here is a picture of the 1st floor of a building (groundfloor removed from the picture). A new ceiling will be installed as close as possible to the underside of the strings / chords (not sure what they're called), insulation added between and above the strings, insulation added between and underneath the rafters of the skeiling (approximately 300mm total) and on the internal walls (80mm). EWI not possible in this case. If I am re-roofing, is it worth making it a warm roof? Say adding 100mm insulation over the sarking before battening and tiling? Or will this add a lot to the cost of re-roofing with small benefit?

The strings/chords are usually called raised collars, tie beams or ceiling joists. The key things to decide are:

1. Where's the internal air barrier?
2. Where's the insulation zone?
3. Where's the external wind barrier?

Ideally you want the three to be tight against each other to minimise wind washing & thermal bypass, where air moves through or around the insulation zone.

Putting the ceiling insulation between & over the ceiling joists minimises the amount of insulation required because it follows the line of the heated space. It also simplifies the air barrier design because it can be fitted below the rafters & ceiling joists before the ceiling is fitted. However, if the wind barrier is formed by the sarking membrane then it will need to follow the rafters up to the ridge. So the insulation between the ceiling joists will be more vulnerable to wind washing or thermal bypass. This risk can be managed by using breathable sarking membrane, taping the membrane laps &/or using sarking boards, but old school building control officers may insist on ventilating the loft.

Putting the insulation between & over the rafters allows the internal air barrier & external wind barrier to follow the line of the insulation from eaves to ridge, reducing the risk of wind washing & thermal bypass. However, this uses significantly more insulation. For the same heat loss you will need (Cos(Roof_Pitch))^2 times the volume of insulation, but in exchange you get a larger heated volume & putting half of the insulation on top of the rafters should avoid any impact on ceiling heights. In addition, forming the air barrier becomes a lot more difficult. It needs to be cut around every one of the ceiling joists & sealed around them with tape or glue.

A couple of things to think about:

1. Are you going to use the loft for storage? If so, an insulated loft inside the air barrier may be worthwhile & will avoid the problems caused by items compressing insulation placed over the ceiling joists.
2. If not, is a ridge beam roof an option? A ridge beam with I beam rafters allows you to have as much insulation as you want between the rafters without the problem of sealing around the ceiling joists.

David

This is good advice. There are two ways of achieving this:

1. Ventilate below the sarking membrane
2. Use breathable sarking membrane & ventilate above the membrane

Using the second approach allows you to seal the sarking membrane & greatly reduce the risk of thermal bypass.

David

Posted By: spoonandforkDoes anyone install lights over rafters (to avoid cutting rafters) or is it case of significantly altering the structure of the roof to create a large opening? My other option being veluxes...

Whether fitting roof lights or veluxes, you would normally adapt the structure to suit. This normally means doubling up the rafters either side of the opening, cutting the rafter going through the opening & tying the floating ends to a trimmer hung off the rafters either side. Be sure to leave lots of space for insulation between the plasterboard lining the opening & the double rafter as it represents a significant thermal bridge. There are details around for minimising this for roof lights based around an externally insulated plywood box.

Posted By: spoonandforkhow do you make your breather membrane airtight at the eaves? Tape it to the sarking?

If your sarking board is sealed to the wall's outer leaf then this should work. The sarking membrane is normally designed to drain into the gutter, so it may help to have two runs of sarking membrane at the eaves: one draining into the gutter & one wrapped around the rafter ends & sealed to an outer leaf wall plate.

David

If the underside of the sarking board is sealed to the outer leaf then this works as well.

David

The main one for me would be level of illumination. You're paying for a roof window which spans (say) two rafters but getting the illumination of two smaller windows. The difference immediately below the window will be (rafter_width/2xrafter_separation)x100. However, either side of the opening the rafter will tend to completely obscure the more distant half of the window. So I think the overall difference will be greater.

Perhaps he could line the opening & cover the rafter with highly reflective foil, so obscured light is replaced with reflected light?

David