Loft ventilation

[Broadback]

Recently I have had loft insulation installed, the "surveyor" maintained that even though there was no obvious form of ventilation it was not needed because the roof felt was a breathable membrane, looks like ordinary roof felt to me. Anyway 2 questions, when is the best time to check for condensation, in warm damp weather, cold, or when? Second if I need ventilation I intend to put through air bricks at either end of the gables, does it matter how high they are placed? TIA

[Christian McArdle]

It looks pretty similar. Are you sure you haven't got breathable membrane?

There no point in insulating the rafters and then putting loads of ventilation into the warm loft. The loft itself should have as much ventilation as a habitable room, i.e. a trickle vent, perhaps.

Christian.

[marvelus]

Doubt he has insulated the rafters.

[marvelus]

Cold, any humidity in the loft will condense on the coldest places.

I cant think of a reason why it would matter. I put one in lowish as a trickle vent an am going to see what happens. If I still get condesation I'll put a tile vent in the other side so I have hot air leaving at the tiles cold air entering at the air brick convection thing going.

[manatbandq]

Do you have other reasons to doubt the surveyor?

When the roof is cold enough for condensation to form, I found it worse in the winter.

As low as possible but I doubt a few air bricks are sufficient. You need continuous eaves vents (or equivalent) so that the air enters at a low level and flows across the roof space.

See

and many other such pages.

MBQ

[Broadback]

Thanks all for your replies. No the insulation is on the floor of the loft. It was not that I did not believe the surveyor, just that he was not a surveyor in the true sense, so thought I would keep an eye on it myself. I was offered (by a different installer) the option of simple plastic wedges that hold the roof felt away from the tiles. If I install the vents the plan was one in both gable ends, which are at either ends of the rather long and narrow loft. If I need to do that I cannot complain as I had full loft insulation and cavity wall insulation installed free!

[The Natural Philosopher]

1/. Condensation will most likley occur on still cold days when hot steamy air from the bedroom ;-) gets into the loft and condenses on anything below dew point. 2/. Beware of end gable vents...they do little for rafter ends in the eave area..the normal recommendation is to vent the eaves, because they are likely the coldest place, and the ridge, because this is the warmest. Then a slow convection can draw air from outside into the eaves and up through the ridge.

[<me9]

I would be wary of airbricks in the gables, unless there is also a snow barrier. I had one at both ends of my extension, and snow blew in one of them and ruined the ceiling. Far better to ensure that the insulation isn't blocking ventilation from the eaves, and fitting soffit vents.

[keith_765]

If you have just had insulation in the loft and the underlay felt is a breather type, This sound rather quire, as breather underlay felt is a modern idea. Therefore why have you had to have insulation in a modern house. Either the property is older and as been re roofed with modern breather underlay. As regards holding the roof felt away from the tiles, this comes natural when the lath are fixed and the tiles laid. The underlay should sag away from the under side of the lath by 10mm to prevent, what is known as ponding, ponding is condensation that runs and drips of the back of the outer roof covering either slates or tiles. It as been found that venting should also be carried out, not only to the roof space, but also venting between the felt underlay and the roof covering.