Category: Ducts

Replacing ceiling insulation
Our current ceiling insulation is a patchwork of old solutions. Today we had much of the old insulation removed, a general cleanup, and the ductwork for the evaporative cooler removed. We will have new insulation installed soon after we install the HVAC system next week.

State of the current ceiling insulation

Our house has an original part about 70 years old, and an addition which was constructed around 30 years ago. The insulation in the addition is polyester batts around 180mm. The insulation in the original roof is a mixture of very thin degraded batts and some blown in insulation.

Image of the house addition ceiling before cleaning

Removing the old insulation

The team arrived this morning to remove the old insulation, clean the roof, and remove the old evaporative cooling ducts. They had a hose which they threaded into the roof space and essentially hoovered out the dusty old insulation. They also removed by hand the old very thin batts and the ducts.

We decided to keep the polyester batts in the addition as they in good condition even though they are a bit dusty.

Roof space in addition after duct removal

Roof space in original home after removing ducts and all insulation

Calculating the improvement in R value and performance

In order to calculate the improvement we expect, I first want to calculate what the current insulation performance is. I will do this only in the addition where the state of the insulation is better understood. This is also the part of the house we use the most (and is very cold in winter).

The addition has polyester 180mm batts. These have an R value of 3.0¹. In order to calculate the actual R value for the ceiling I have to take into account the joists which are wood beams. These have an R value of about 0.9, and they act as thermal bridges, carrying heat into the roof more easily than the areas which are covered with insulation. I then have to work out how much of the area of the ceiling is covered by insulation, and how much is joists² . Joists are typically 140 mm deep, 45 mm wide and have a span of around 600 mm. A rough approximation is that between 8% and 12% of the roof space is covered by joists. I will assume as an approximation that this is 10% and then setting

$R_b = \text{batt R}, R_j = \text{joist R}, f_b = \text{fraction batt}, f_j = \text{fraction joist}$

$\frac{1}{R_{total}}=\frac{f_b}{R_b}+\frac{f_j}{R_j}$

Using the values above gives an R value for our ceiling of 2.43.

Adding more realistic assumptions about ceiling insulation

The ceiling insulation is missing in fairly large parts of the ceiling. For instance where there were vents and ducts, down lights, extractors. There are also just gaps.

To make a more realistic estimate of what the current performance might be I’m going to add another term for areas with no joist and no batt. These have only the plasterboard which has an R value of a little less than .1 and I assume that 10% of the ceiling is uncovered with batts.

The model is now that the fraction of batts is 80%, 10% is joists, and 10% is only plasterboard.

Then with $R_p =0.1$ for plasterboard R and $f_p=0.1$ the fraction of plasterboard the formula for calculating the overall R value is :

$\frac{1}{R_{total}}=\frac{f_b}{R_b}+\frac{f_j}{R_j}+\frac{f_p}{R_p}$

This gives a total R value of only 0.725 – ouch! Lots of little thermal bridges really kill the performance of ceiling insulation.

Modelling the improved performance of ceiling insulation

The key thing to calculate for the insulation performance is how well it does at preventing heat flow. To calculate the heat flow per unit of area Q from a warm space to a cold space through insulation which resists the movement of heat with an R value we need to calculate:

$Q = \frac{\Delta T}{R}$

where $\Delta T$ is the tempearture difference between the hot and cold area

We can use this to calculate the heat flow through our ceiling. Assuming the temperature difference between the inside of the ceiling and the roof space is 10 degrees C (or K) the heat loss for a total area a is given by:

$Q*a = \frac{a \Delta T}{R}$

Using an area of 50 sqm for the ceiling area of our lounge/kitchen and the R value of 0.725 we calculated for the current state gives a heat flow of 689 W from our lounge into the roof. No wonder it is so cold.

We can look at how improvements in the R value will change this heat flow.

R Value Heat Flow out of lounge
0.725 689 W
1 500 W
2 250 W
3 166 W
4 125 W
5 100 W
6 83 W
7 71 W
8 62 W
Heat flow from our lounge for various values of effective R value for insulation. Temp difference = 10 degrees.

There are obviously huge gains in getting from R below 1 to R = 4, and doubling the thickness of insulation from R=4 to R=8 only reduces the remaining heat loss by a factor of two at a very high cost. There is an exponential at work here.

Resources
1. A good resource on insulation is ICANZ ceiling insulation guide ↩︎
2. See Nathers: Thermal bridging information
  also abcb – Thermal bridging case studies ↩︎
April 9, 2026
Getting ready to replace ducted gas heating

We need to replace our gas heating system

About five months about our ducted gas heating system broke down. This was not the first time it has done so as the unit is now 30 years old. We called a technician to look at it, and he determined one of two components had failed. The problem was, that the manufacturer no longer supplies parts for this system and we need to replace the gas heater.

No going back

We had been considering saying Goodbye gas and had been looking at what would be involved both price and renovation. The only way to repair the heating system was to replace it with a newer model. This was the push which got us to start the journey to electrification.

We have gone in steps, first installing installing solar and a battery and then determining what heating to install. We have a single story home, which has around 200m2 of floorspace. This gas heating ducts run under the floor, and our first consideration was to re-use the ducting, or at least continue to heat with ducts running under the floor.

Underfloor ducting ruled out

We discussed our options with a few installers. Because electric ducted heating needs larger ducts, we would have to replace all the underfloor ducts. There is not easy access to our underfloor space, which is quite tight and this would have been a lot of work.

We have a large roof space, and there is plenty of room for the HVAC unit to go there and run heating/cooling ducts to all the rooms. We have vents in the ceiling and ducts from the evaporative cooling at the moment. Unfortunately, the vents are in the wrong places in the ceiling for heating so, and we will also need to use different ducting.

Placement of heating ducts

The current evaporative cooling vents are away from the windows, towards the centre of the home. Evaporative cooling blows moist air through the home, and we open the windows to allow the air to escape. With heating, we want to place the vents near the walls and windows. The return air is in the middle of the house, so the warm air is drawn to the return.

We will use an 18kW Panasonic HVAC system and have 4 zones in the home with dampers in the ducts to allow the zoning. We will place temperature sensors in each of the zones, and the overall control will be run by an Airtouch 5 controller.

Time to replace gas, preparation and arrival of winter

We will install the HVAC system next week. In advance of the installation, we removed much of the old insulation and cleaned the roof space. The reduction of the insulation and the lack of any heating means the house is very cold this week while we are waiting for the installation next week.

Goodbye old gas heater!

April 11, 2026