Building Design, Drafting & Energy Efficiency



What is a building envelope?

What is a building envelope?

This is a question that nobody asks. Frustratingly, they don’t ask it because they have never heard the term, ‘building envelope.’

This is a reflection on how little we have been educated in the field of building science over the last 20 years, and the main reason we are doing a poor job with energy efficiency. As building scientist Joe Lstiburek says, “If you don’t get this concept, nothing else really matters.”

Here’s a clue. The red line in the provided image represents the building envelope. The concept of addressing Water, Air, Vapour and Thermal applies to this line.

A building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to water, air, heat, light, and noise transfer.


What’s a heat load?

Unravelling the mystery of the heat load.

We are often told about building heat loads, simulations etc. but what does this mean? It sounds awfully complicated. Let’s get back to basics and see what’s happening.

The basic calculation for a heat load is:
q = U A ΔT
q = heat transfer (Watts)
U = overall heat transfer coefficient (U-value is the Conductance, which is the inverse of an R-value), 1/U-value=R-value
A = Area (m2)
ΔT = (Delta T is the temperature difference between inside and outside)

In other words,
Heat Transfer = Conductance x Area x Temperature difference.

When we do this calculation for the floor, walls & ceiling of a conditioned space and add them together we get the amount of energy in Watts needed to heat the space. The smaller the numbers, the less energy is required. (We have more control over the Conductance and Area than we do over the temperature difference).

In the attached image I have written the U A ΔT formula inside a box, and yes, there’s a reason for this. The box represents an airtight envelope. It’s no good if the air that we are trying to heat inside the house doesn’t stay inside the house. The purpose of the insulation is to keep the warm conditions inside the house but this is very difficult to do if the air constantly escapes. Try keeping your coffee warm with the lid off the thermos.

There you go, That’s a basic explanation. Of course there are factors that affect the temperature difference (Delta T) such as surface type, surface colour, absorptance, solar heat gain etc. but all of these end up as figures that are plugged into the basic U A ΔT formula.

Just remember U A ΔT in a box, and work on keeping the controllable figures small.



Is your house a leaky bucket?

Is your house a leaky bucket?
What would you do if you had a leaky bucket and the experts told you to fix it by making the bucket thicker?
Now imagine having a leaky house and being told to fix it by making the insulation thicker.
Here’s a crazy idea. How about we fix the holes first? Just something to ponder.
leaky bucket

Water, Air, Vapour, Thermal

Do you want an energy efficient house?

To ensure an energy efficient house you must deal with the following items in the order as listed

1. Water

2. Air

3. Vapour

4. Thermal (star rating)

Hang on! Why is thermal at number 4?

It is at number 4 because it won’t work unless you address 1, 2 & 3. 

1.If water gets into the structure via rain, flood or leaks, air and vapour will also certainly get in. Wet insulation doesn’t work. A wet structure causes health issues and structural damage. (Thermal doesn’t work).

2. If Air gets into the structure it carries dust, toxins, pollen, and MOISTURE which in turn causes health issues and structural damage. Air moving through insulation also dramatically decreases the performance of the insulation. Air leakage from the building can also equate to up to 60% of your heating/cooling bill. (Thermal doesn’t work).

3. If Vapour gets trapped in a structural assembly it can condense on an internal surface. Again, this MOISTURE causes health issues and structural damage. (Thermal doesn’t work).

So, when we talk about increasing the thermal performance of a building it is important to get our priorities right. Increasing from 6 stars to 7 or 8 stars won’t work unless we address items 1-3.

Increasing insulation values can actually make our moisture problems worse if we treat ‘Thermal’ in isolation and don’t address moisture & air management. At the international Builders’ Show I had a discussion with building scientist, Mark LaLiberte, from Construction Instruction . He said, “Treating Thermal in isolation is insane. You need to address Water, Air and Vapour first then thermal will follow nicely. If you focus on thermal without a good moisture and air managment strategy you’re asking for trouble.”

Window air infiltration

I have had a few questions lately regarding window air infiltration. This is an important component of energy efficiency but one that is often overlooked. Window manufacturers have their windows pressure tested at 75Pa. The result of the air infiltration test is listed on their window data as AI. The AI figure represents Litres per Second per square metre (l/s/m2).

As an example, a typical awning window and a typical double hung window might have the same thermal performance figures, (Uw and SHGC) but have wildly different air infiltration rates. Awning window AI = 0.3 l/s/m2 Double Hung AI = 4.2 l/s/m2 I have known of people who diligently selected windows with excellent thermal values only to have their curtains flapping (with the window shut) due to the high air infiltration…….not an energy efficient outcome.

LESSON! Make sure you consider air infiltration when selecting your next windows.

International Builders’ Show

As part of my continuing path of learning in the field of building science, I will be attending the International Builders’ Show in Las Vegas in February 2019.

In Australia we are still in our infancy when it comes to energy efficiency, so much so that we have not yet adequately addressed the effects of moisture management and indoor air quality. Moisture and air quality problems are both unintended consequences of insulating our buildings.

In Europe and North America, energy efficiency measures have been developed over decades, and over that time problems such as moisture and air quality have arisen and been dealt with. I an attending the International Builders’ show with an intent to learn more, to speak to industry experts and come home  more knowledgeable and capable of directing my clients adequately in the pursuit of building  energy efficient, durable and healthy buildings.

The show in Las Vegas is huge. It runs for 3 days and expects over 60,000 visitors. Click on the link below to check it out.




Window heat loss:

Windows are great, they serve to provide us with natural light and natural heating via solar heat gain. Windows also allow us to see outside and they  facilitate natural ventilation when operable.

As good as windows are they are really bad at one thing – ‘insulating.’

When we consider our thermal fabric, (floor, walls, ceiling) the weak point is where this fabric is compromised. Windows represent a wound in the thermal fabric.

The NCC minimum thermal requirement, (R-value) for external walls is *R2.8. The R-value of a typical aluminium framed, single glazed window is R0.15, (Uw6.5). This 18.7 times less thermally efficient than the wall! Even a typical aluminium framed, double glazed window at R0.25, (Uw3.95) is 11.2 times less thermally efficient than the wall.

Now that we recognise that windows are a major source of heat loss, what do we do about it?

When designing your house consider where the major heating load is, (typically in the living areas/ kitchen etc.) and ensure that the windows in these areas are thermally efficient in order to minimise heat loss. Double glazing is a starting point. Good thermally broken, double glazed units are the best choice to minimise heat flow between inside and outside. Drapes with pelmets also add insulation to windows, (makes a big difference on cold nights).

How do you know what a good thermally efficient window is? The important thing to look for when choosing a window is the U-value of the whole window. This figure appears as Uw-.-. The lower the U-value the better. Typical Values:

  • Uw6.50 = aluminium, single glazed.
  • Uw3.95 = aluminium double glazed.
  • Uw.2.3 = aluminium framed, thermally broken, double glazed.

Futher explanation:

The U-value is measured as W/m2K. Lets look at the aluminium framed single glazed unit as noted above.

Uw6.50 = 6.5Watts per 1m2 of window area per 1 degree of temperature difference, (temperature difference between inside and outside).

Now lets plug this U-value figure into a house with 30m2 of window area with a temperature difference of 12 degrees, (thermostat set to 21 degrees inside with an outside temperature of 9 degrees).

6.5 x 30m2 x 12 = 2730Watts of energy loss.

Putting this figure into perspective, 2730Watts or 2.73kW is equivalent to running 27 **Television sets. That’s a lot of energy.

Running the calculation again with the thermally broken window gets us down to 8 TV’s.

2.3 x 30m2 x 12 = 828 Watts of energy loss.

It’s not all bad news as of course windows also act to provide solar heat gain. This is heavily dependant on careful consideration when placing the windows. It’s important to get the orientation and shading right to maximize winter heat gain and summer shading to ensure a warm house in the winter and a cool house in the summer.

When you are looking for windows for your new home, or more importantly when you are at the design stage of your new home, have a careful look at where the windows are, which way they’re facing and address them appropriately. As a rule of thumb, north facing windows to living areas with well designed shading is optimal. Large expanses of south facing windows are primarily a heat loss issue.

Remember, the U-value is the measure of how much heat is transferred through the window. The lower the U-value the better the insulation properties of the  window – the better it is at keeping the heat or cold out. In all cases regardless of climate zone a window with good insulation properties will help to improve the comfort of your home.

Note: The latest version of our glazing calculation includes window heat loss data, (measured in Watts) for the total window area of the house. This data is on the Window Analysis page and has scenarios for temperature differences from 6 – 21 degrees.

*R-value based on NCC Climate Zones 6 & 7

**Television sets assumed at 100W.

Example data from spread sheet below. Figures based on 30m2 total window area and Uw2.3 window units.


Is your new house insulated?

Is your new house insulated? The answer should be, “of course it is.” Insulation is something that is well and truly covered in the design and documentation stage of your new house, so when you look at your building permit drawings and of course, your energy report there is reference to the amount and location of the insulation. Also, when you see the house being built there is evidence of the insulation arriving and being installed.

So, what’s the problem here?

The problem is not the specification of insulation but the ‘correct’ installation of it. The correct installation of insulation involves installing it without gaps between batts and ensuring that the entire thermal envelope is covered.

You would think this would be simple enough, especially considering it is a Building Code requirement. However, after observing a number of houses being constructed I have found evidence that leads me to believe that most houses are not insulated properly.

Here is a list of identified problems:

  • Wall insulation is squashed into place, (squashing the insulation reduces its potential insulation value).
  • Wall insulation is carelessly placed leaving large gaps between.
  • Wall insulation is NOT installed over windows where there are lintels.
  • Wall insulation is NOT installed at external wall corners and external junctions where internal walls meet the external wall. This problem is due to the wall wrap being placed prior to the insulation contractor appearing on site. Once the wall wrap is placed, these external junctions cannot be accesses for the installation of insulation.





Seal gaps & cracks?

One of the throw-away notes you will sometimes see on construction drawings is, “seal gaps & cracks.”

This simple statement is meant to cover a range of air infiltration/leakage issues in the building fabric. It has been condensed to this simple statement because it is assumed that these air infiltration issues are dealt with as part of ‘good construction’ practice. For this reason, the sealing of gaps and cracks is not part of house energy ratings, (thermal performance assessments).

The assumption that the sealing of gaps and cracks is dealt with as a matter of course is a major problem. It is a problem because most of the time it is NOT dealt with and most building practitioners and energy assessors are unaware of the basic requirements.

Ceiling insulation installed with 5% gaps can cause a 50% reduction in thermal effectiveness. Ineffective sealing of windows, doors, roof lights exhaust fans etc. can also lead to excessive air leakage. This  drastically reduces the efficacy of the building’s thermal envelope and causes a greater heating/cooling load than technically necessary, hence higher power bills.

As a part of our energy reports we include National Construction Code Part 3.12.3 – Building Sealing. This part of the code addresses all items of potential air leakage. By addressing air leakage and providing recommendations in report form there is less likelihood that this important component of energy efficiency is overlooked.  Properly dealing with ‘gaps & cracks’  during construction will ensure that all insulation and energy efficient measures perform to their full potential.

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