Water Vapour

Question 1

How is the water vapour related to air temperature?

Answer 1

The warmer air is, the more water vapour it can contain.

Air rarely contains its maximum capacity of water vapour.

Question 2

What are the two ways in which you can raise relative humidity?

Answer 2

1. If you add water vapour to the air in a sealed space, you will increase the relative humidity. Boiling water which releases steam, bathing, breathing, having lots of indoor plants etc.
2. Reducing the temperature in a sealed room, so that Air cannot escape, will raise relative humidity. Though the temperature is falling, the water vapour mass in the air remains constant, raising the relative humidity. If temperature drops to dew point, Water vapour condenses.

Question 3

Is dew point a fixed temperature?

Answer 3

No, Each mass of air has its own dew point, which is determined by the proportion of water vapour it contains.

It is important to realise that the relationship between different Dew point is what results in the use of vapour retarders.

Question 4

What happens in the summer when humid air comes into contact with cold surfaces.

Answer 4

Humid air can be dropped the beneath it’s dew point when it hits a cold surface.

Cold water pipes, and porcelain tanks drip causing water staining, mildew and decay in buildings.

Window panes fog up and drip, causing rusting and rotting of their sills and frames.

Question 5

What are the temperature differences in hot and cold climates internally.

Answer 5

In hot climates, our temperatures are likely hot and humid outside, and cool dry and dehumidified inside.

In cold climates, a temperature is likely cold and dry outside, and warm and humid inside.

Question 6

What drives the momentum of water vapour?

Answer 6

Water vapour in Air exerts a partial gas pressure called vapour pressure.

The more humid the air is, the higher the vapour pressure.

Vapour pressure moves from areas of high pressure to low pressure in an attempt to reach equilibrium.

In a situation where there is a moist side and a dry side separated by a wall, vapour pressure will try to move through the wall from the humid to the dry side.

Question 7

What happens to vapour pressure if there are leaks in a wall?

Answer 7

The airleak can carry you through far more water vapour through an air channel than vapour pressure can on its own, seeking equilibrium.

Question 8

What sort of resistance do common building materials have to vapour pressure?

Answer 8

Most common building materials have little to moderate resistance to passage of vapour.

Substantial amount of water vapour passing through walls with little move towards equilibrium of pressure.

This hesitant change towards equilibrium is represented by a gradual decrease in dew point temperature as vapour moves from moist side to dry.

Question 9

Assuming that the building is in a cold climate, what happens to the temperature of materials as vapour pressure is exerted?

Answer 9

As this happens, the temperature of the one swarm interior wall drops.

Through the various layers within the wall assembly, the temperature drops at different rates, dependent on the thermal resistance of each material.

The cold exterior is slightly warmer than the cold air.

Question 10

What problem occurs when the temperature of the location falls below the dew point of that location?

Answer 10

When a location temperature falls below a location dew point, the water vapour in the air condenses.

A slight local decrease in vapour pressure occurs and water vapour from higher vapour pressure areas keeps moving towards this location to equalise the pressure. This inflow of vapour relentlessly occurs until temperature decrease or difference in vapour pressure stops.

What results is saturated and saggy insulation which is ineffective, walls which then conduct a lot more heat, increased cost of artificial air conditioning and extensive hidden structural damage.

Question 11

What is a vapour retarder made of?

Answer 11

Vapour retarder is a continuous sheet of polyethylene plastic or metal foil which is vapour tight.

Question 12

What is the relationship between high and low dew points.

Answer 12

Moist air has a higher dew point.

Dry air has a lower dew point.

Question 13

What happens when migrating vapour hits and impervious layer, like a good coat of exterior paint.

Answer 13

Vapour pressure can blister the wall paint.

This is the way the outsides of bathrooms and kitchen’s peel, because they have higher levels of internal vapour pressure.

Question 14

When is the vapour retarder applied?

How is the vapour barrier then sealed?

Answer 14

The vapour retarder should be applied across the interior face of the wall and ceiling framing after the insulation has been installed. The resulting seems should be sealed with mastic or tape.

Question 15

What is mastic?

Answer 15

A type of Rezin.

Question 16

What is the result of the vapour retarder with in the wall assembly?

Answer 16

The vapour retarder prevents water vapour from entering the wall assembly. This is to maintain the dew point within the wall assembly at a low enough temperature so that it does not exceed the temperatures of the wall component materials and enclosed air. Hence preventing condensation forming on the inside of the wall cavity.

Question 17

What is relative humidity?

Answer 17

Relative humidity is the most convenient way of defining water vapour content of air.

It is a percentage figure which represents the quantity of moisture contained in the air, divided by quantity of water vapour that air could contain at a given temperature.

Question 18

How do you prevent water vapour damaging dripping cold-water pipes.

Answer 18

Dripping cold water pipes need to be first insulated, and secondly jacketed in vapour tight plastic or metal.

The insulation keeps the jacket above dewpoint, and the jacket prevents the water vapour from penetrating through to the assembly.

Question 19

In older buildings, is it possible to add vapour retarders?

Answer 19

No, this is considered too costly or ‘non-feasible’ to add vapour retarders to older buildings.

Better to plug the air leaks and then apply a good coat of paint on the warm side.

Special vapour retarding interior paint is available for this purpose.

Question 20

How to protect dripping water closet tanks?

Answer 20

In humid weather, tanks of water closet tend to trip.

You can place an insulating liner made of plastic foam in the tank which acts to raise the outside surface temperature of the tank above dew point temperature.

The porcelain tank itself acts as a vapour barrier.

Question 21

Two ways in which air-leaks waste energy in buildings.

Answer 21

1. By letting outside air in and inside air out.

2. Transporting water vapour to places where it can condense.

Question 22

How do you add an air barrier to a masonry wall?

Answer 22

In a masonry wall, the air barrier is a sticky layer of mastic spread over the surface of the interior layer of masonry within the cavity.

Question 23

What complications can you expect when adding an air barrier to a framed wall?

Answer 23

The air barrier has two roles, the first of which is to prevent air passage through the construction.

But, because it is often installed on the colder side of construction, it must allow the free passage of water vapour. There are materials are for this purpose.

If it needs to be installed on the warm side, it is possible to use a vapour tight air barrier and have it serve as both air barrier and vapour retarder.

Question 24

How to add an air barrier to a framed wall.

Answer 24

In many types of framed wall situations, a continuous sheet of material is wrapped around the building just before the exterior cladding is added.