Vapour Barrier is it needed?

[rockinter]

Hi,
I am about to top up the loft insulation. But i have just seen the insulation that was put 15 years ago does not have vapour barrier underneath. Do i need to put a vapour barrier?
thanks

 

[noseall]

A vapour barrier prevents moist air entering the loft space and needs to be fitted immediately beneath the insulation i.e. sandwiched between the plasterboard and the joists.

This is usually done via foil backed plasterboards, foil backed insulation or a simple polythene sheet.

Do you need it? Just read some of the posts on this forum regards 'condensation in lofts', then make up your own mind.

 

[PrenticeBoyofDerry]

You wont need a vapour barrier, if the instulation is above the ceiling in between the loft joist, but is an option as noseall says.
http://www.energysavingtrust.org.uk...lazing/Loft-insulation/All-about-installation

 

[rockinter]

i guess, as i haven't had any condensation problem in the last 15 years with the current insulation. I won't need to put the vapour barrier.

 

[mointainwalker]

IMO it would be very worthwhile to lay a thin poly sheet - taped where necessary - over the existing insulation before adding the new.

 

[rockinter]

Surely if you add the poly sheet on the existing insulation you will get condensation problem as it will be on the cold side instead of the warm side?

 

[jeds]

i guess, as i haven't had any condensation problem in the last 15 years with the current insulation. I won't need to put the vapour barrier.

You don't need a vapour barrier as long as the loft space is well ventilated. You also can't rely on the experience of the last 15 years as adding more insulation changes the situation. Your loft space will become even colder and the liklihood of condensation will increase.

For the same reason you should also consider adding more ventilation if reasonably practical. Or at least keep a close eye on the situation to make sure you are not getting excessive condensation in the timbers.

 

[rockinter]

Would vapour barrier make the rooms underneath more warmer than without it?

 

[noseall]

Would vapour barrier make the rooms underneath more warmer than without it?

Probably marginally warmer with a vapour barrier as the moisture will be trapped within the room below and moist air will hold more heat than dry air. Damp air takes more energy to heat though, so as a consequence background ventillation is a must, as you don't really want this moist air hanging around within the building.

 

[mointainwalker]

The vb in itself adds next to nothing to the insulation equation, however - and in this I disagree with some other posters - its presence is vital.

Without it, moist air will rise into the loft and somewhere within the thickness of your insulation the moist air will reach its dew-point as it cools getting nearer the cold air above.

When this happens the air relases the moisture it is carrying and the result is wet insulation (wool-type). This means that not only do you have potential problems with damp ceilings, but the insulation value of the insulation drops dramatically as water is a superb conductor or heat i.e. makes your heated spaces below lose heat very quickly.

 

[Chukka63]

spoke to a Building inspector today and a vapour barrier isnt required but he advised it was probably a good idea over "wet" rooms.