Will improving loft insulation reduce upstairs condensation?

[burn_rachel]

I have a 3 bedroom 1950's ex council house. Since we have had double glazing fitted - we have had mould spreading in outside corners of the bedrooms (the bathroom is fine!)
I have taken down the fitted cupboards over the airbricks to increase air circulation + am arranging cavity wall insulation
My loft has minimal insulation of fibre matting aprox. 3-4 cm. I was hoping that by increasing the insulation to recommended depth would be sufficient to stop the condensation problem in the bedrooms.
I know that the eaves need to be left to allow for ventilation and was planning on raising joists ( at right angles to existing joists) in centre run of attic to allow for boarding over.
Am concerned that I could be transferring the condensation problem to the loft, Do i need to use any special membrane under the planned chip board?
Will the existing airflow through the eaves + occasional rips in roof membrane be sufficient to stop this?
Do I just insulate the sides and leave the central run with its minimal insulation and boarding?
Any advice or suggestions gratefully received - thanks

[Perry525]

Mould is the result of water vapour created by washing, drying towels and other things on radiators, cooking, breathing.

The thing with water vapour/moisture in the air is, that it always seeks a cold spot to condense.

It is forming in the corners of your bedroom because they are the coldest part of the room.

In the normal way, water vapour condenses on the windows first, as they are usually the coldest part of a room, especially if you draw the curtains at night, creating a closed area.

Now that you have double glazing, the coldest part of the room is the corner, where there is little or no air circulation.

It will help if you do not turn the heating down or off, enabling the warm air to circulate, warm the corners and move the wet air to somewhere else.

Warm air holds more water vapour than cold, keeping the room temperature steady, will keep the moisture in the air.

Opening the window and letting the warm wet air out and replacing it with outside air that will usually be colder and drier will also work.

The air outside is usually drier and colder, not always, we are going through a period at the moment where the air outside is a similar temperature and humidity to that indoors and humidity controlled bathroom fans are running achieving nothing.

Opening air bricks is not a good idea!
From 2016 all new builds will have to be of Passive House standard. This means that homes will be air tight to save the waste of heat that air bricks cause.

Once an air brick is uncovered, it is uncontrolled and the heat from a home will quickly disappear, especially when there is a wind.

It is better to ventilate by opening a window, at least doing this means you will adjust the opening to fit the weather.

About 25 years ago the Government recommended that homes in the west of the UK or high up in the middle should NOT have cavity wall insulation.

The cavity is there to stop the water/rain that comes through the outer wall making the inner wall wet.

When you fill a cavity, the content usually transfers the rain to the inner wall making it wet. Unless you have polyurethane foam or similar that completely fills the space. As a closed cell insulation this is water proof.

Water is 4000 times better at conducting heat than still air, having a wet wall will cost you a lot, when trying to keep your home warm.

3 to 4 cm of insulation is next to useless!

Adding insulation will help to keep the heat in the bedroom and as such will help the air to hold more moisture.

You should add more insulation merely to save money and improve your comfort.

If you insulate using sheets of polystyrene carefully cut to a push fit, you will get twice the benefit for the same space.

Then you can board over the loft using the existing joists.

Polystyrene has an extra benefit, in that you can stand things on it, where the loft has no boards without compression and loss of insulation.

The best practice is to fill the whole space between the joists with polystyrene, then after fitting the water vapour proof membrane to fix another two inches of polystyrene sheet across the ceilings before adding the plaster board and decorations.

If you only place insulation between the joists the joists themselves become the weak link, they are after all exposed to the cold of the sky and they pass your expensive heat direct to that sky.

Measure and add together the total area of wood that makes your ceiling and then realize how much heat is lost through the joists.

You have several possibilities, one open the windows for 10 mins or so every morning and evening to let the moisture out.

Another, keep the temperature of your home steady.

Buy and use a dehumidifier, making sure to keep it working when the home is cooling down.

Use extractor fans in the kitchen and bathroom when washing, drying clothes, cooking.

An extractor fan with a built in heat exchanger is best, it will save 80 to 90% of your heat and will prevent the fan from pulling cold air into your home, creating drafts and possible cold feet.

A water vapour proof membrane fitted below the joists in the upstairs rooms to stop the water vapour moving through the holes in the light fittings and through the plaster board is essential.

The idea of eaves ventilation, is based on the idea that the passing wind will create an area of low pressure to the lee of a home, pulling the warm wet air from the loft. (And the rest of the home)

In reality, there are many days and nights when the wind does not blow, and the idea doesn't work.

Fitting a water vapour proof membrane and sealing all the holes and cracks into the loft works. (And making sure the door into the loft is an air tight fit.)

No!