noticed some materials claim to be breathable.
My theory is that heat escaping through current insulation is then dissipated through the roof as it is just slates over roofing felt. If I put a layer of insulation over this it would mean the loft space was warmer. If the loft is warmer then less heat will flow into it as the temperature difference will be smaller. Therefore more heat stays in the rest of the house. 

I was brought up in a 3 storey Victorian detached house which my mother recently downsized from so from those 50 years of experience here are the things that really made a difference to how warm that house felt in no particular order.

Cavity wall insulation - this made an enormous immediate difference and is really worth the money
Double glazing on every window - some difference but not as much as we'd expected but did reduce noise considerably
Lined curtains that covered window completely - immediate very effective difference and if all the way to the floor even better
Loft insulation - very effective when done well with no gaps and real thickness of best quality insulation, also insulate top of trap door/access so that there are no gaps but you must be finicky about doing it very well and leaving clearance around the edges of roof so either do it yourself or pay for a really good job and nitpick all the way through
Heating - radiators must be suitable for size of room and sited well and furniture should be arranged so as not to block air circulation and boiler should be sized to cope well with everyday demands
Chimneys - should be properly blocked off or have liners for fires/wood burners etc so they are not making draughts
Floors - should be covered with nice thick underlay and good quality carpets and if you can stuffing the gaps between the floor boards helps too
Door curtains for external doors - very effective especially when lined and well sited so that whole doorway is covered
Draught proofing - very effective especially around sash windows where locks were changed to the type that hold sash sections together and great when blocking draughts around external doors

All these things above actually worked and many of them can be done yourself relatively easily and cheaply. All of this cost my parents much money and trial and error and now it seems peculiar that it was so hard for them to get good information about what to do and how. 

stuart45

You can have the insulation at rafter level and create a warm roof, but then you don't need the space vented as you would lose the heat.
When I built my extension, it was on the end of a flat roof extension, so I went over the lot with a pitched roof. 
As my wife wanted a vaulted ceiling I ended up sticking the 120mm insulation boards on top of the rafters, with is better for the cold bridging problems but more difficult to do when fixing the battens to the rafters.
However to put insulation at rafter level either on top, inbetween, below or a mixture of these you should have a breather membrane under the tiles. Also on a new build a VCL in fixed on the upper floor ceiling.

Jeepers_Creepers

If you overboard (underboard?) the rafters with insulated p'board, leaving the space above - between it and the slates - open and ensure that each end of this void is taken down and ventilated to the eaves on both sides to ensure a free passage of air up, across at the apex, and down t'other side, then - yes - you can insulate your loft and should have no concerns about condensation causing rot. You will have effectively turned the loft into another 'room'.

As with all other rooms in your house, tho', if you don't also provide some ventilation for it, then you'll get some condensation forming in it from warm/moist air coming through from the rest of the house. In this new scenario, however, there wouldn't be a risk of rotting timbers ('cos they are on the other side, and ventilated) but of mould and mustiness.

stuart45

Mine was 120mm Celotex boards on top of the rafters. One of the chippies at work told me he had a lot of trouble knocking the fixings through the counter battens, then through the insulation and then finding the rafters below.
I decided to make a jig for the counter battens, drive the fixings into them on the ground , stick 3 fixings up through the insulation from below next to the rafters to mark them out and then stick the counter battens in place and drive them home.