An Insight Into Insulation
We all know what insulation is and those who have homes that are not insulated, tend to find the winters extremely cold and the summers unbearably hot. As told by our year nine science teacher, hot air always moves towards cold air – so in winter, the warm air will try to escape out of your home and in summer it will weasel its way in. The indoor comfort levels become so poor that you have to crank up either the heater or aircon, resulting in your energy bills going through the roof.
So how do we make the most of our insulation?
Insulation is the fourth layer in our wall control system and works by slowing conductive heat-flow through your building envelope. Bulk insulation – which is the most commonly used insulation material in Australia, has millions of little fibres that are poor heat conductors. The criss-cross pattern forms tiny air cells that slow down the transfer of heat into your home.
The effectiveness of the insulation material is measured by its R-Value, which is determined by the insulation’s level of resistance to heat flow as well as its thermal resistance. The higher the value, the better the thermal resistance a material has. This rating system is commonly used in Australia, however in Passive House we consider the insulation’s U-Value. This describes the quantity of heat that passes through one square meter of a building component in one second. The U-Value is dependent on the thickness and thermal conductivity of the material chosen, meaning well-insulated building components will have a low U-Value.
The most important part of insulation is making sure it’s continuous. We want to make sure we have no gaps in our insulation, no sections missed, with insulation batts fitted nice and snug, and not overly compressed. It means having the insulation in your wall meet the insulation in your ceiling.
Four main areas must be considered when insulating to Australian Passive House standards:
Exterior Walls
In a Passive House, you will generally find a thicker wall system that accommodates for more insulation. You will often find studs spaced further apart, meaning more room for insulation and less timber. At Passive House standard, we look to achieve a U-Value of less than 0.15W(m2k), which roughly equates to a 240-300mm wall assembly. When compared to a standard house, you will find that they usually have a U-Value of 0.28W(m2k), which would equate to a 120-160mm wall assembly. This is why we should be aiming to have thicker external studs on all projects so we can beef up the amount of insulation on all external walls.Roof
A roof should perform just as well as a wall. Again, we want to achieve a U-Value of less than 0.15W(m2k). We usually have more space in our roof compared to a wall, which can make it easier to create a well-insulated assembly. As previously mentioned, it is extremely important that the insulation from your wall continues all the way up and connects to the roof system, as insulation should be continuous with no gaps!Floors
20% of our heating is lost through our floors and for some reason most people don’t consider getting insulation in their slab or under the subfloor. Our floors must achieve a U-Value less than 0.25W(m2k), equating to roughly 150-300mm of insulation, which will act as a thermal bridge layer to stop any heat lost between the slab and the cold ground. XPS insulation can be installed in the slab, at the bottom of beams and around the external edges of the slab to achieve this.Windows
We have spoken about windows in detail in a previous post (which you can read here), but you must also make sure that these perform as well as your wall assembly. The still air/gas acts as insulation between the triple glazed units, which helps to produce a lower U-Value.
We know a Passive House demands that you build to a higher level of construction standard. Although those mentioned above are the obvious areas, extra care needs to be taken when installing insulation. There are also many weak spots that we need to consider when installing insulation. These may be areas that are hard to get to or we forget about.
A commonplace that lacks insulation is the opening around windows. To allow for easier installation, this opening is always bigger than the window itself. Most builders will leave the cavity around the window and use only the architrave to cover up the gap. However, best practice would see spray foam insulation installed around the window to cover any gaps and prevent air from getting through as well as insulating around the window. Window lintels are another area that rarely gets insulated. Some lintels will only protrude 45mm into the stud, leaving a 45mm gap. This is too small of a space to install an insulation batt, however, some XPS foam can be installed over the lintel to give it a thermal break and provide some form of insulation.
We still commonly see cut-outs in the insulation for downlights and power points even though this is no longer needed. In the past this was done due to the use of halogen lights, which would expel an excessive amount of heat. If you had anything next to the light, there was a chance it would catch fire. There were regulations that required cans to be installed over the recessed light and insulation could not be installed around the light. These days, downlights come IC4 rated, meaning recessed light fittings can be safely installed without minimum clearance for normally flammable building elements. This allows for a continuous run of insulation without having to leave space for downlights to be installed. One area that still requires a cut out is your manhole hatch. A simple way to effectively insulate this area is to cut an insulation batt that sits above the manhole to slip in or attach some XPS foam to your manhole that won't get lost!
Second storey floor joists can also be forgotten, especially where the joist meets the external wall, it is common to see weak spots when looking under a thermal camera in these areas. You need to make sure insulation is pushed into those tight spots or placing insulation before cladding being installed. By thinking ahead, it allows us to get to all areas that will not be accessible later down the track.
In general, we want to make sure we insulate every corner. It’s typical for insulation to be installed a day or two before plaster is hung. This means our external walls already have their membrane and cladding on with no access to our corner studs and wall junction studs, resulting in multiple spots with no insulation. A simple way to solve this is to have your carpenters carry around a bag of insulation and install this once the frame is built and before bracing ply is installed.
Most of the requirements surrounding insulation in a Passive House are not expensive or difficult to execute. With a slightly thicker wall assembly and using the correct insulation products, this can be easily achieved. We are already installing insulation in our homes, so why not make sure it is installed correctly. Those little bits of effort go a long way in making sure the client has a high-performing home and their energy bills are dramatically reduced.