Heat loss and Windows

Question 1

How do you reduce heat loss through radiation in window?

Answer 1

1. 2/3 of the total heat loss in standard window from radiation
2. Ordinary glass readily emits heat to a colder surface; lowering glass emissivity reduces heat loss

Question 2

What is the best spacing to reduce convection heat loss in windows?

Answer 2

1. Air movement in spaces between flass paves results in heat loss due to convection. space too small: conductin through air is significant. Space to big still air rises when heated on interior(warm side) and falls as it is cooled on experior side, therefore the convection movement of air passes heat tot he exterior.
2. Best spacing to minimize convection losses is 12 to 16 mm (1/2 to 2/3 ) between glazings.
3. Also, gases like: Argon and krypton, are often used to reduce convection heat loss. Optimum spacing for these gases varies.

Question 3

Explain conduction heat loss in window assemblies.

Answer 3

Window frames and sashes are the primary elements that conduct heat.
New technology and design can use insulating materials to greatly reduce heat loss from these sources.

Question 4

How does airleakage contribute to energy loss in window assemblies?

Answer 4

1. Most of the air leakage of operable windows (windows that can be opened) occurs between the window sash and frame.
2. Bigger windows tend to leak less air per unit area.
3. Air leakage can also occur in poorly constructed fixed windows between the insulated glass unit and the frame.
4. Windows that have the lowest leakage rates, regardless of the type, tend to be fixed windows (ie they cannot open)

Question 5

What do U-factor values represent?

Answer 5

The rate of transfer of energy through conduction, convection and radiation.

Question 6

List components and formula for thermal transmittance (U-factor) calculations for windows.

Answer 6

1. Centre-of-glaze transmittance(centre glass in HOT2000)
2. edge-of-glaze transmittance(centre glass in HOT2000)
3. Frame thermal transmittance

Formula:

Uoverall = (Uframe xAframe)+(Uedge x Aedge)+(Uglaze xAglaze)
——————————————————————————-
(Aframe + Aedge + Aglazing)

Question 7

What is the function of low-emissivity (low-e) coating?

Answer 7

Allows light to pass through windows, but reduces radiation heat losses or gains. It reduces radiation heat loss in the winter and heat gain in the summer.
It improves the window U-factor by limiting radiation losses

Question 8

How does low-e coating work?

Answer 8

It is a thin metallic layer applied to the surface of the glazing to improve energy performance.
1. reflects long-wave infrared radiation (heat) portion of the spectrum, while still allowing part of the solar spectrum to pass through it.

Question 9

Contrast LSG and HSG coatings.

Answer 9

Only a small portion of the solar spectrum is visible light, and how low-e coatings deal with the remaining invisible portion of the solar spectrum varies.

Low solar gain: reflects most of the invisible solar spectrum, helping keep solar gain to a minimum. Typically located on exterior to reflect heat out of buildings.
High solar gain: transmits most of the invisible solar spectrum and accompanying heat gain, and is typically applied to interior of windows to reflect heat into the house.

Question 10

What does SHGC stand for?

Answer 10

Solar heat gain coefficient (SHGC).
It is a number between 0 and 1
LSG products have SHGC of less than 0.3
HSG products have an SHGC greater than 0.3

Question 11

Where is the greatest heat loss in windows?

Answer 11

center and edge of glass. Because of this, glazing is contained in something called an insulating glass (IG) unit which consists of at least two panes of glass separated by a spacer bar and sealed around the edges to ensure airtightness. IG units are typically filled with an inert gas, such as argon or krypton, to reduce heat transfer throught the glazing.

Question 12

What causes heat loss in IG units or insulation glass units?

Answer 12

air movement in the spaces between panes of glass resulsts in heat loss through convection. The size of the space is important. If too small, conduction through air is significant. If too big, the still air rises as it heats up on warm interior side and falls as it is cooled on the cold exterior side of the window. This convection air movement passes heat to the exterior.

Question 13

WHat is best spacing to minimize convection losses?

Answer 13

1/2 inch to 2/3 inch (12 to 16mm)

Question 14

What gases reduce heat loss in IG units?

Answer 14

Argon and krypton are used to reduce convection heat loss.

Question 15

What are the three factors for an energy rating (ER) for residential windows and doors?

Answer 15

Measure of performance based on:

1) heat loss (U-Factor);
2) air leakage loss;
3) potential passive solar gain of a fenestration product

Question 16

What type of fenestration system is an ER intended for?

Answer 16

Vertically installed in low-rise residential buildings.

*skylights are not included

Question 17

The higher the ER….

Answer 17

…the more potential energy gains the product will allow

Question 18

Higher ER indicates…?

Answer 18

…a slower heat transfer without significantly reducing the amount of solar gain.

Question 19

How is residential fenestration Energy Rating calculated?

Answer 19

Unitless value derived froma formula that balances heat loss, air leakage loss, and the potential passive solar gain of fenestration products:

Simplified ER equation:

ER = (57.75 x SHBCw) - (21.90 x Uw) - (1.97 x Lw)+ 40

*review page 61 Study Guide

Question 20

What is visible transmittance?

Answer 20

a fraction of incident visible light that passes through fenestration products expressed as a value from 0 to 1. the higher this value, the more light passes through fenestration products.

Question 21

What is the compliance path for Zone 1, 2 and 3 climate zones for Canada, with Energy Star certified windows, doors and skylights.

Answer 21

Zone 1: ER min 25 or Max 1.6 W/m2K (0.28Btu/hft2*F), and
Min 16 ER for sliding windows and doors only

Zone 2: ER Min 29 or Max 1.4, and 16 (sliding windows/doors only)

Zone 3: ER min 34 or Max 1.2, and 24 ER

Question 22

Energy products may be ENERGY STAR certified of they have:

Answer 22

1. A high ER that balances heat loss with heat gain. The higher the ER value, the more the product is energy efficient (ER do not apply to skylights)
2. A low U-factor which measures the rate of heat transfer. The lower the U-factor value, the better the product insulates.
3. Windows, doors and skylights must have an air leakage value of 1.5 L/s/m2 or less