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.


See Insulation for similar Posts

For Posts about Green Building see Sustainability


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.

See Insulation for similar Posts

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.

See Insulation for similar Posts

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.


See Insulation for similar Posts

For Posts about Green Building see Sustainability

Rendered Foam Walls

This addendum was added to an original Post from 2014 as there has been a lot of publicity recently (late February 2019) about foam panels and certification has been removed from certain types of panels.

A particular issue for apartment blocks has been related to high speed spread across the surface of the panels to other flats.

On a standard house the render should protect the insulation from external flames. (If flames penetrate the plasterboard, from the inside, it is likely that the occupants will either have already evacuated, or be dead before the insulation ignites) n

Nevertheless you should review whether the potential risks from foam panels are acceptable to you.

Original Article

Rendered Foam walls are becoming much more common, particularly in the upper floor of 2 storey homes. They offer a real advantage in situations where it would be difficult to provide adequate suppport for a heavy brick wall (For example when the upper floor needs to be set back from the ground floor)

If you are worried about strength you need to be aware that the real strength of the house is in the frame. (see: House Construction – The Frame)


  • The Foam boards, which are manufactured with an external mesh face, are fixed to the frame with special galvanised screws that incorporate spreader washers.
  • Joints are sealed with a polyurethane foam and have mesh jointing tape.
  • External corners are reinforced with metal strips.
  • A minimum of 5mm of  acrylic render  is applied, normally in a three layer system.

Polystyrene Foam

There are 2 different types of foam used in this construction method:

  • Expanded polystyrene( EPS) – Good thermal performance but limited impact resistance/structural strength.
  • Extruded polystyrene (XPS) – Similar thermal performance and looks similar  but the production method is different which results in increased impact resistance and structural strength. Higher cost

Insulation values for the various board thicknesses are:

  • 50mm    – R 1.2
  • 75mm    – R 1.8
  • 100mm – R 2.4

Final Thoughts

Although there are some advantages in this system it does require careful detailing and construction otherwise leakage can occur damaging your house.

The advantage of masonry on the lower part of the house is that it is less likely to be damaged by the bumps and bangs of daily life. Once the wall is above head height damage becomes less of an issue and the rendered foam board should be fine.

I’d prefer XPS to EPS.

Although the insulation values are good the builder will most likely want to save the cost of the insulation batts in the frame. If you ask for the wall to include insulation batts you will have an exceptionally well insulated wall at very little extra cost.


For similar posts see Insulation

For more about house design see Choosing a House . . . A new E-book for only $4 to help plan your new house


Don’t Forget Curtains!

It’s well known a lot of heat is lost through windows.

With single glazing the Thermal Resistance, ‘R’ Value  is 0.16.

Although they aren’t considered in energy calculations (see this link: Energy Rating) curtains can significantly decrease the heat loss from a window.

They also improve the feeling of comfort.

Insulation Value

With different materials its not possible to give a definitive value for the insulation value of curtains  however there are some indicative values in this post.

An effective curtain can increase the ‘R’ value of the window to between 0.3 – 0.5.

Even with a double glazed window curtains will typically further reduce the heat loss. (‘R’ value increased from around 0.33 to around 0.6).

Effective Curtains

To be fully effective curtains must:

  • Provide a ‘seal’ around the window to stop air movement at:
    • Top – Pelmet
    • Bottom – Overlap window sill by at least 300mm, or down to the floor
    • Sides – Overlap edges of window by at least 300mm.
  • Use heavy close weave material, preferably lined.


What Does The Energy Rating Mean?

The ratings are calculated by a NatHERS (Nationwide Housing Energy Rating Scheme) computer simulation of an individual house design to estimate the thermal comfort.

The calculations are based on 69 different ‘Climate Zones” and allows comparison of different properties in the same climate zone.

The following table shows the Energy Load (heating,cooling, lighting and hot water) for various capital cities for each of the star rating band.

  6 Star 7 Star 8 Star 9 Star 10 Star
Adelaide 27 19 13 6 1
Brisbane 12 10 7 5 3
Hobart 43 31 20 9 0
Melbourne 32 23 15 7 1
Perth 19 14 9 5 1
Sydney 11 8 6 4 2

The values are in kilo watt hours/m2/annum.

I haven’t shown less than 6 stars as that is the minimum for new houses.

As you can see if you go all the way to 10 stars the energy load will be virtually nil.


If you are building a 200m2 house in Melbourne at the minimum 6 Star Rating your annual energy load will be:

200 x 32 = 6,400 kilo watt hours

By increasing the rating to 8 star the calculation becomes:

200 x 15 = 3,000 kilo watt hours

a significant reduction of more than 50%!