Thermally Insulating Window With Aluminium Foil

[boblalux]

Our upstairs room is directly under the inclined roof in which an inclined window has been built.  Although there is a sunlight blind attached to the window (roll up - roll down), it is dark-coloured, and gets very hot during the day.  The room is now nearly 30°C!!! I get the impression that most of the heat comes through the window and heats up the blind, which radiates into the room.

If I fit a 'panel' of aluminium foil between the blind and the window (inside),  will this allow the room to cool down? 

Edited July 10, 2016 by boblalux

[CraigD]

Welcome to hypography, boblalux! :) Please feel free to start a topic in the introductions forum to tell us something about yourself.

 

If I fit a 'panel' of aluminium foil between the blind and the window (inside), will this allow the room to cool down?

I expect it would, as would a panel of white paper or fabric. :thumbs_up

 

It a pretty easy experiment, so I hope you’ve already hung the aluminum foil, and will have an experimental answer to the question by nightfall. Let us know your results.

[boblalux]

Welcome to hypography, boblalux! :) Please feel free to start a topic in the introductions forum to tell us something about yourself.

 

I expect it would, as would a panel of white paper or fabric. :thumbs_up

 

It a pretty easy experiment, so I hope you’ve already hung the aluminum foil, and will have an experimental answer to the question by nightfall. Let us know your results.

 

The temp. is now 31.6°C!  I think the problem must be elsewhere - eg bad insulation under the roof tiles, although I don't get the impression that the walls and ceiling are 30°!

The Al foil is quite hot on the inside; would it make any difference if the foil was fixed outside on the window, ie OVER the glass window?

Bob

[GAHD]

Run your hands along the walls(&ceiling), if they feel warm it's likely heat-transfer from the shingle. Ash shingles are particularly bad for heating up, just like ash roads. Painting them a bright red or yellow will help a lot in that instance(white even better, but it'll yellow and show dirt more rather quickly,) If you have central-air, check the baffles in your ducts to see if you can divert more cold air up to the top, which will then fall to lower floors(remember to re-set the baffles in winter for heating if your climate is like that.).

Also if you recently has roofing done, make sure the vents at the eaves and cap are installed correctly and unplugged: Airflow is important in ALL seasons.

 

Edited July 11, 2016 by GAHD

[CraigD]

The temp. is now 31.6°C! I think the problem must be elsewhere - eg bad insulation under the roof tiles, although I don't get the impression that the walls and ceiling are 30°!

The Al foil is quite hot on the inside; ...

I didn’t expect that.

 

A bit of websearching reveals some practical engineering knowledge: that high albedo – reflecting light – is not the only quality needed by a material to prevent heating by sunlight through a window. The material also needs to have a high heat emissivity, to emit absorbed visible light as infrared light, rather than reaching a high surface temperature, which heats the surrounding air via convection.

 

I found a chart titled “Spectral characteristics of building materials” at this energy utility trade website. It is about roof, not window shade material, but I think the principles apply to both.

 

Notice that while “polished aluminum foil” has a high albedo (“solar reflectance” in the diagram) of 0.85 to 0.95, its “far-infrared emissivity” is low, 0.05. This explains why aluminum foil gets hot, and perhaps does more to convectively heat the room than it does to cool it by reflecting incoming light.

 

... would it make any difference if the foil was fixed outside on the window, ie OVER the glass window?

Yes, I expect it would make a big difference, as the hot aluminum foil would then be heating air on the outside of the window, not the inside.

 

Also, glass is less transparent in the infrared spectrum than in the visible (source: commercial webpage “Measurement of Solar Transmittance through Plate Glass”), so much of the infrared light radiated by the foil will be kept on the side of the glass the foil is on.

 

Since it’s usually easier to hang stuff inside a window than outside, maybe you should keep your shield on the inside, but switch to a high albedo/high emissivity material. On the diagram, “white plaster” is highest for both of these – my guess is a piece of white sheetrock would be good. If you don’t have that handy, I’d try white paper.