Double Glazing Or Not

Double glazingI posted this on the Alternative Technology Association forum ( ):

“We are renovating a house. According to our ‘first rate’ energy report we can use single clear glazing. Doing so means that we will use 41000 mega joules per year for heating and cooling.

We will use natural ventilation to cool the house and hydronic heating (radiators) to heat the house. The energy rating gives our heating component of energy use at 30540 mj/annum. At 1.8c per megajoule we are looking at $550 per year to heat the house (no cost to cool)

If we use double glazing throughout then our heating consumption changes to 24170mj/annum. This translates to $435.

This is an annual heating saving of $115

The single glazing option is $15941 + gst
The double glazing option is $20311 + gst

A difference of $4370 + gst or 38years of $115 to pay off?!

Have I miscalculated?”

I received many replies generally agreeing with my calculation but recommending double glazing never-the-less. The arguments put forward were reducing CO2 emmissions, better sound insulation, increased resale value and reduced temperature fluctuations internally. These were all valid to some extent but not what drove us to choose double glazing. The reason we went for it is best articulated by ‘Phil A’ who wrote:

“If you go with the double glazing you will have a really good set up.
Imagine the place in ten years time and power has doubled again. You will be glad you did. The place will even “feel” better with double glazing, being warmer and quieter.
Hydronic under floor heating over insulation and double glazing are three things I will never be without again. Magic.”

A nebulous sense that we will have ‘a really good set up’ in ten years time rather than a ‘we could have afforded double glazing, I wonder if we should have’ in ten years time.


Drawing Drawings So Builders Can Build


Now that we had received our planning permit it was time to draw the building so that any builder could understand how it is put together, with what materials and to what standard. I had jumped the gun and started doing this while we were waiting for planning. I did this because I was confident enough that we would get our town planning permit without to much trouble. This would speed up the process. During this faze of the work we employed a structural engineer and an energy rating consultant. Engineering is needed so that the building stands up and it is required as part of obtaining the building permit. The energy rating, which here in Australia is starting to catch up to the rest of the world (i’ve worked in the UK and Singapore), ensures that the proposed house maintains its temperature efficiently – generally through good insulation and a favourable orientation. When I finished drawing everything up I decided to send the drawings back to the costing expert. When we sent our sketch design in to him the cost was about $350K including builders margin and GST. I knew when I was drawing this house up that I was adding bits in here and there but I hoped when we tendered the price would be about $350K because the market here was going down and this means builders become more competative as less work is available. I was a bit shocked when the estimator’s cost came in at $450ish K. I had added some items but I can’t imagine they would have added up to more than 30K. There was also more steel than was originally thought and the slab was more complicated due to the ground condition, but again this would only account for $10K at best. I believe the original sketch design estimate was too optimistically priced – hopefully this newer estimate was high. Nevertheless it was very useful to get the design recosted because it is important to get as much of the documentation right before going to builders. So, having seen the new estimate, I set about redrawing again. This time my wife and I were as merciless as we could be but in the end we felt we could only save about 50K with the changes we were making. If we wanted to reduce the costs further it would mean a substancial redesign involving new town planning. The 50K we hoped to save came from removing a fireplace I’d added between the meals and sitting room – 19K, minimising joinery – maybe 10K, painting floorboards rather than stripping them back, staining and oiling them – maybe $4K, reducing the amount of lighting – $2K and various other changes. We also thought we would do the project as owner builders to get rid of the%10 – %15 percent margin on some items. (more about tendering in the next post). Once we had made all these changes I prepared the cocumentation to go out to builders along with a contract for some of the work. Other contracts would be sent out by us directly to trades for some items of work. Finally we sent the drawings out.