Week 12

Installation of all delivered windows and external doors was completed.  An adhesive backed tape is placed over the outside of the window bucks to the outside foam as a water guard against any rain driven penetrations. Much shimming and leveling is necessary to mount the windows and doors straight in their rough opening.  The remaining gaps are filled in with expandable foam, casting the windows in place and creating a well-sealed and insulated space surrounding the window frame.  The green water impermeable membrane was used to cover the wood portion of the window bucks that had angled internal sides.

The wallboard guys breezed in and out and within what seemed like an hour had the ceiling boards in place.  This ceiling board had a water impermeable aluminum layer on one side to prevent humidity from the living space from penetrating into the cold attic.

A visit by an energy inspector from Efficiency Vermont to inspect the house for Energy Star Rating, pointed out that one of the biggest leaks of energy out of the living space into the attic takes place at the interface of the ceiling board and internal walls.  (European readers may wonder how this could be as they may not realize that we have a tradition of building hollow walls out of 2" x 4" "studs" or more precisely by 1.5 in x 3.5 in wide lengths of lumber).  These hollow walls may have electrical wire or plumbing holes in them which allow air entering the wall from the attic to leak into the living space or vice versa.  Traditionally, these houses were said to "breathe well".  But to build to new standards, these gaps as well as all wall penetrations into the roof need to be sealed with a flexible caulk for air tight construction.

Miss Inspector took pictures of every glass penetration of the outside walls, as these contained the U-vale and SHGC numbers for heat loss/gain calculations.  She was happy with the fresh air supply for the wood stove as now required by Vermont law.  The solarium air preheat ability for the HRV supply air was also appreciated but she was concerned about possible blockage of inlet air by the solarium design.  The use of a  Heat Recovery Ventilator is a requirement for air tight houses to maintain proper humidity and oxygen levels.  These units inhale fresh outside air, exhale stale inside air, and recover most of the energy in the exhausting  air by using it to heat the inhaled air in an isolated aluminum cross-flow chamber.  This device is clearly still a source of heat loss, however with a perfectly tight house design, the air heat loss can be controlled by the HRV's frequency of operation.

Miss Inspector took issue with my plan to electrically heat my house, with wood heat backup for electrical power outages and extra cold days.  She conceded that Propane heat is nearly as expensive as electricity and is also not considered a future energy source.  She suggested more usage of the wood stove though I had to inform her of the smoke concern in the community.  She said that the electrical utility is concerned with excessive demand.  I wondered if she had the same concern about the demand placed on the utility by the mega-mansions going up around some parts here as she had about the minimal estimated demand of a small energy tight house.  She looked at her watch and had to suddenly leave for another meeting in her fancy crossover vehicle......

Speaking of wood stoves, I had a difficult time making up my mind on a suitable wood stove for backup heating in case of power failure, extreme cold, or romantic setting needs.  Initially I looked at a Danish made RAIS stove, however, the price tag for the stove itself plus the expensive stove pipe installation coupled with an inflexibility in installing the stove pipe in a timely fashion made me shop around at a much closer store overlooked before.  Bingo! They had a better stove for the same amount of money and they were very accommodating to our schedule needs.  The stove is from a German Company, Hase and it is the Bari model.



Window Heat Gain
In week 9 we looked at the heat requirement of the windowless house and found that at 32 degF 2800 BTU/hr would be required to keep it at a comfortable 68 degF.  In week 10 we looked at the solar energy or insolation (BTU/sqft-day), available in this part of Vermont. Today we will look at the many factors affecting passive heat gain from windows in the house.  We will not consider the reduced heat loss through the warmer walls on sunny days.

In week 11 we detailed four parameters for all of the "windows" or more precisely for all of the window and door penetrations of the house wall.  The rough opening (RO) defines the area of wall penetrated.  The U value is the parameter that defines the averaged heat loss through the complete window opening including the frame, sash, and any hardware (usually metal) for opening and closing the windows.  The SHGC  gives the percentage of normally incident solar energy falling on the complete window that actually manages to penetrate to the inside.  Non-glazed portions of the window and glazing losses such as reflection and solar heating of the glass will reduce this number.

To obtain the total heat loss due to the windows and doors of the first floor we just sum their UxRO products from week 11.  Doing this we arrive at 67.7BTU/hr-degF  which points out when compared to the heat loss of the walls, floor, and ceiling (72 BTU/hr-degF), that the windows are 48% of the heat loss at night.  This clearly shows that good shutters should clearly reduce night time heat loss.  Of course, the windows are also the main heat gain during the day, especially on cloudless days.

The north facing window will only see diffuse light most of the year with some north light at low incidence in summer.  The east and west windows will see nearly direct incidence light mostly in summer which is nice in the morning but may lead to too much heat later in the day.  Since there will be no active cooling (AC), inside summer temperatures will be modulated by opening windows during the cooler nights and closing them during the day when necessary.  Window shades will assist in diminishing the western sun. Having said all of the above, only the heating season needs will be addressed with only the south facing windows assumed to have solar gain.

To help us estimate the maximum amount of heat gained by the south facing window we first sum up their glazing area x SHGC products.  Doing this we arrive at 26.5 sqft.  Now, were direct sunlight to shine onto all windows at normal incidence on a clear day, we could capture around 8000 BTU/hr, considering that solar energy is optimistically around 300 BTU/hr-sqft at noon.  But as that isn't ever the case  for vertically mounted windows, we must subtract the reflected light which will vary as the sun travels on its path across the sky.  Our tax supported government offers for free a solar position and intensity calculator which shows all the factors involved in arriving at actual values of insolation for any place on earth.  Furthermore, our government provides for free, maps that show the energy incident on various types of fixed and tracking collectors for any period of the year, based on historical data. From this we can obtain, for example, monthly passive heat gains for vertical unobstructed south-facing windows. We can then compare that heat gain to the actual heating requirement for maintaining the house at 68 degF.   That is a task for the next posting.

Please send me comments that add to the discussion and correct any errors of comprehension.