Loft insulation: Condensation concern.

[MROY4391]

I am contemplating 'topping up' the insulation in my loft by using a proprietary product which consists of fibre glass enclosed in polythene to form 'pads' of various thicknesses. The 'pads' are designed to be laid on top of existing insulation.
My concern is, by using these, I would be forming an impervious moisture barrier over the ceiling rafters which may cause problems.
The 'pads' have a few minimal perforations on what would be the top surface of the polythene when laid, which I suspect is to allow air entry to enable the fibre glass filling to relax back to normal size. The polythene outer is not vapour permeable.
Your comments are invited please.
Many thanks.

 

[Deluks]

You mean space blanket? Does the job just fine. If you get any moisture condensing on the underside then that means you have a ventilation problem. A house should be well ventilated, open windows and doors, use extractor fans and trickle vents. No drying washing on radiators etc.

 

[PerryOne]

Since the Government in all its wisdom came up with the idea of everyone adding to their loft insulation -problems have occurred!

Plasterboard is transparent to water vapour!
An 8 by 4 foot sheet of plasterboard will allow 2.5 litres of water vapour to pass through it in 24 hours.

This is leading to wood rot in lots of lofts. Due to lower loft temperatures and condensation.

The best practice is to line the underneath of the ceiling with a water retardant membrane. (Plastic sheet) Note: There are very few things that are water vapour proof -but this is a matter of degree.

 

[MROY4391]

Deluks & PerryOne,
Thank you for your comments, I will report back when concern resolved.

More opinions invited please.

 

[PerryOne]

Perhaps you will expand your concerns?
This is a fairly large subject where one can pontificate all day and bore people to death.
I will mention that fibre glass lets moisture vapour through quite readily as it does the heat you have paid for. (Fibreglass is rubbish as insulation)

Condensation is caused by people breathing, sweating, washing, leaving bathroom doors open, leaving wet clothing and damp towels on radiators.

Ventilation removes water vapour as most times the air outside is both colder and dryer - however, letting air that you have paid good money to heat up and keep you warm, out into the garden is both foolish and expensive.

The best solution is a de-humidifier, you keep the bad habits and the de humidifier looks after the water vapour.

 

[MROY4391]

I now have sufficient information in order to proceed with my insulation project. Many thanks to those who have posted replies.
Regards.