Roof vent slates after loft insulation - necessary

antiskeptic

Been told its necessary to add roof ventilation tiles to an existing pitched tiled roof post attic insulation.

This to offset risk of condensation.

I don't quite understand. Why would loft insulation increase the risk of condensation? Is it that the roof will be colder now the loft is insulated?

The roof has roofing felt as a secondary membrane. I'm not supposing this is to be puctured so that ventilation can be improved below the felt. So how do the vent tiles improve ventilation below felt (on the underside of which you might expect vapour to condense?

Wouldn't it be the case that a tiled (as opposed to slated) roof would be thoroughly vented anyway - you can see plenty of light between the tiles?

Calahonda52

antiskeptic wrote: »

1 Been told its necessary to add roof ventilation tiles to an existing pitched tiled roof post attic insulation.

This to offset risk of condensation.

I don't quite understand. Why would loft insulation increase the risk of condensation?
.
2: Is it that the roof will be colder now the loft is insulated?

3: The roof has roofing felt as a secondary membrane.
I'm not supposing this is to be puctured so that ventilation can be improved below the felt. So how do the vent tiles improve ventilation below felt (on the underside of which you might expect vapour to condense?

Wouldn't it be the case that a tiled (as opposed to slated) roof would be thoroughly vented anyway - you can see plenty of light between the tiles?

1: by whom?
2: yes
2a are soffits ventilated?
2b: poor detailing may have stuffed the soffits vents, if any
.
.
3: secondary?
3a, felt will be cut to allow air into attic space

antiskeptic

1. Surveyor

2a. Yes

2b. The required 1 foot of rockwool did indeed restrict the amount of air coming in via soffet (its one of those roofs where the top floor ceiling joists tie into the pitched roof rafters a couple of foot up from the wall plate). There is space but not anywhere near as much as before.

3. Secondary as in secondary barrier to water ingress (the first being the roof tiles)

4. Felt cut? If it's function is to act as a secondary barrier for water ingress then how can it be cut?

If it is acceptable to cut it in order to allow a vent tile to function, then my query on why a vent tile at all, given there are substantial air gaps between the overlapping tiles. If there's a thousand tiles on the roof, what advantage is adding a few vent tiles? Why not just cut the roofing felt and let the air which will flow up between the roof tiles do the job?

Gumbo

antiskeptic wrote: »

1. Surveyor

2a. Yes

2b. The required 1 foot of rockwool did indeed restrict the amount of air coming in via soffet (its one of those roofs where the top floor ceiling joists tie into the pitched roof rafters a couple of foot up from the wall plate). There is space but not anywhere near as much as before.

3. Secondary as in secondary barrier to water ingress (the first being the roof tiles)

4. Felt cut? If it's function is to act as a secondary barrier for water ingress then how can it be cut?

If it is acceptable to cut it in order to allow a vent tile to function, then my query on why a vent tile at all, given there are substantial air gaps between the overlapping tiles. If there's a thousand tiles on the roof, what advantage is adding a few vent tiles? Why not just cut the roofing felt and let the air which will flow up between the roof tiles do the job?

The vent tile should have flaps where the felt overlaps.

Meningate

The vents are necessary. There would not be enough draft under the tiles to just put a hole in the felt. BUT remember the felt is there to collect any water that gets past the tiles and carry it down to the gutter. People fitting vent tiles don't always seem to be aware of that. Just cutting a hole in the felt and shoving the vent through is not good. You need a flap of felt turned up above the vent to deflect any water from above the vent, to the side.

antiskeptic

Meningate wrote: »

The vents are necessary. There would not be enough draft under the tiles to just put a hole in the felt.

There would with these tiles. A small section of the felt was damaged and I cut it out and laid in a lapping repair piece. Not before noticing that the amount of light showing between one tile laid upon the other. Add up all the roof tiles and you could free flow a force 10 gale through the attic if there was no membrane

BUT remember the felt is there to collect any water that gets past the tiles and carry it down to the gutter. People fitting vent tiles don't always seem to be aware of that. Just cutting a hole in the felt and shoving the vent through is not good. You need a flap of felt turned up above the vent to deflect any water from above the vent, to the side.

..where it presumably drips into the attic having made it to the edge of the flap? Maybe, given the felt belly's down slightly, the flap could be cut to one side, high on the slope so that there's a fall from the flap edge towards the belly.

I've seen some kind of felt insert that tackles this to a degree. The tile then matching up with a hole in the insert.

Wondering now whether I can combine: just use the available under tile ventilation above membrane with a few of these inserts installed from the attic. Save having to get up on the roof.