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© 2014 Copyright True Turtle            Website by Design Lab 443

WINDOWS

High quality windows & doors means more comfort and lower energy use

Windows, doors and skylights are holes in your superinsulated envelope

Even though windows, skylights and doors are a small percentage of a wall, their low performance drags the overall R value of a wall down significantly True Turtle uses both higher amounts of insulation AND high performance windows and doors.  Together, our walls let 60% less heat loss for our new constrction walls and 35% less heat loss for our retrofit walls than a typical wall.  Our higher quality assemblies make more comfortable, lower cost home to live in. 

High performance window definitions that matter...U value, R value and how they're related

The windows True Turtle uses and how they stack up by the numbers

We install very good, high performance windows in order to make the highest performing wall assembly we can. 

 

We pay particular attention to:

 

the rated performance of the window (U value and SHGC),

 

the kind of window (casements if we can, double hungs if we must)

 

 

For a quick primer on window components and why we chose what we do keep reading below. 

 

 

 

This page is a work in progress...please excuse all dust and debris!

U value

 

Lower U = better insulator

 

Used when discussing:  windows and doors

 

U value describes how fast heat is lost through a window assembly, so it is a measure of how well a window assembly insulates.

 

The lower the U-factor, the less heat flows through it and the better it is at insulating. 

 

 

R value

 

Higher R = better insulator

 

Used when discussing: walls, roofs, foundations

 

R value describes how much a material resists heat loss.  The greater the R value the more insulative it is. 

R is the inverse of U

 

In general...R=1/U

 

The relationship of R and U is actually more complicated than inverstion.   For discussion purposes we're keeping it simple.

 

What's useful is expressing window performance as an R value so you can compare the wall performance to window performance.

 

It's just easier to compare apples to apples. 

What makes a high performance window and skylight?

There are three main components to a window:

 

Glazing, Frame and Spacer

 

Windows have become sophisticated technological marvels over time.  Each component has undergone enormous technological advances....and each brings strengths and weaknesses to a window assembly.  Selection of each component makes the window assembly either higher or lower performing.    A survey of components and their contribution to the performance of a window assembly are below.

 

Image:  National Fenestration Rating Council

Window frames are the weakest link in window performance. They provides the least insulation. Improving the thermal resistance of the frame can contribute to a window's overall energy efficiency, particularly its U-factor. Vinyl, insulated vinyl, wood, fiberglass, composite frame and metal are common frame materials each with their own advantages and disadvantages.

Window frames

Frames

Window frames provide the least aount of insulation in a window assebmly.. They are the weak link in window performance.

 

Improving the thermal resistance of the frame can contribute to a window's overall energy performance, particularly its U-factor.

 

Vinyl, insulated vinyl, wood, fiberglass, composite frame and metal are common frame materials each with their own advantages and disadvantages.

Low emissivity (Low-e) coatings

Low-emissivity (low-e) coatings on glazing or glass control heat transfer through windows with insulated glazing. Windows manufactured with low-e coatings typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.

 

A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The low-e coating lowers the U-factor of the window, and different types of low-e coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A low-e coating can also reduce a window's VT unless you use one that's spectrally selective.

Multiple panes

Two panes of glass with a gap inbetween them insulate much better than a single pane of glass. 

 

Performance is increased with additional panes of glass, but weight becomes a factor.  Many manufacturers chose to do multiple coatings in lieu of triple pane glass.

 

 

Gas fills

To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the panes with inert gas -- commonly argon or krypton -- that has a higher resistance to heat flow than air.

 

 

Warm edge spacers

Low-emissivity (low-e) coatings on glazing or glass control heat transfer through windows with insulated glazing. Windows manufactured with low-e coatings typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.

 

 

Air leakage and window type

Heat loss and gain occur by infiltration through cracks in the window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. An AL rating is required for ENERGY STAR certification. This rating must be 0.30

 

 

 

 

Window orientation and the limits of high performance design in an urban environment

A discussion about high performance windows would not be complete without a discussion about orientation.

 

Without question, the biggest impact you can have on buildings energy use is it's orientation.  The optimal orientation is the long axis facing south...ie providing the most south facing glass you can.

 

Why south facing glass?

 

It lets in light and therefore heat...which cuts down on heating bills in heating dominated climates such as DC.

 

Just as importantly, south facing glass can be shaded appropriately on the exterior...allowing good control to minimize oveheating in summer while maximizing heating potential in winter.

 

What's wrong with north facing glass?

 

It doesnt' get any sunstrike, so all it does is lose heat.  These should be used sparingly.

 

What's challenging about east and west facing glass?

 

It is a struggle to shade east and west facing glass because the sun hit it at such low angles when the sun rises and sets.

 

The homes we buy are what they are...we cannot chose their orientation.

 

Most of the housing stock in Washington DC is 100 year old brick row homes.  The District of Columbia is an extremely competitive environment for real estate.  There are many companies who buy to gut renovate homes to sell.  We do not have the luxury of selecting homes we purchase based on their orientation.  We buy what we can, model what we get and make them as high performing as possible.

 

 

 

 

 

 

Many thanks to...

Efficient Windows Collaborative 

 

National Fenestration Rating Council and

 

Energy.gov...Windows, Doors and Skylights

 

All have informed this page and give a deeper analysis of window technologies.  If you want to learn more about windows start with those resources.