Flat roof heat loss

I recently wanted to find out how much my poorly insulated flat roof was costing money-wise in terms of heat lost to the outside, and also how long it would take to pay back if I insulated it. This was mainly because the quotes I had from roofers for the recommended 120mm PIR board insulation was much more than other insulation alternatives, e.g. Rockwool, or a smaller depth of board insulation. Would upgrading to this board insulation pay for itself in a reasonable timescale?

I created a couple of calculators to help me with this:

Firstly, the roof heat loss cost calculator will tell you how much, (based on some very rough average numbers), your roof is costing you for a period of time. It doesn’t take into account temperature variations over the period of time specified, so bear that in mind.

Secondly, the insulation payback comparison calculator will compare two roof insulation options, and the price difference between them, and work out how long it would take for the better insulation to pay for itself compared to installing the poorer alternative.  Unlike the previous calculator, this gives a better estimate as it uses the concept of Heating Degree Days to model outdoor temperature changes through the seasons etc.

Note: Don’t rely on these for accurate numbers, they are will give very ball-park figures, because, as usual in real life, there are all sorts of details and nuances to consider.

Things you need to know to do the calculations:

The roof area that heat is lost through.

Hopefully you are able to work that out yourself.

The type of roof you have, or want.

This page (http://www.greenspec.co.uk/timber-flat-roof-insulation.php) gives a good overview of the types of flat roof construction and common materials. It also has a quick-reference table for typical U Values of insulation types

The U-value of your roof.

  • This is a number representing the rate that heat, i.e. energy, is lost through your roof (or other surface such as floors, walls, windows). It is also called thermal transmittance. It must take into account the different materials making up the layers of your roof, including insulation, and any thermal bridging elements, e.g. wooden joists in a cold-deck roof construction.
  • Fill in the “details of structure” in this simple calculator to give you an indication of the U Value of a roof: http://www.vesma.com/tutorial/uvalue01/uvalue01.htm.
  • Note, this assumes no joists, so can’t give you a good estimate for a cold-deck construction, but you can work out that soft wood at 6inches (150mm) thick has a U Value of around 0.77W/m2K, so at least it gives you a guide at if 25% of your roof area was joists, you could add up the total heat losses for the different areas to get a total heat loss and therefore cost.
  • See the “Thermal Properties” section on page 14 and 15 of this PDF (http://www.the-flat-roof.co.uk/Sect1_2.pdf) to see how you arrive at a U Value given the material’s thermal conductivity and taking into account the material’s depth.

Other configurable options

A couple of settings in the calculator are set at default values, which you can change if you know more accurate info. The default values will give results based on 1 year of English weather, and using a gas central heating system of 90% efficiency (if you’re boiler was made before about 2000, 80% might be more realistic, and if it was made before 1990, 70% might be more realistic!)

Concept of a Heating Degree Day, or HDD:

  • What is a “heating degree day”? http://www.degreedays.net/introduction
  • Heating degree day calculator for your area: http://www.degreedays.net
  • A value for my area (East of England) for the period March 2012 – March 2013 was 3043, when using a base temperature of 17°C (this is the temperature you want to keep inside temperature of the building at), so I have used that as the default in my calculator.

Cost of fuel:

  • Some typical costs per KWh can be found here: http://www.confusedaboutenergy.co.uk/index.php/domestic-fuels/fuel-prices
  • At the time of writing, natural gas, which is the most common fuel for domestic heating in the UK is on average 4.5pence per KWh, so I have used this as the default in the calculator. If you live in a different country, just ignore the pounds and pence figures and substitute your own currency. As long as is is decimal you are ok!