Replacing felt with breathable membrane from inside loft

[craig1234]

I am building an extension and have created extra space in the loft in the process with all the new roof space being covered with breathable membrane/tiles but leaving a small section (approx 3m x 3m) of original roof with felt. I wish to insulate the new loft space (between and under rafter insulation as per the rest of the new habitable area) which should present no issues where there is already breathable membrane but I need to do something where I have the original roof felt. I don't want to remove the tiles and replace the felt from the outside so I am thinking that as it is such a small area then from the inside I could cut away the felt in the "empty" area between the rafters and replace this with strips of breathable membrane. If I am careful I should be able to just squeeze the edge of the membrane between the rafters and what is left of the original felt (on top of the rafter). I could then silicone the edges to provide a seal. Alternatively, I could cut away the old felt and simply fit the membrane as a continuous sheet underneath i.e. it would touch the underside of the tiles and then run underneath the rafters. Does anyone have any views or suggestions on my thoughts? I have actually bought the insulation (Celotex) already.

 

[freddiemercurystwin]

Yes, you're bonkers.

 

[craig1234]

Thanks for your lack of useful information/input

 

[Alastairreid]

Put high and low vents in the old part of the building.

Or you may get away with opening the flaps in the felt to create an airflow.

 

[craig1234]

Thanks Alastair.
Are you suggesting that I don't put the breathable membrane in at all i.e. just rely on the existing felt with added ventilation?

 

[Alastairreid]

No reason why you cant retain the existing felt as long as you add a means of venting the space.

Something like These.

 

[craig1234]

Alastair,
Great - those look the part - I will go with that - thanks very much for your help.

 

[Guitar_man]

I have tried those vents. They are very dependent on how tightly fitting are your tiles. If you have gaps they may work as clay tiles usually have. If you dont ie have fibre cement tiles or equivalent tight fitting they wont work.
My investigations have found that the most effective type of ventilation is cross ventilation, ie install vent tiles on opposite sides of the roof at the same height at low or mid level.

 

[craig1234]

Jimbo, Alastair,

Thanks for your help - I have fitted the vents and they seem to work - bit tricky getting them to fit and they were a little brittle I thought but they seem to have done the job!

 

[Alastairreid]

Fingers crossed craig.