Low-E ( low emissivity ) is the new big thing to lowering your utility bill. There are Low-E glass windows, Low-E window film, Low-E radiant barriers, Low-E house wrap and Low-E guitars. Okay, the Low-E in guitars are not relevant but you get my point everyone is talking about Low-E. Mostly it is about windows but Tyvek does have a Low-E house wrap and there are a number of different radiant barriers you can get for you attic.
What is Low-E
Well there is a great scientific explanation over at Wikipedia but if you are physically science challenged or just don’t feel like wading through it, the simple explanation is the amount of heat absorbed and re-emitted (emissivity). If we had a perfect emitter it would have an E of 1 and if we emitted nothing our E would be 0. The interesting thing is most low E materials are also highly reflective. For the purpose of reducing heat flow (insulating) the lower the E number the better. Of course this varies by frequency and we are primarily interested in the infrared (heat) in this case. As further point of clarification different material emit heat better or worse and reflect heat better or worse. Ideally we want something that reflects heat and if heated through conduction will only emit its heat slowly.
To be a little more detailed visible light has a wave length in about the 450 to 750 nm range, room temperature infrared is around 10,000 nm and typical window glass (soda lime glass) is over 80% transparently from around 400 to 2800 nm then drops to around 30% from 2800 on. So glass would mostly pass higher temperature heat (infrared) from the sun but would absorb and re-emit the lower temperature heat from let say your house to the cold out side. Glass has an E of .91 compared to polished silver of 0.02, so glass is 91% of a perfect emitted. Low-E window coatings range from an E in infrared from .10 to .50 where lower is better. You can see having a Low E coating or film would be reflecting a lot of the heat and emit it more slowly than plain glass. This translates into less heat moving through the glass.
To clarify one the point Low-E is not high reflectivity or at least not exactly. If you are reflecting infrared you are not emitting it. However highly polished metal or metal coatings which are highly reflective are also low-E. I am sure a physicist would say I am wrong in some way but the material is basically reflecting the heat back from the exposed surface and enhance do not emitted it as readily .
Why is it helpful
Have you every seen in the movies where the sniper with his infrared scope sees their target on the other side of the wall? This is the infrared emission of that person passing through the wall. In really life how well this would actually work depends on what the wall is built out, specifically the transparency of the wall material at the infrared frequency. If you had some form of radiant barrier it would block much of the infrared and make it hard to see hot objects with the infrared scope. Needless to say without a radiant barrier heat can pass through the walls and that is you loosing heat in the winter and gaining heat from the outside in the summer.
Here is a very nice demonstration radiant barrier versus non-radiant
What are the options
Today there are Low E option for windows, doors, walls, and the attic in the follow section we will look at each of these.
Replacement low-e windows
The Most expensive option it to replace your windows with low-e windows. Most sources estimate that the return on investment (ROI) is about 25 to 30 years for this. If you have old windows that have issue like air leaks, won’t open or close correct or have lead paint this is probably an option to consider. If you windows are somewhat newer and function well you should consider some of the other options.
Low E Storm Windows
If your windows are okay and you want to add some additional insulation you might want to consider storm windows and while you are at it you might as well get low-e storm windows. Storm windows are around half the cost or less than a replacment window and something you can easly install yourself saving additional money your ROI is going to be in the 10 to 15 year range.
Low E window films
If you have pretty good windows but they don’t have a low-e coating or you just don’t want to spend much money you might want to consider a low-e film (See my review Gila Window Film HRT361 Heat Control Review – Save of Utility Bills). You can get it in 15 foot rolls that are 36 inches wide which is usually good for about 3 windows. They also sell 100 foot rolls and your can optionally get 48 inch wide. Films are fairly easy to install and the ROI is about 2 years. If you don’t want to replace you windows for a while and you already have storm windows that are in good shape this is a very good solution.
There is a number of house wraps out there the most popular of which is Tyvek ™ from Dupont ™. Dupont has come out with a number of Tyvek variations one of which Tyvek Thermawrap ™ LE which is Low E. Another popular Low-E house wrap it ESP Low-E House Wrap which is rated at blocking 97% of radiant heat in the hot some months. The most important thing to remember about low E house wraps is to get the low e you need to install them correctly. This mean having gap of usually 1 to 2 inch between them wrap and the out site wall, other wise you get conductive heat flow. If installed correctly you will get reflection of heat in the summer and low emissivity of heat from your house in the winter.
My house is so old there originally was no insulation in the attic. Then it seems in 1945 about someone put newspaper on the front and back wall and on parts of the attic floor. Don’t believe me? See the picture. I have been doing a lot of research into attic insulation since I have a few odd feature, that is besides the newspaper insulations. Most newer houses have soffit and ridge vents whereas my house the roof line extends 3 feet down into the second floor on both side with blown in insulations and I have blown in the attic floor (over the news paper).
There are several options for the attic. First before you run out and buy some radiant barriers you should first have some form of attic insulation in place. Attic radiant barriers of a great addition but should be use with other insulation products for optimum effect.
If you have vented soffit and ridge vent you will one to put a radiant barrier over the insulation on the attic floor making sure you do not block the soffit vents. You can optionally run a radiant barrier along the rafters leaving a 6 inch gap at the top and bottom.
If you do not have soffit and ridge vents you defiantly what to run the radiant barrier along the rafters and there is no need to leave gaps at the top an bottom. You will also want the run it over top of the insulation on the attic floor. If you want a useable floor space for storage you will want to but the flooring over the radiant barrier but you will one an inch or two space. If you have a sold contact with the radiant barrier is will start conducting heat instead of reflecting it.
Where and What to Purchase
So where can get all the Low produce you need? Home Depot and Lowe’s carries most of the items. For a list of low-e products and the tools you will want go to the What You Need page.
- R-Value Table – Insulation Values For Selected Material.
- Emissivity Values for Common Materials.
- ESP Low-E Insulation
- Low-E Reflective Insulation.
- ESP Low-E House Wrap.
- Dupont Tyvek Thermawrap LE.
- Low-E Housewrap.
- Insulation Ebook from home exterior guru.
Let me know what you think in the comments below.