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  • Writer's pictureAdrian W. Osgood

Lets Talk Windows.

The soon to arrive energy code updates in California are going to make window efficiency requirements even more stringent. Let's discuss some of the science behind them so we can choose the best windows for a project.


So before I begin I wanted to give full disclosure. A large portion of this came from a recent conversation with one of the architects that we work with here in Sonoma County. He was hoping for some clarification on how specific windows attributes will effect the Title 24 (energy) compliance of a project we were working on. After writing up a long and drawn out email about energy code, window technology, compliance modeling, etc. I thought, "What the heck? I've spent all this time amassing this information. I might as well put this out there for others as well," and so another blog was born. Oh, and I wasn't an English major so don't expect to be bedazzled by my grammar.

Lets first get an understanding. How are homes modeled in California?

In California all new construction must go through the Title 24 process. Basically, all homes have a digital model created of them which determines whether or not they meet the energy code requirements. In this discussion we are focused on performance modeling which allows the energy consultant to decide how the energy saving components will be integrated into the design. The energy calculations performed are Time Dependent Variable (TDV) based. This means that for each conductivity, thermal resistance, or general efficiency value input into the calculations there is an algorithm designed to determine the amount of energy each material will either release or contain. When all aspects of the construction are taken into account we are left with the home's usage of TDVs (i.e. gas or electricity). The TDV usage of the proposed home is then compared to the standard home as defined by the CEC. The comparison results in the compliance %. Essentially it is how close the proposed design comes to the required standard home. Anything below 0% compliance is bad news. This means some aspects of the proposed home need to be upgraded and that is exactly the issue we ran into.

What is the relationship between TDVs and windows?

For this home we were using heat pumps. The heat pump systems we looked at use electricity for heating and cooling so the primary TDV in focus is electricity. Now when we discuss windows one of the primary things you will hear is Low-e. The term Low-e stands for low emissivity. Emissivity is a measurement of how much radiant energy a material releases. So when we say Low-e windows we are basically saying they have coatings that don't allow radiant energy to pass through them. Generally, Low-e is a single coating while Low-e2 has two coatings and Lowe-3 has -you guessed it- three coatings. By coating the different faces of a dual pane window with specific low-e coatings you end up with the glazing U-value and SHGC. U-value is the windows conductance rating which basically means how much conductive energy the window allows to pass through it. The SHGC or Solar Heat Gain Coefficient is the ratio of solar radiant energy passes through a window vs. how much radiant energy is actually hitting a window. National Fenestration Rating Council tested U-value and SHGC takes into consideration low-e glazing, what type of gas is between the panes, thickness of panes, air gap, and the framing materials conductivity. Each play a special role in determining how effective the windows are at keeping energy in or out. In some cases Low-e might be implemented to permit visible light to pass through while blocking out lower frequency infrared radiant energy, therefor, allowing the room to stay nice and sunny while also keeping it cooler. Alternatively low-e coatings can be used on the inside of the home as well to block radiant energy from leaving.


So how do different windows stack up?

For the project in question the designer had originally had spec'd out thermally broken metal windows. They look fantastic but as you all know metal is a highly conductive material so it doesn't make for particularly great window framing material. The area weighted U-value and SHGC of the home was extremely high due to the windows and therefore our heating and cooling TDV usage was very high. Initially the designers wanted to keep the poorly performing metal framed windows. From a Title 24 modeling perspective this was basically the worst thing we could hear. The metal windows were destroying the compliance margin and we needed to come up with ways to make up for it.

We did two things. First, we suggested a solar/P.V. array for the home. It would offset a large portion of the homeowner's energy costs and had a nice federal rebate to go along with it while also bumping up the compliance. That wasn't enough to get the home over the line though. In order to offset the high heating/cooling loads we needed to take credit for additional upgrades. We decided to go with some additional HERS testing. The HERS QII (Quality Insulation Installation) test and HERS blower door test are assigned a quantitative value in Title 24 compliance modeling. They are designed to improve the overall functionality of the home thereby reducing the TDV usage. Therefore, with those implemented in the Title 24 we see a great improvement in the overall compliance which offsets the poorly performing metal windows.

In the meantime we also looked into some windows made of other materials. The aim was to do some value engineering and see which options worked best for long term energy savings and immediate investment costs. For this we ran the model with two alternative window/door designs. Both had Low-e coatings with a manufacturer specific interior coating. The first was performed with all Pella windows and doors. The Pella windows were wood framed with Lowe coatings. Modeling with them we found that our average U-value across all glazing in the house is considerably lower. This means that we no longer needed to take credit for those two HERS tests in order to maintain the required compliance margin and the homeowner would save a lot of money in the long run on energy bills -about $600/yr.

When then looked at a Marvin/La Cantina combination of doors and windows. Marvin windows were selected with La Cantina bi-fold and sliding doors. With this combination we saw the same thing. The glazing performs far better than the metal windows but not quite as well as the Pellas. This is due to the La Cantina windows having a U-value of .38 which is considerably higher than the Pella at .29 U-value.

One important thing to note is that every company has its own trademarked coatings so when you see something like Marvin Low-e/ERS the ERS typically stands for that companies final interior pane coating. It can get a little confusing. Similarly La Cantina has the Low-e/i89 coating and Pella has the Low-e / IG coating. For those of you in the building industry you will likely see window call outs from your energy consultant that say something like (Manufacturer) Argon: Low-e2: i89 to describe a window. That means it has an Argon gas fill, two Low-e coatings, and a final interior coating specific to that company.


How exactly do the coatings translate into the model?

Well they don't translate directly into the model. The complexity of the calculations in the Title 24 compliance manager makes it very difficult to determine exactly how a change in U or SHGC value will alter the compliance margin without actually running it. However, you can say with reasonable certainty that if your heating TDV usage is too high you can select a lower U-value window and reduce the overall heating load in turn improving your efficiency. The same could be said for SHGC. If your cooling TDV usage is very high then your Solar Heat Gain Coefficient may be too high and you could improve the overall compliance by selecting a lower SHGC window.

Currently we are still waiting on price points for the different arrangements of windows. I'll be sure to give an update as soon as we find out. If you have any questions or comments please leave them down below. Additionally, if you have any other ideas you would like modeled let me know.

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