Window film latest

[lizzie]

5 minutes ago, JSHarris said:

On the topic of overhangs, because "normal" glass has a high emissivity, it will absorb a lot of heat when exposed to long wavelength IR from the sun and get very hot.  This means that glass external sunscreens can work very well - they will absorb the heat from the sun but let the light through, so as long as the external sunscreen has a gap between it and the house it will work just as well as any other sort of external shade.

 

Glass canopies are an option, perhaps.  Not massively expensive, won't reduce light levels, tend to look OK on a fairly contemporary house design and can be easy to fit.

Thanks for that Jeremy, an avenue to explore?

[Dreadnaught]

20 minutes ago, JSHarris said:

because "normal" glass has a high emissivity, it will absorb a lot of heat when exposed to long wavelength IR from the sun

 

And if they were blue-tinted glass, they could work even better. The 150 Cheapside building in London got there first…

 

150 

Edited August 4, 2018 by Dreadnaught

[Jeremy Harris]

The new building that was built at Porton as a part of the rationalisation programme I managed in my last post, used a mix of glass and metal slat brise soleil screening.  there was a lot of glazing on one office elevation and the screening helped a great deal to reduce the cooling loads.  It's government policy to avoid the need for comfort cooling in all new buildings, as well as reduce the heating demand, which is one reason this was designed in. 

 

Clear glass was used for the canopies, and that seemed to work very well.  IIRC, it had a very faint greenish tinge, probably just from the thickness and size of some of the glass sheets used (they were very large).

 

 

[laurenco]

Argh, we are getting privacy film on our Velfac triple glazed windows. How do i know if they're laminated or toughened? My window film people haven't mentioned anything about it breaking the glass (nor asked what type of glass is it). The film company are Smashgard Window Films Limited https://alternativetonetcurtains.co.uk/window-films-gallery/

[Ed Davies]

On 04/08/2018 at 09:45, JSHarris said:

when exposed to long wavelength IR from the sun

The sun emits very little long-wave IR (longer than, say, 5 µm compared with its short-wave IR, visible and UV output). Of the little it does put out, most is absorbed by the atmosphere. The long-wave IR reaching a window will come from the local atmosphere and surrounding objects. This has implications for positioning of sunscreens: e.g., a glass canopy might only protect from some of the LW radiation from the sky but not that from the parts of the atmosphere at lower angles and nearby houses, trees, etc.

[A_L]

10 hours ago, laurenco said:

Argh, we are getting privacy film on our Velfac triple glazed windows. How do i know if they're laminated or toughened? My window film people haven't mentioned anything about it breaking the glass (nor asked what type of glass is it). The film company are Smashgard Window Films Limited https://alternativetonetcurtains.co.uk/window-films-gallery/

 

Laminated glass will be etched BS EN 14449, toughened BS EN 12150

[laurenco]

great, thanks A_L 

[Jeremy Harris]

2 hours ago, Ed Davies said:

The sun emits very little long-wave IR (longer than, say, 5 µm compared with its short-wave IR, visible and UV output). Of the little it does put out, most is absorbed by the atmosphere. The long-wave IR reaching a window will come from the local atmosphere and surrounding objects. This has implications for positioning of sunscreens: e.g., a glass canopy might only protect from some of the LW radiation from the sky but not that from the parts of the atmosphere at lower angles and nearby houses, trees, etc.

 

 

Good point, that was sloppy of me.  As far as very long wavelength IR we are talking about re-radiation from objects all around that have been heated the sun, in the main.  However, glass does still absorb IR, in the case of some types of glass it can absorb a lot, and thicker glass, of the type used in canopies, tends to absorb a fair bit.  The solar canopies we used had a slight greenish tinge to them and were, I suspect, made from glass that absorbed pretty much everything below the visible red wavelength, and maybe even a bit of that end of the spectrum, given the colour tinge..

[Ed Davies]

19 minutes ago, JSHarris said:

and were, I suspect, made from glass that absorbed pretty much everything below the visible red wavelength,

Yes, quite plausible. The glass on solar thermal collectors is usually low-iron to make the best use of the available short-wavelength IR in the solar spectrum so perhaps your canopies were just thick ordinary glass or perhaps “high-iron” if that's possible.

 

AIUI the sort of solar glass typically specified in places like California is designed to reflect short-wave IR but I'm not at all clear what these add-on solar films try to do.

[Jeremy Harris]

I've been looking around at the absorption spectra of glass, and it seems to vary a great deal.  Quite a lot of "normal" glass seems to roll off quite sharply above about 3µ wavelength, from what I've been quickly been able to find out.

 

Emissivity is the other interesting point.  I took some thermal images using my relatively cheap thermal imaging camera, which is calibrated for a "standard" emissivity of around 0.9, I believe.  When we were looking at external IR reflective film, I had two samples stuck to our south-facing glazing, and then tried to take photos and thermal images of them for comparison, during bright, sunny, weather:

 

 

This is an external view of the front glazing, with a patch of the darker shade of 3M IR reflective film on the left and the lighter IR reflective film on the right (the film patches are stuck to the side panes, either side of the door and just above the level of the door handle).  It  seems  that the camera can read right through the triple glazing to record a reasonably accurate house internal temperature of 21 deg C (the house stays at 21 deg C pretty much all year around), which is curious.  The hottest point it found looks to be close to the dark grey alloy external glazing frame cladding (the "cool" object to the left is a shiny stainless steel mailbox, with a low emissivity), which is what I'd expect, as the sun has definitely warmed up the alloy frame.

 

 

 

 

This external view had to be taken at an angle, because the reflections from a head-on shot were dominated by me holding the camera!  However, it seems that the patches of reflective film are significantly more reflective at even optical wavelengths than the glazing alone.  The best illustration of this is with the patch nearest the camera, where there is a sharp contrast between the reflected image of my car st the junction of the film and the glass, as well as the film reflecting the image of the trees opposite quite clearly.