Insulation Basics – Ventilation

A significant factor in the comfort of you house is the ‘Ventilation Rate’.

No one likes cold draughts in winter.. . . and hot air creeping in during summer can be equally unwelcoming.

So how do you calculate heat losses due to ventilation?

Well it starts by deciding how many air changes per hour (ac/hr) you have.

That is how often is the air in the areas you want to heat, or cool, replaced.

Typical ventilation rates

Its hard to measure the actual changes without specialist equipment however here are some typical values:

    • Old weatherboard property – More than 2ac/hr
    • Typical new house – Around 1 ac/hr
    • Well draft sealed house – 0.5ac/hr


To calculate the ventilation heat loss the following formula is used:

Qv = 0.36 x V x N

Qv – Ventilation heat loss in in W/degree C
V – Volume in cubic m of space being heated or cooled
N – Number of air changes per hour

So for a typical house with a floor area of 250m2 with 2.3m ceilings:

Qv = 0.36 x (250 x 2.3) x 1 = 207W/degrees C

To keep this house at 20 degrees with an outside temperature of 10 degrees would need 207 x 10 watts = 2.07kw/hour…… just for ventilation losses

A well draft sealed house will reduce the heating required to 1.04kw/hour. (You would be saving $1 every five hours, with more saving on colder nights)


If you have got a flue less heater (such as portable gas heaters, kerosene heaters or bio ethanol heaters) you do need some ventilation to keep a safe level of oxygen in the room.

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Insulation Basics – Reflective Finishes

There are a number of insulation products with reflective surfaces now available but how do they work?

Why do the instructions tell you to put the more reflective surface on the underside rather that the top?

Here is some information to help you understand what’s happening.

All surfaces have different levels of ability to reflect heat with black and matt finishes being very poor and shiny metal surfaces being very good. Most reflective surfaces are also poor emitters of heat. This characteristic is called emissivity (normally written as ‘e’).

An example is clean aluminium foil has an ‘e’ value of around 0.03. This is much less than common building materials such as:

  • bricks ‘e’ = 0.9 ; or
  • Building membrane ‘e’ = 0.5

Low emissivity finishes are generally considered to have an ‘e’ value of less than 0.05.

As dust can increase emissivity by 0.25 Aluminium foil is only considered to be low emissivity on vertical surfaces or the underside of a inclined or horizontal surface such as a roof or ceiling, where it will remain clean.

The emissivity acts with the surface layer of air on the building material to give an overall R value for the surface.

The following table shows how the ‘R’ values increase with a Reflective (low emissivity) surface compared with a high emissivity surface.


Cavity Width

Direction of

Heat Flow

Low Emissivity

R value

High Emissivity

R value








Sloping 45 degrees











You will see that low emissivity insulation such as aluminium ceiling insulation has better performance (Higher R values) when the the Hotter air is above the insulation than the reverse.

This means that in summer it resists heat during the day but allows the house to cool more quickly at night. In winter, however the performance in keeping the heat in the house will not be as good.


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Buffer Zones

Planning a house layout? or considering builders standard layouts?

Thinking about buffer zones when you are will save you on heating and cooling costs.

So what are buffer zones?

Buffer Zones are basically rooms and spaces that may be heated to a lower temperature, only heated occasionally, or even left unheated in winter.

In summer the situation is reversed and these rooms and spaces will not need to be cooled to the same extent as the main rooms.

As these rooms and spaces are at a temperature between the main rooms and the outside they act as additional insulation reducing the cost of keeping the main rooms at a comfortable temperature.

Examples of Buffer Zones

Typical  ‘Buffer Zones’ are:

      • Roof space
      • Garage
      • Guest bedrooms
      • Laundry
      • Study
      • Toilet
      • Bathrooms
      • Porch
      • Conservatory

An example of using buffer zones can be seen on the following floor plan.

The study, laundry, main bathroom, toilet, en-suite are all enclosed rooms on the South side of the house.

They don’t need to be heated/cooled all the time.

For instance in winter the bathroom only needs to be heated for around half an hour in the morning.

All this reduces the volume to be heated in winter and stops heat loss to the North.

A popular buffer zone in England is a Porch.

We haven’t had one in an Australian house yet and they don’t seem to be very common in standard designs. . .but if you live in the High Country, or Tasmania, one could be well worthwhile.

Conservatories are also less common in Australia probably because of the overheating risk in summer.

They can however be useful for increasing ventilation through the house if properly designed with large top vents.

Has a buffer zone worked for you?

For more posts about plans see the Design Category.

To save money on Heating and Cooling see Insulation


Insulation Basics – Weather Effects

When you are considering how much heat you will gain in summer, or lose in winter its not just the outside temperature you need to think about.

It’s actually the surface temperature of the outside wall, or window as it can be different to the outside measures air temperature.

Here are some thoughts on how weather can change the thermal performance of your new house exterior.

Wind Chill

Although people normally talk about wind chill with respect to clothes it applies to buildings as well.

Typically the most affected surfaces are windows.

For an exposed windy site the R value of the windows can be 25% lower than on a sheltered site.

Things such as changes to the micro-climate of you house will reduce wind chill.

Rain Wetting Walls

Once a wall gets wet the rate of heat loss to the outside in winter will increase due to evaporation cooling the wall.

This affects porous surfaces such as brick and wood more than impermeable surfaces like steel.

For a typical brick wall a moisture content of 5% can lower the termperature of the outer skin by around 25%.

The most effective way of avoiding this by keeping the walls dry is to have at least 450mm wide eaves.

Summer Radiant Heat

Have you ever felt a brick wall after the sun has been shining on it for a while?

The radiant heat absorbed by the wall can make the wall surface 5-10 degrees hotter than the surrounding air.

If you are doing calculations for air conditioning this can make a difference to the required cooling capacity of the unit.

So keeping the summer sun off the north and west facing walls with wide eaves, verandas or pergolas will help keep your house cool.


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Insulation Basics – Double Glazing

Large single glazed windows are one of the biggest reasons for heat loss in a modern house.

It is also a major source of heat coming into the house in summer.

So what can you do?……………here is a comparison between single and double glazing.

Single Glazing
A single glazed window with an aluminium frame has a U value of around 7watts/degree C/m2 (an R value of 0.14 )

So if your house has got 30m2 of windows and its 5 degrees C outside.

You will be losing the following amount of heat through the windows if you keep the house at 20 degree C

30 x (20-5) x 7 = 3,150watts = 3.15kW/hour

For refrigerated cooling from 300C to 200C you will need the following amount of cooling to balance heat gain through windows:

30 x (30-20) x 7 = 2,100watts = 2.1kW/hour

Double Glazing

If you have double glazing with timber or uPVC frames you will reduce the U value to around 3 watts/degree C/m2 (An R value of 0.33 ).

For the same conditions as the above example the heat loss through the windows will be reduced to:

30 x (20-5) x 3 = 1350watts = 1.35kW/hour

For refrigerated cooling from 300C to 200C your heat gain through windows will be reduced to:

30 x (30-20) x 3 = 900watts = 0.9kW/hour

Other ways to reduce heat loss are

  • Reduce window size. As walls are better insulation than windows this can offer significant reductions in heat loss
  • Curtains or Blinds. Will provide similar performance to double glazing. . . but only during the time when they are closed.

Extra Benefits of Double Gazing

Improved Security: Its much more difficult, and noisy, to break in through a double glazed window.

External Noise Reduction The bigger the gap between glass the better the performance.

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For Posts about Green Building see Sustainability


Insulation Basics – Vapor Barriers

When you are thinking about installing insulation you also need to think about Vapour Barriers.

Although I frequently saw mention of vapor barriers in books and articles it was a long time before I understood why there were needed……………. Here is my explanation:

  1. Warm air, inside the house, contains a lot of moisture (water vapor) which comes from people breathing, cooking, showers, flueless gas or oil heaters, and house plants.
  2. If this air is allowed to pass through the building structure it cools.
  3. As the air cools it can’t hold as much moisture and water condenses in the structure and in the insulation.
  4. The water can:
    • Waterlog the insulation reducing its effectiveness.
    • Cause rot in wood.
    • Cause corrosion, Particularly on the underside of a metal roof.

To stop the problems you put a vapor barrier………….Which is really an airtight barrier……….on the inside face of any insulation.

An Exception

You Don’t usually need a vapor barrier on the ceiling if you have a ventilated roof space as the air flow above the insulation will dry out any moisture in the insulation.

Cathedral ceilings  and flat roofs are a different matter and Do Require a vapor barrier as there is no ventilated space above the insulation.

Types of Vapour Barriers

Vapor barrier don’t have to withstand any pressure so they can be quite thin. Examples are;

    • Polythene sheet,
    • Reflective foil,
    • Foil backed plasterboard,
    • Water resistant painted surfaces,
    • Impermeable insulation such as sheets of polystyrene.

All joints and overlaps in the vapor barrier should be taped or glued to make sure no air gets through.

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For Posts about Green Building see Sustainability

For more about Moisture Problems see this link: Condensation.


Rooms over Garage

Will your new house have a bedroom above the garage?

If so you should check what the builder specifies in the way of Insulation between the garage and the room floor.

Although regulations quite often specify insulation on external walls they usually don’t consider above the garage.

Why this matters

In winter garages can get very cold, particularly if the garage door is left open.

In summer the garage can get very hot. . . more so if the garage door faces west and absorbs the afternoon sun..

This means that in both summer and winter the lack of insulation can make the room uncomfortable!

What You Can Do

Make sure you discuss with the builder the option of putting insulation in the garage roof before the ceiling is installed.

Doing it then is going to be much cheaper than trying to do it later.

Already built? . . . perhaps you could add Insualtion to the garage door.

Microclimate, And Why It’s Important To Your New House

A microclimate is a local atmospheric zone where the climate differs from the surrounding area.

The term can refer to areas as small as a few square feet (for example a garden bed) or as large as many square miles.

When we are talking about providing a microclimate for a house the things can do for are things like:

      • Providing shade trees to keep the summer sun out of the house, particularly on the west side where eaves don’t work as well. (see Photo)
      • Overhanging eaves, and verandas to keep the walls dry. (Once brick walls get wet winter winds cause evaporation which chills the brickwork speeding the loss of heat through the wall)
      • Providing shrubs and plants close to the walls to retain a layer of still air which slows heat loss in winter. The plants also help shade the walls in summer.
      • Soft leaved plants that can help reduce summer heat due to evaporation from their leaves.
      • Fences, bushes or trees to deflect or break up winter winds.


Have you provided a microclimate around your house?

For More post about the garden and finishing off a house see Settling In


Insulation Basics – Calculating R Values

A previous post –introduced R and U values

In this post we will demonstrate how to calculate R values.

Well it all starts with the thermal conductivity of the building material. The ‘k’ value. This is a measure of how fast heat travels through a material.

k values are usually stated as Watts / m2 / degree C.

Tables of typical values for ‘k’ have been provided in Design Tables. Actual values may vary from these values depending on material density and manufacturing techniques.

For any component of the building R = Thickness in metres /‘k’ (The units are square metre, degree C per watt [m²·°C/W]).


An example of the calculation follows:

Material is single skin brick with a density of 1800kg/m2 (protected from rain)

R = 0.110m / 0.71( from  Design Tables.)

= 0.155

Remember that when calculating R values the thickness counts, so if you have insulation batts that are compressed they becomes less effective.



When it comes to cavities there are generally accepted R values as follows:

Cavity Width

Heat flow

Horizontal or Upwards

Heat flow




20mm or more



Typical loft between tiles and ceiling


Between roofing material and sarking



Behind tiles (or shingles on wall)



The effects of reflective finishes and weather will be discussed in future posts.

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For Posts about Green Building see Sustainability


Insulation Basics – ‘R’ and ‘U’ Value

R Value

R1.5 Batts, R2.0 Batts……. all the way up to R6.0 Batts, but what does it all mean?

The R value is a measure of the thermal resistance of the component of the material. In other word how hard it is for heat to pass through that component.

U Value

Once you have the R value  you can calculate the Heat Transfer Rate, the U Value.

The calculation is U = 1/R (The units are watts/degree C for each square m)

To get the R value of a structural element, for example a ceiling, you add the total of all the R values of each of the components.

The following table shows the effect on the value of ‘U’ for various levels of Insulation for a ceiling.


Table 1. Ceiling ‘R’ and ‘U’ values


Total R value

U value

No insulation













So how do you use these figures?

The following two examples are for a house of 150m2, which you want to keep at 22 degrees C

  1. On a summers day the temperature in the roof space is 50 degrees C (not unusual in Australian summers) and you want to cool it to 22 degrees C, a difference of 28 degrees C.

Heat transfer through ceiling = 150 x 28 degrees x ‘U’

  1. On a winters day the temperature in the roof space is 5 degrees C and you want to heat the house to 22 degrees C, a difference of 17 degrees C.

Heat lost through the ceiling = 150 x 17 degrees x ‘U’

The results of the heat gains and losses for the various R levels of ceiling insulations are shown in Table 2 below.


Table 2. Heat Gain / Heat Loss Through Ceiling.


Summer Heat Gain


Cooling Required

Winter Heat Loss


Heating Required

No insulation



R1.5 Batts



R2.0 Batts



R4.0 Batts



You can see from the above table that by providing insulation you will need considerably less cooling in summer and less heating in winter.


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