DIY Green Home Improvement » Building Code

SHGC, Visible Light Transmission and U-Value

rcovington — Sat, 06 Jun 2009 03:47:00 +0000

The key factors to understand when judging the expected performance of a window or door are:

Solar Heat Gain Coefficient (SHGC)
Visible Light Transmission (Vt)
U-Value (U-Value)

These are the standards by which most test protocols evaluate glazing from.

Solar Heat Gain Coefficient (SHGC) -- is the measure of relative heat gain (compared to a piece of 1/8″ thick clear glass) that a particular piece of glass will absorb under exposure to the sun. Reducing the Solar Heat Gain Coefficient (SHGC) is a function of either tinting the glass and making it darker or coating the glass with low-emissivity coatings to reflect the amount of infrared spectrum most responsible for heating that the glass absorbs.

Visible Light Transmission (Vt) -- is a measure of the amount of total visible light that will pass through the glass, again relative to a piece of 1/8″ thick clear glass. Adding low-emissivity coatings coatings to glass will affect the Visible Light Transmission (Vt) factor pretty dramatically depending on the specific glass manufacturer’s method of coating.

U-Value (U-Value) -- is a calculated measure designed to represent the ability of the assembly (both pieces of glass, the air space, the frame components, etc.) to resist the transmission of cold and heat through them. This is a composite value of all the components and is not a relative measure. The lower the U-Value, the better able the assembly is capable to resist the passage of heat and cold. U-Value is the inverse of R-Value in measuring the same physical characteristics and thus the higher the R-Value, the better able to resist the passage of heat and cold.

Traps With Solar Heat Gain Coefficient -- (SHGC) and Visible Light Transmission (Vt)

Looking at the Energy Star website will tell you the recommended SHGC and U-Value you should have for your specific Climate Zone. But these recommendations do not take into account daylighting and the effect that Visible Light Transmission (Vt) can have on the performance of the overall structure during the heat/cool/light cycles. If you lower Visible Light Transmission too much, the daylighting in the interior will be reduced to a level that may require supplemental electrical lighting for some functions, or to make the environment enjoyable to the occupants. In evaluating the overall structure from a green building perspective, you must look at all the factors here.

Adding electrically generated light, uses energy for the lighting, but it also generates additional cooling loads on the HVAC system adding to the overall electrical usage. There have been studies done particularly on skylights, daylighting and electrical usage in commercial buildings which attribute a very significant increase in electrical usage to this phenomenon. So make sure you do not overshoot in reducing the daylighting in the home in pursuit of the gremlin of the lowest Solar Heat Gain Coefficient (SHGC).

U-Value is the relative constant in all this. Lowering Solar Heat Gain Coefficient (SHGC) will not in and of itself have a large impact on U-Value. Nor will lowering the Visible Light Transmission (Vt). The U-Value is a function of the physical properties of the glass, wood and/or metal that compose the composite of the unit and these are not affected by changes in the other values.

U-Value and Solar Heat Gain Coefficient in Climate Zone 4

If you live in Climate Zone 4, then U-Value becomes a more important consideration in judging the window performance overall. But in this zone, higher Solar Heat Gain can add incrementally to the heat inside the home and reduce the heating demand placed on the heating system. Thus, a SHGC of .65 will be of more benefit to you . You will also finsh that the High Solar Heat Gain low-e coatings will typically have higher Visible Light transmission in the .7 range.

So, you must understand the impact of all these factors as it relates to your specific location. When you are through evaluating on this basis, you will need to look at the air infiltration ratings. I will discuss this in another article.

This added 7/7/2008

Light to Solar Gain Ratio (LSG)

I ran across a new measure that helps you guage the relationship between SHGC and Visible Light Transmission. The new ratio is known as the Light to Solar Gain (LSG) and is a ratio of the Vt to SHGC. For instance if the Vt is .66 and the SHGC is .40 then the ration of .66/.40 or 1.65. The recommended ratio is 1.25 or higher. The higher the ratio, the higher the daylighting benefit. It also means that the glass transmits more light than heat to the interior, again a positive in the Southern climate zones 1,2&3.

Solar Heat Gain in Energy Star Zone 4

If you live in Zone 4, then it is desirable to have a higher Solar Heat Gain ratio in your windows. Otherwise your heating system will work harder in the winter and diminish your energy efficiency. The standard, 1st generation pyroltyic Low-e coatings (also known as High Solar Heat Gain Low-e, is the easiest method of obtaining a higher SHGC and still retain the reflectivity of the low-e as it pertains to winter infa-red heat retention in the interior of the home.

You may run into a problem in this regard as some window companies have settled on the more efficient (for zones 1,2,3) sputter coat low-e‘s as their “standard” offerings. They prefer to narrow the choices because it is less costly to offer fewer choices. Do not be disuaded. Stick to your guns and insist on the specs that meet the Energy Star qualifications for your home. Any window manufacturer can get the pyrolytic coatings. If they tell you they cannot, move on to a manufacturer that will give you what you need. Buying windows is like anything else, you will either get what you “buy” or you will get what you were “sold”. In the end, you must be responsible for the choice you make.

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Coming Changes To Building Codes and Energy Codes

rcovington — Sun, 31 May 2009 10:39:10 +0000

Photovoltaic Shingles

Massive changes are coming to an energy code near you. Under a Federal Law under consideration in the Congress (H.R. 6279). The law requires the model energy codes establish minimum targets of increases in energy efficiency as mandates of the Federal government on the order of :

30 percent in editions of each model code or standard released in or after 2010
50 percent in editions of each model code or standard released in or after 2020.

The law also mandates that if the body that governs the IECC model energy code that is the basis of almost all energy codes in the various states do not achieve these goals, the Secretary of Energy is commanded (within 12 months) to institute a Federal energy code that achieves the mandates. The law will have the effect long term of mandating that all new buildings have solar panels, photovoltaics or wind turbines installed on all new construction.

The law will also mandate the enforcement of the new mandates on states could risk the loss of Federal funds. It also does not fully fund the regulations and structure to oversee all the enforcement that will occur under this law.

Funding shall be considered adequate, for purposes of this paragraph, when the Federal Government provides to the States at least $50,000,000 in a year in funding and support for development and implementation of State building energy codes, including for training and enforcement.

Of course, the problem with all this is that what is good for one area of the country is not good for another, but the imposition of Federal regulations on issues that are local in nature many times distorts the solutions and results and brings massive inefficiency to the markets. The other problem, it adds costs to the system that will in all probability far exceed the laudable goals.

There are provisions in the code that will make it necessary for all new “buildings” to consume “zero net energy”, by the year 2050. Well this will be enormously expensive to attain. If they include residences in this provision, imagine the cost of adding solar and wind systems and what that will do to the cost of building that home. It will exclude many first time home buyers from being able to purchase or build their own home and will drive rents up dramatically.

You should get educated about this bill. It will start to have an immediate impact in the next 2-4 years. It couls cost you personally and it is not necessarily the best method to move the country towards energy efficiency. In fact there is no technology that gets us to these goals without dramatically higher costs of energy. This bill is more about a stealth method of impsoing limits on green house gasses than it is about energy efficiency.

Solar, Photovoltaic, Wind and other methods of generation that are available to make a new building “zero net energy” have not reached the stage where they can fill this role in an economically efficient manner. The reason we do not have them in place in a massive way is that the market is waiting for the economics to change for that to take place. No amount of government intervention can force that to change. Only massive investment in the technologies and adoption that brings economies of scale that lowers the cost of these technologies can do this. Otherwise we could end up with massiv energy inflation along the lines like we have seen sscoiated with food and corn ethanol. This is the example we must all consider when moving down this road.

There are other methods of moving forward towards these goals. The better approach would be to embody incentives towards the investment in these technologies. Tax credits and other mechanisms, modulate investment into technology in a market efficient manner and avoid the kinds of distrotion you see with mechanisms like mandates and regulation.

Expanding the credit for Solar generating systems will do more towards these goals. Adding a credit for the installation of wind turbines and establishing a registry for manufacturers to concentrate information on available systems would increase the use of this technology. There is a serious shortfall on readily accessible information on wind generation systems and there is apparently a lack of independent independent research and ratings for systems that consumers like to see before committing to such a large investment. There will also have to be a change of heart in zoning commissions and HOA committees to allow expanded use of Solar Panels, Wind Turbines and Photovoltaic. This is one area that Federal Legislation could be of benefit in supporting and expanding investment in these technologies.

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