Building Science and Technology

We are lucky to have a team of experienced builders and technical advisers on our project, two of whom are Jeff Rhodin from Sustainable Analytics (HERS/Energy Star Certifier) and Ken Neuhauser at Building Science Corporation.  Sometimes I ask a seemingly innocent question and I get a detailed answer that makes me wondering whether I should be paying these guys tuition.  It makes my brain hurt but it's educational and has help us make informed decisions along the way.  The building techniques that have gone into our home is not a finger to a wind exercise but rather construction based on sound science and building principles.  Below are some of the technologies and products that are important components to our energy strategy.

Insulation

Lapolla Industries

There are many different forms of insulation of different thicknesses with each having pros and cons.  There are many considerations that go into selecting the most appropriate for a particular application including cost, long term efficacy and air sealing.  Not all R-values are the same and some of the pitfalls of the most common form of insulation, fiberglass has pitfalls such as the lost of R-value when it is compressed.  In addition, any form of insulation needs to be installed correctly to reap its benefits.  Where possible we opted for closed cell foam from Lapolla Industries as it offered the best combination of R-value, minimal thickness and air sealing.  For cost reasons, we were not able to insulate everything with foam but used foam products extensively including 4" of rigid foam on the exterior of the home.




Insulation R-Values


HRV

Heat recovery ventilation (HRV), also known as a Mechanical Ventilation Heat Recovery (MVHR), is ventilation system that employs a counter-flow heat exchanger between the inbound and outbound air flow. In other words it transfers heat in cold weather from stale air being vented out and warms the fresh air before introducing it into your home.  The reverse occurs in warm weather.  An HRV provides fresh air and improved climate control while also saving energy by reducing the heating (or cooling) requirements. Energy recovery ventilators (ERVs) are closely related, however ERVs also transfer the humidity level of the exhaust air to the intake air.

Considerations in selecting an HRV:

When keeping energy use in mind, it's important to select the appropriate sized HRV or ERV.  The following is how we (by we, I mean Building Science Corp) determined the appropriate ventilation rate:

The ASHRAE standard that applies to residential ventilation is ASHRAE Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. The ventilation rate is calculated by the formula:

Qfan = 0.01Afloor + 7.5(Nbr + 1)

where

Qfan = fan flow rate in cubic feet per minute (cfm)
Afloor = floor area in square feet (ft2)
Nbr = number of bedrooms; not to be less than one.

For our home the ASHRAE 62.2 rate is:

(0.01  x 2,410) + 7.5 x (3 + 1) = 54.1 cfm

Under the ASHRAE standard, this ventilation rate is applicable as long as the full ventilation rate is delivered every 3 hours.  This means, the 54.1 cfm could be met by a ventilation system flow rate of 162.3 cfm for 1 hour every 3 hours.  And also, the rate could be met by operating that same 162.3 cfm system for 20 minutes every hour (or 10 minutes every half hour).  Note, the measured air leakage rate for a home does not impact the recommended ventilation rate unless the house is very leaky. 

Here's the really important part.  "Through the experience of over 100,000 homes in the Building America program with distributed ventilation systems, it was found that ventilation rates at 50-60% of the ASHRAE 62.2 ventilation rate provided satisfactory moisture and odor control.  If local code does not dictate a higher ventilation rate, BSC’s recommendation is to design a distributed ventilation system to be able to deliver the ASHRAE 62.2 ventilation rate and then operate the ventilation system at 50-60% of that rate.  The ventilation rate can then be increased if deemed necessary (e.g. during a house party, contaminant/odor event) but will not, by default, use more energy for ventilation than experience has shown to be necessary.  The ventilation rate of the ventilation system is modulated by either speed control (for multiple speed systems) or duty cycle.

There is a small power spike incurred when electric motors start.  This power (measured in Watts) spike will be of very short duration and, therefore, translates into very little energy (measured in Watt-hours) relative to a period of steady state operation.  For a given fan effectiveness in cfm/Watt, there is virtually no difference between operating a fan to give 3X cfm for 20 minutes and hour on the one hand or X cfm continuously on the other.  Operating the fan on a duty cycle that is less than 100% provides flexibility to increase the ventilation rate if desired.  The duty cycle control also allows the ventilation rate to be “trimmed” to an appropriate rate without excess.  A unit that delivers 1 cfm/Watt will use twice the energy for the same ventilation rate as one that delivers 2 cfm/Watt.  Therefore, a larger ventilation unit operating at 50% duty cycle and 2 cfm/watt is going to use half the energy energy as a unit that must run continuously at 1 cfm/Watt to achieve the same average ventilation rate."

Considerations in selecting an HRV vs an ERV:

"The primary means of controlling excess humidity in a heating dominated climate is through ventilation.  By recovering moisture from the exhaust air, an ERV will be inherently less effective at removing moisture from the home and will, therefore, require longer run time to achieve the same level of moisture control.  If an ERV is used for the ventilation system, there should be separate exhaust fans for the kitchen and bathrooms: an ERV would redistribute a portion of the excess humidity from these spaces back into the house. 

When a home is mechanically dehumidified below the (absolute) humidity of outdoor air, an ERV can reduce the latent (moisture) load of ventilation by exchanging some of the incoming moisture with the exhaust stream.  In heating-dominated climates any dehumidification-season advantages of the ERV are typically going to be vastly overwhelmed by the need for more run time in the heating season to control humidity when outside air is drier."

Andrew's Simplified Answer:

Use a slightly larger HRV (not an ERV) than the expected capacity and run it at 50-60% capacity to hit target ventilation rates.  Select a unit with an ECM motor to minimize energy use.

Air Duct Sealing

The current Energy Star for Homes standard for total duct leakage
is 6 cfm per 100 sq/ft at an air pressure of 25 pascals.  In other words if you pressurize your air ducts (see photo on right), you should only lose less than 6 cfm per 100 sq/ft of air to gaps or air leaks in the ducts throughout the house.  So in the case of a 2,410 sq/ft house, divide the sq. footage by 100 and then multiply by the measured air pressure at the ducts.  The Version 3 energy star standard further reduces this standard to 4 cfm per 100 sq. ft.

As of 10/1/10, our ducts were tested at 3.48 cfm and it is expected the leakage rate can be further reduced.  Cutting air loss to gaps in the ducts were achieved by ensuring ducts were properly sealed with mastic and tape at every joint.  Gaps around returns and air registered were sealed with copious amounts of foam, caulk and tape to ensure there was a good seal preventing air loss to gaps between the vent and the subfloor.