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How do Window Films Work?

Dustin_7022224 | Posted in Energy Efficiency and Durability on

So I’ve gotten myself into a discussion, perhaps a disagreement or misunderstanding, about window films for the benefit of improved thermal comfort/performance.

Specifically, we are talking about a 3M Thinsulate product, although the literature he cited (government contract, written by manufacturer) refers to EnerLogic.

My confusion in part stems from what I think is the definition of R-value and by extension U-value.  I understand R-value to be a measurement of resistance to conductive heat transfer.  I do get that these films, just like low-e coatings, can have a big impact on radiant heat transfer, but I cannot wrap my head around how they could actually effect conduction, and thus reported improvements in U-value.

Is it just a matter of how the U-value of a window is measured – thermal performance, resistance to heat loss, but not specifically conduction?  I guess resistance to heat transfer is insulation all the same, I just didn’t expect it to effect the U-value.

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Replies

  1. charlie_sullivan | | #1

    U-factor for a window is an all-in number. The way heat transfers through a window is complicated: first heat transfers from the room to the first pane through convection and radiation. Conduction spreads that heat throughout the volume of that pane very easily as glass is highly heat conductive. That warm pane then transfers heat to the next pane, also by convection* and radiation, spreads though it by conduction, and then finally that pane transfers heat to the great outdoors, mainly by convection. Meanwhile heat also flows through the frame.

    U-factor lumps all of that into one number, allowing you to calculate heat transfer through that whole system just by calculating Q = ΔT⋅U⋅A.

    A common misconception is that there's additionally radiation heat transfer through the window, but in fact, ordinary window glass is completely opaque to thermal radiation. Thermal radiation from the room that hits the window will not go through it, but will be absorbed, warming the window, leading to the process described above.

    So you can change the U-factor by changing the frame or by changing the in the gap(s) between panes, or by changing radiation to or from any of the multiple surfaces involved.

    *Technical side note: The heat transfer through the gas between panes is sometimes more conduction than convection, conduction through the gas which isn't moving much in a narrow space.

    R-value differs from U-factor in that it doesn't include the "surface film" effects. If you have a R-8 insulation board instead of a wall (maybe a doghouse made of foam?), you'll actually get R-9 because of the surface films adding to the insulation value of the board itself.

  2. onslow | | #2

    Dustin,
    Late to the thread thanks to other events, but here's the TL/DR edition.

    I think it helps to first clarify NEAR IR energy (not FAR IR) as relates to windows and heat loads on buildings. Near IR energy, which is a large fraction of the sunlight we get down here on earth, does indeed go through window glass. Different window glass types will pass different amounts. Different coatings will reflect different amounts. The optical/visual effects of different glass and coating selection results in a myriad of window IGU choices to consider.

    Window companies ostensibly test and certify the performance of their windows based on a controlled hot box test much like the ones used for wall assemblies. At least the now old references I have seem to indicate their use. Perhaps by now, enough data has been collected to make rating individual window selections possible by simulation alone. Since windows can have different orientations to sun load and also spend about half the time in the dark, the advertised "R" value of a window will need to be a pretty fair representation of how much heat will flow through just like rating a wall cross section. Window manufacturers more properly express the heat flow value using the U value, which is how one adds up all the thermal properties of all the elements in the window, (or wall - or roof) which I presume includes air films and coatings. Selling by using an R value, while technically incorrect, makes the window performance easier to understand and sell to the lay public.

    Window companies and magic IR bean salespeople part company on how they deal with the variable infrared energy presented daily by our sun (which I am looking to see if it is addressed in a hot box test or simulation). Window companies express the variable energy gain/rejection as the SHGC. The magic IR bean people pitch "effective R value". Radiant energy is a frequently misunderstood component of overall heat flow in building design. The presence of two different terms being used to express heat flow resistance which is often equated with insulation value is an additional confusion point that only aids salesmen.

    The trap your friend has fallen into is frequently used by magic IR bean salesmen to hawk supposedly high R value nonsense. The window film does have a conductive R value - of about zip. If you put a piece over the back of your hand and put an ice cube on it, you will find the cold transmits across it very quickly. The advertising might suggest the film has an R value of X. As you note, the same window film can reflect radiant heat energy, which is its most useful feature. How you characterize the heating or cooling value of that radiant energy reflection ability is where the salesmanship sets in.

    If I tell you that my magic window film will lower your AC costs by keeping out "heat" as a generic concept, I would then next suggest that my magic film has an "R" value based on how much "heat" I have spared you. I will of course look to use the largest value I think I can safely convince you is credible. I might start with a graph that shows over 50% of sunlight is infrared energy and only 42% is visible light. My magic window film blocks 97% of IR energy, so obviously you will save 50% on your air conditioning costs. By extension, my magic window film has "effectively" increased your windows R value by at least 50% maybe more! SO BUY NOW BUY NOW.

    Like the lady in the TV ad a few years back said, "That's not how it works!"

    For one thing, the heat film is only usefully functional during daylight hours when light is on it. If you put it on a north window the net energy rejection will be quite small. Did this window suddenly lose R value? No. None of the windows will benefit from the "magic R value" I sold you on when it is night. At best you will have gained some tiny "R" value provided by the film thickness. Come winter you might find that my magic film is now a negative effective "R" value that is costing you dollars because the film blocked a positive source of energy (near IR) from getting through the window. The window film has indeed affected your total energy use, but not by altering the "R" value of any part of the window in a significant manner. It does however, affect your U value when placed in the system. But only for the time period when external near IR energy is present. I am sure that every pitch includes "up to" language, even if in small type. The classic version of "your mileage may vary".

    The NFRC uses U values when discussing windows and doors for good reasons. R value should really only be used to express the heat flow resistance for bulk materials like foam insulation. This can be a bit confusing since windows and doors are of course made up of glass, wood, plastic, foam, gas fills and the like, which have thermal properties often expressed as R values. The U value is the sum of the component R values from one side of the window to the other. This sum also includes the value C sometimes used in reference to air films, just to add to the alphabet soup. C is the inverse of R. For practical purposes of discussion I will glide over this one.

    The various coatings do alter conductive energy transfer in the glass elements thanks to reduced emissivity at the interfaces of glass and coatings. If the coating impedes the glass's ability to radiate heat one could say the glass now has a greater R value. Not exactly correct, as the bulk glass thermal conductivity has not changed though the film presence has altered its thermal transmittance properties. In that way the coatings do have a conductive R value of a sort when incorporated into the window as a whole. IF you could pull out just the micron thin film coat and test it for its thermal properties, you would find it has very high thermal conductivity K value(low R) as most coatings are metallic complexes.

    It is poor practise, though committed by many to say the inverse of U values are equivalent to R values. (U =0.10 equivalent to R10) Technically, R values are a materials thermal conductivity (K)value times thickness of a homogenous sample of a material. R value is for individual materials. Once two materials are connected in a system, U value is the correct term. Energy losses across multiple materials via the three modes - conduction, convection and radiant is characterized by U value - the thermal transmittance of a system. Radiant energy just tends to be the less understood component which leaves an opening for specious claims. That is why the window film will effect the system U value without having a physical R value of any significance.

    IR wavelengths are simply those frequencies outside our perception past what we call red. Ranging from near terahertz frequencies to just below perceptible red, IR wavelengths vary by about a factor of 1000. The difference between near IR and long wave or far IR given off by humans and sofas is more like a factor of 100. So saying something is in the IR range is kinda like saying one lives somewhere between NYC and LA. Lots of different habitats to deal with.

    The IR energy from the sun is predominantly of the high energy short wavelength type. On the way down to earth's surface some is absorbed by the atmosphere and re-emitted as longer wavelength IR, but mostly what hits your building is near IR. Similar things happen to the near IR energy coming through your windows. A small fraction of the short IR is absorbed by the glass with the percentage varying by glass type. The rejection/reflection of near wavelength IR varies with incident angle and energy rejection can be greatly enhanced with coatings.

    Whatever fraction does make it through the window will be absorbed by objects and air inside and converted to heat. The warmed objects will re-emit long wave IR to the room and windows. This range of IR wavelengths is absorbed by glass very well. The glass will warm and re-emit long wave IR from both surfaces. Window coatings might have atomic level properties that imbue the micron thin films with resistance to conductive heat flow, but I believe that primarily the suppression of radiant emission by the film makes the coated glass behave as a lower conductance material.

    The position of the coatings on the glass surfaces 1-4 (or 1-6 in triple glazing) is very important. One particular coating option places a far infrared reflecting coat on the interior most surface, #4 or #6. This is the reverse of a window coating being used to reject exterior near IR. Warm people and sofas emit radiant energy in far IR. The coating bounces far IR energy back to people near the window, which increases the perception of the window being warm. The sofa is mute on this subject.

    However, the glass becomes cooler than a #3 surface coated pane would thanks to the loss of energy it would have absorbed from the occupants and furniture. The lack of the coating on #3 surface also means that whatever convective heat is put into the glass will be free to radiate into the gas fill. A #3 surface coat would suppress re-emission so the energy gained by the interior pane stays a little bit warmer. You can search GBA for comments on this coating option.

    The coatings used to control heat flow by rejecting incoming infrared energy and suppressing re-emission have analogs that might help clarify the disconnect between radiant energy paths and conductive and convective energy paths. It may also help amplify on why R values are not equivalent to U values.

    Think of aluminum foil, space blankets, and fire tents. All will redirect radiant energy in very useful ways, but the materials perform very differently when physical (conductive) contact is made with them. A thousand sheets of aluminum foil will not super insulate your house if stacked together. A single layer over the entire outside would certainly lower your heat load from radiant energy. Useful in the southwest, less so in Maine. I can't safely suggest the experiment of putting aluminum foil on your arm and pressing an iron on it, but perhaps a heat lamp and an ice cube will help your argumentative friend to concede the point that R or U aside, radiant energy is not conductive or convective and must be addressed differently.

  3. GBA Editor
    Kiley Jacques | | #3

    This recent post goes beyond the scope of just windows, but I think you might find it of interest: Understanding R, U, K, and C Values.

  4. onslow | | #4

    KIley,

    Edited. Sometimes one's memory capacity sinks too low in the body. Best not to pull them out when that happens.

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