Solar Reflective Glazing

[pauldoc]

I have some large south facing glazing on my project. I have got a certain amount of shading incorporated, but wanted to get some feed back from anyone who has specified Solar reflective glazing. Has it made a difference and how was it cost wise as an 'upgrade'?

[Jeremy Harris]

We're having external reflective film fitted this afternoon, to combat the excessive solar gain we get through our 3G glazing.  Not cheap, and it may well have been cheaper to specify reflective glass in the first place.

I should be able to report on how it works in a week or two, but can say that, based on the samples we have had on the glazing for a few weeks now, it does massively reduce the level of radiated heat through the glass.

[pauldoc]

thanks Jeremy. Does it have a slight colour tint?

[Mikey_1980]

Hi Jeremy, which option did you go for in the end, Sentinel Plus Stainless Steel or 3M Prestige?

 

[Jeremy Harris]

56 minutes ago, Mikey_1980 said:

Hi Jeremy, which option did you go for in the end, Sentinel Plus Stainless Steel or 3M Prestige?

 

Both! 

We're putting the stainless steel finish stuff on the big front gable, primarily because it gives a lot more privacy than the other stuff, yet doesn't look like a mirror-finish office block.  My wife was concerned that anyone walking down the lane (we get maybe two people a day walk down it.................) might see her coming down the stairs in her dressing gown.  The reflective finish does provide a fairly high degree of privacy, much more than the pale grey tint of the other stuff, and performs similarly in terms of reflecting heat away on the outer face of the glazing.

On the East-facing bedroom window, the one where we had over-heating problems from both solar gain and the proximity of the old thermal store in the services room, which leaked a great deal of heat (in passive house terms), is getting the 3M Prestige, as privacy isn't an issue there and it's fairly non-reflective, so unlikely to cause a sun reflection problem for the neighbour, although they are a fair distance away and much higher up the hillside.

[Gone West]

I fitted a Fakro triple glazed roof window six years ago which had reflective glazing with a 'g' value of 32%. It does reduce the light level but that is compensated for partly because it's at 42 degrees.

[pauldoc]

Another question along the same lines

Is there a difference in what type of glazing causes more solar gain...i.e does double or triple glazing cause more solar gain?

I ask because I spoke to a manufacturer and they suggested triple glazing causes more solar gain so to specify only double glazing on the elevations that will suffer with it the most 

[Jeremy Harris]

1 hour ago, pauldoc said:

Another question along the same lines

Is there a difference in what type of glazing causes more solar gain...i.e does double or triple glazing cause more solar gain?

I ask because I spoke to a manufacturer and they suggested triple glazing causes more solar gain so to specify only double glazing on the elevations that will suffer with it the most 

There's slightly less solar gain through most triple glazing, because there are three panes of glass that will absorb more radiant heat than two panes of glass in double glazing.  Also, there are usually two low e coated surfaces inside triple glazing, arranged to reflect radiated heat back into the house, so there is a lot less radiated heat loss.

The fact that triple glazing makes a significant reduction to heat loss, together with the fact that it's usually fitted to houses that are better insulated and sealed than most, means that overall the slightly reduced solar gain isn't that significant when compared with the much lower overall heat loss of the house, so there is almost always a net solar gain, I think.

One thing we noted was that the inside pane of our triple glazing (before the heat reflective external film was applied) would get pretty warm - often up to 35 to 40 deg C on a sunny day.  With the heat reflective film the inner pane is barely warmer than the room temperature.  My view is that this solar heating of the inner pane of glass contributes quite a lot to the heat conducted and convected into the air in the house, as the warm window panes are acting as big convection heaters, in effect. 

Since having the film fitted a couple of weeks ago our cooling systems haven't come on, whereas before they would come on most days by mid afternoon.  To me that's a pretty good indicator that it's working very well.

[pauldoc]

Jeremy

One thing the manufacturer said was that with triple glazed units the sun entering it is in effect being magnified twice by the additional pane, increasing the heat, which is what most people want to increase solar gain on purpose to have that extra free heat. And sounds like what was happen to your units with the high temp of the inner pane. They also said that reflective glazing can cause the house to be too cold as it cuts too much solar gain out and that double glazing is a good happy medium as the solar gain is less than triple glaze.

Whether this is true or not I don't know. They recommended a mix of glazing units, with triple to the rear northern elevations and double to the south, which has large expanses of glass. I am not building to especially passive standards, just a decent level. I am more concerned about the solar gain at this point and am worried the house will be unbearably hot.

They have quoted solar reflective glazing approx. 20% higher than standard glass, not sure if that is to the overall cost or just the units I didn't check. Sounds a lot if its to the overall price.

[Temp]

On ‎7‎/‎7‎/‎2016 at 12:22, JSHarris said:

My wife was concerned that anyone walking down the lane (we get maybe two people a day walk down it.................) might see her coming down the stairs in her dressing gown.

Tell her not the wear it :-)

[ragg987]

1 hour ago, pauldoc said:

One thing the manufacturer said was that with triple glazed units the sun entering it is in effect being magnified twice by the additional pane

Does not make sense to me. The amount of solar energy that falls on the outside of the pane will be fixed based on surface area, angle, cloud, shade etc conditions. The amount that subsequently enters into the room will depend on other factors e.g. emmissivity, reflectivity, U-value. The factors that affect how warm your room will become are separate again and include external temperature, internal temperature, ventilation, U-value and probably a host of others.

Over-heating is not likely to be black and white. In cold winter months, autumn and spring you probably welcome the solar gain, but in summer you do not. So perhaps a solution would be external shading (which you already mention - leafy trees or bushes might work well) or increase ventilation when at risk of overheating (open the windows!).

I have only seen reflective windows in commercial buildings. Mobile and wi-fi signals both suffer. Only you can decide if this is a good thing or not!

Question - is yours a generic concern about solar gain or is there anything to support? Might be useful to get a more objective view, e.g. PHPP provides an estimate of days of over-heating.

[pauldoc]

11 minutes ago, ragg987 said:

Does not make sense to me. The amount of solar energy that falls on the outside of the pane will be fixed based on surface area, angle, cloud, shade etc conditions. The amount that subsequently enters into the room will depend on other factors e.g. emmissivity, reflectivity, U-value. The factors that affect how warm your room will become are separate again and include external temperature, internal temperature, ventilation, U-value and probably a host of others.

Over-heating is not likely to be black and white. In cold winter months, autumn and spring you probably welcome the solar gain, but in summer you do not. So perhaps a solution would be external shading (which you already mention - leafy trees or bushes might work well) or increase ventilation when at risk of overheating (open the windows!).

I have only seen reflective windows in commercial buildings. Mobile and wi-fi signals both suffer. Only you can decide if this is a good thing or not!

Question - is yours a generic concern about solar gain or is there anything to support? Might be useful to get a more objective view, e.g. PHPP provides an estimate of days of over-heating.

I guess its a generic concern. I was initially worried about it because of the extensive glazing we have on the south elevation. basically its double height glazed gable- entrance/hall and bedroom upstairs. The roof over hangs by around a metre.

When I spoke to the window manufacturer about solar reflective they suggested double glazing will cut the heat down slightly, and in the winter when you want heat from the sun the reflective glazing would make the house too cold. I think as you suggest I have to look into other ways of shading. We do have trees, but I think we wll need blinds of some kind too

[ragg987]

Personally I would be reluctant to spend thousands of pounds on a solution unless I KNEW there was a problem. Random google search shows this calculator (not used and no affiliation). http://www.cchrc.org/solar-heat-gain-through-windows-calculator

[pauldoc]

3 minutes ago, ragg987 said:

Personally I would be reluctant to spend thousands of pounds on a solution unless I KNEW there was a problem. Random google search shows this calculator (not used and no affiliation). http://www.cchrc.org/solar-heat-gain-through-windows-calculator

that's my thoughts to! Thanks i'll take a look

[willbish]

On 15/07/2016 at 15:57, ragg987 said:

Personally I would be reluctant to spend thousands of pounds on a solution unless I KNEW there was a problem. Random google search shows this calculator (not used and no affiliation). http://www.cchrc.org/solar-heat-gain-through-windows-calculator

 

Would anyone know how to alter the Azimuth on this spreadsheet to give figures for windows other than true south?

[ragg987]

Looks like you do not directly enter the window orientation, but use the data from the linked PVWatts calculator to reflect that the solar energy is reduced if the orientation is not true south.

 

Enter your postcode on the website linked in the spreadsheet and go from there.

[Jeremy Harris]

19 hours ago, willbish said:

 

Would anyone know how to alter the Azimuth on this spreadsheet to give figures for windows other than true south?

 

 

As an alternative, you could misuse PVGIS ( http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php?lang=en&map=europe ).  It's intended to predict PV panel output, but it also allows the solar radiation to be estimated for any given lat and long, azimuth and elevation.  You could use this data to calculate the solar radiation striking the outside face of any window, then work back from there to get the solar gain.  It has climate data for the whole year, too, so with some work you could probably work back to get the solar gain at any time of the year reasonably accurately.

 

The advantage of PVGIS over other models is that there is a fair bit of supporting data from those with solar PV systems that shows it's pretty accurate, certainly better than 10% in the majority of cases, I believe.  It's also flexible enough to accept any elevation or azimuth angle, so could be used for vertical windows or angled roof lights.  Use the monthly or daily radiation tabs to get what your after, rather than the PV estimation tab.