ThermaWrap foil under tiled roof - no roofing felt

[croz]

1920s house - no roofing felt under tiles. Planning to reinsulate at joist level plus add ThermaWrap foil (or equivalent) stapled to rafters for additional insulation but also to reduce general dust and dirt blowing in through tiles.
Have read elsewhere that foil wrap under tiles can lead to damp etc ....so is this a sensible plan (re-roofing with felt added is not an option) ?

Was assuming that gap between foli and tiles would be adequate for "ventilation".

All advice appreciated

[olly22]

Hi croz.

the first question would perhaps be why do you want to do this? Are you planning to use the space as a habitable room or are you just trying to keep it cool in the summer?

By insulating at joist level the loft space becomes prone to large temperature fluctuations. ie, very cold in the winter and very hot in the summer. This is how the majority of houses are insulated and is called "cold roof construction".

I planned to do a similar thing to you to try and reflect some of the heat back in the summer. (I decided not to bother in the end).

The space between the foil and the tiles would be enough, you need about 50mm. Felt or breather acts as another weather barrier, if water gets blown up under the tiles it would hit the felt/breather and run down into the gutter. However, as you dont have either I wouldnt have thought that adding foil would make much difference. Adding the foil may make it cooler in summer but do very little in terms of warmth in the winter as you insulating at joist level anyway.

[croz]

Hi croz.

the first question would perhaps be why do you want to do this? Are you planning to use the space as a habitable room or are you just trying to keep it cool in the summer?

By insulating at joist level the loft space becomes prone to large temperature fluctuations. ie, very cold in the winter and very hot in the summer. This is how the majority of houses are insulated and is called "cold roof construction".

I planned to do a similar thing to you to try and reflect some of the heat back in the summer. (I decided not to bother in the end).

The space between the foil and the tiles would be enough, you need about 50mm. Felt or breather acts as another weather barrier, if water gets blown up under the tiles it would hit the felt/breather and run down into the gutter. However, as you dont have either I wouldnt have thought that adding foil would make much difference. Adding the foil may make it cooler in summer but do very little in terms of warmth in the winter as you insulating at joist level anyway.

Thanks - no plan to use the roof space for habitation ..two reasons for planning to use the foil

1. To try and stop some of the dust and dirt that blows in through the tiles (I use the loft for "storage" - ie it's full of junk that eventually I will get around to sorting out after 25 years of accumulaion).

2. as an easy way to add SOME insulation (keeping warm rather than cool is the objective) BEFORE I manage to add to the joist level stuff (which - to do it effectively - of curse means clearing the junk ...and of course IF I insulate at joist level in the latest recommended fashion - laying at 90 degrees to and across the joists that - as far as I can see, then makes it pretty difficult to use the space for storage ....hard to board over it etc ?)

[olly22]

Ah I see. Well thermawrap is a pretty cheap pointless product. It's basically bubblewrap with foil on each side. You need to be looking at products by actis or gen-x which are much more expensive. To stop the dust you could buy a roll of breather membrane and just staple to the rafters. I believe current building regs suggest 270mm insulation. I think in conclusion adding thermawrap to the rafters won't do much as the warm air will just sit in the loft. I suggest clearing the loft and insulating at joist level asap. The winter is on it's way :-)