Window sills.

[omits]

I have a 70s house and had the windows done in UPVC. The internal sills are thin tile with the cavity right underneath. Very cold in winter (there is a 4 degree C diff on surface of tile with the outside wall internal surface!). I am going to replace the tiles with oak say 20mm thick. What material should I use under the wood to fill the cavity to reduce the cold hitting the sill? Thanks.

 

[tell80]

Rip a length of polystyrene & jam it into the cavity.
Use sealant to seal any suspect area at the bottom rail of the frame.
MDF window board is available from any supplier or shed.

 

[OvertonPlumber]

Use Rock wool cavity slab to bridge the cavity