Friday, May 24, 2013
Passive House Windows- Energy Balancing
The recommended window insulation value (resistance to heat transfer in hour square foot degrees Fahrenheit per British Thermal Unit or, hr ft2 F/ BTU ) for a window off the shelf is a minimum R 7.1 or an installed minimum of R 6.68. This R6.68 number holds true for the installed value of the glazing components of a Passive House door as well. These numbers are recommendations for Central European climate and thus a higher R value for Maine is certainly preferable. And certified Passive House windows are best.
Now part of a Passive House strategy to achieve low energy use is to gather as much solar heat gain as is practical without overheating. To this end the glazing in windows must be significant on south sides of the building and somewhat east and a bit on west sides with minimal on north. And the glazing in the windows must be shaded with overhangs tailored to each compass orientation to avoid overheating in summer and let lots of sun in, in winter. And this glazing must be designed to allow a certain amount of solar heat gain without reflecting too much so the heat passes into the building. This solar heat gain has a number called the solar heat gain coefficient or SHGC which represents the amount of solar heat allowed in with 1 representing all of it and 0 meaning none. A good SHGC for Maine would be between 60-62% or .60-.62 SHGC. A minimum of 50% solar heat should be allowed or .50.
Shading devices like overhangs or attached fins to keep summer sun out are important as a Passive House cannot have temperatures indoors exceed 77 degrees Fahrenheit for more than 10% of the occupied year (recommendation is 5% as global warming continuously upgrades the average temp).
A window is "energy balanced" if it takes in the same amount of energy in a year as it loses. It is "positive" energy balanced if it takes in more energy in a year than it loses and a Passive House wants to have a positive energy balance for each window without overheating beyond the above limit.The calculation for energy balance involves a calculation for window transmission losses (QT) and one for window solar gains (QS).
The formula for window transmission losses is QT (in kBTU/yr) = sum of (Uw, installed x Aw x ft x Gt) where Uw is the U value (U= 1/R) of the window installed in BTU/hr ft2 F; Aw is the total window area in ft2 (square feet); ft is the temperature correction factor usually 1.0 and is unitless; and Gt is heating degree hours in kFhr/yr (thousand degree Fahrenheit hours per year) for your area.
The formula for window solar gains is QS (in kBTU/yr) = r x g x Aw x G where r is the solar reduction factor plus the glazing fraction so a multiple of 4 factors and is unitless; g is the "g value" or solar transmittance also unitless; Aw is the area of the whole window unit in ft2 (square feet); and G is the global solar irradiation during heating period in kBTU/ft2yr (thousand BTU's per square foot year).
These windows are key whether building to meet new Passive House standards or retrofitting to meet the EnerPHit Passive House renovation standard. I'll talk about the meaning of the formula components and more in future.