Multifoil insulation: warm in winter yet cool in summer


Too hot in summer?

In recent years, with the Building Regulations requiring loft conversions to be well insulated, freezing attic rooms are things of the past. However they can be still be draughty, and stifling hot in summer even with the windows open.

Time to cool it down...

Using multifoil insulation as part of the overall build-up can provide more benefits than just reducing heat loss. Being a vapour barrier it also provides airtightness (if it is correctly taped) – so no more draughts. Its shiny surface reflects 95% of radiated heat back into the room across an air gap next to it – a heat mirror in effect.

But how does the same shiny surface also keep things cool?

TLX Silver helps keep things cool tooPhysics coming up… A material that reflects 95% of heat radiation absorbs the other 5% - but then it only emits 5% of heat as radiation. That’s why these surfaces are known as low emissivity surfaces. So having a material with such a low emissivity surface as part of the insulation build-up means that the heating effect of the sun’s rays is partially blocked, since most of the transmission by infra-red radiation across the air gaps is prevented. The same principle is used in the internal construction of the product, with internal shiny layers reflecting heat back across the air spaces between the polyester fibres.

It’s all about reflectance

A polished aluminium surface has an emissivity of around 0.05 – in other words 95% of the heat is reflected, and surfaces metallised with aluminium are the best heat reflectors that can be made.

It’s very difficult to measure this, and in fact the relevant standard relating to the thermal performance of reflective insulation productsHow a Silver Multifoil works BS EN 16012:2012 notes that measurement errors increase significantly below 0.05, and that even if the average of the measurements made does end up less than 0.05, the value should only be declared as 0.05.

So why do some products quote lower surface emissivities – 0.02 or 0.03 – than this? Well, it will boost the calculated thermal resistance (R value) of the air gap used in the U-value calculation, which will make the product appear to be better.

Science bit follows… (definitely optional!)

The method used to measure emissivity shines a heat beam at the shiny surface and measures the amount reflected.

The heat and light (really the total EM energy) radiated from a surface (per unit area per second) is given by the Stefan-Boltzmann law: Power per unit area =  εsT4 where ε is the emissivity and s is the Stefan-Boltzmann constant

Note that this increases with the fourth power of the temperature in degrees Kelvin!

So a tiny temperature shift causes a big increase in the amount of heat radiated from the surface, which, if you are not aware of it, you would think was due to a decrease in emissivity. But since the method relies on shining infra-red radiation at the shiny surface, this immediately causes the surface temperature to heat up…

Also the calibration points (made using a very shiny and a very black surface) are ε=0.02 and ε=0.96 with a straight line graph between the two. The inevitable drift of the low setpoint won’t affect measurements somewhere in the middle too much, e.g. ε=0.5, but will have a big effect on measurements <0.05.

So the experimental method is inherently difficult to control – and that’s not even considering the statistical methods for considering how to get the most accurate value! Experimental scientists come equipped with a big pinch of salt to use when they see ε values <0.05!

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