Owners' Comment...

"Before moving here we lived in a reasonably nice 1920's home.  We were used to the drafts, noise levels, and maintenance issues.  We had been living in this new home for about 2 weeks when that record cold spell known as the Siberian Express occurred in January 1989.  Extreme winds and below zero temperatures came on hard just after dark.  We were inside, and we remained unaware of this until several hours later when we were sitting quietly,  gazing out the window at the stars.  We noticed an odd flickering light on the horizon.  At first, not yet familiar with the new place,  we thought it might be the lights of a faraway car.  Moments later we finally realized that the 40 foot tall spruce trees on the southern edge of our lot were whipping wildly  in the wind and causing a nearby light to "flicker."   We were astonished that it could be so windy (and cold) because we had felt no drafts and had had no acoustical clues.  That very sensory experience brought home to us the extraordinary difference these homes can make.  We never got cold during the Siberian Express..."

1. Overview
• Awards Relevance
• Optimization as a System
2. Performance Details
• Energy
• Structural Moisture
• Radon
• Ventilation
• Cost

Awards Relevance

This home received two national awards in 1991.  The awards were nearly simultanr knew of the other. One was for indoor air quality, specifically for control of radon. The other was for energy efficiency. The awards occurred at a time when there was significant industry debate about the compatibility of energy efficiency and indoor air quality in buildings. The home's performance has shown that a home need not sacrifice one for the other, but can have both in abundance.

Documentation

The home's ability to perform was measured. The Bonneville Power Administration, the Washington State Energy Office, and others installed monitoring equipment and data was collected to measure several aspects of performance over a one year period:

• Energy use for heating, cooling, and hot water.
• Air exchange rates for ventilation.
• Relative humidity.
• Moisture in walls, floor and attic.
• Radon gas in house and crawlspace.
• Cost of the mostly hidden energy features.

The building's physical systems had to satisfy each of several design goals. Energy performance was a critically important goal but could not be emphasized at the expense of durability, esthetics, etc.  The basic design goals were:

• Safety
• Durability
• Comfort
• Desired Aesthetics
• Indoor Air Quality
• Energy-Efficiency
• Economy

Insulation, efficient windows, and an airtight building shell to stop drafts: all are necessary for thermal comfort at a reasonable cost.

Appropriate ventilation is also necessary to provide some fresh outside air and remove indoor air containing moisture and pollutants typically generated in the home. Only a small amount of air exchange is needed, typically less than the amount transferred through an 'untight' building shell on a cold winter day, yet more than that transferred through the same shell on a windless summer day. Pressure differences caused by wind and temperature raise your heat bill in winter, and may raise your doctor bill in spring, summer and fall. The key is to tighten the shell and ventilate with a fan you can control. A small amount of continuous ventilation is usually best for economy, health, and comfort.

The outside air required for ventilation is provided with fans. If the shell is tight enough, this action will actually pressurize or depressurize the house, depending whether air is pushed into the building or pulled from it. These pressure effects can be strategically used to achieve desirable effects.

In typical older homes this issue of pressure impacts has been mostly ignored, occasionally with undesirable results. And in rare extreme cases the results have even been deadly. Typically radon gas has been sucked from the soil into homes by ignored pressure impacts. Also, ignored pressure impacts have sucked harmful combustion exhaust from furnaces, fireplaces, and gas water heaters into the home's occupied zone.

• The living space (occupied zone) is depressurized relative to the outdoor air, so that fresh outdoor air is continuously pulled into the home.  Most of the air is pulled in through a supply duct where it is filtered and then heated or cooled if necessary.  A small amount of outdoor air is pulled in through the inevitable cracks and openings that remain in the airtight building shell, with the very desirable result of preventing that moisture typically generated in the home from getting into the walls and attic.
• The crawlspace below the occupied zone is pressurized relative to the air pressure outdoors and in the occupied zone. The primary benefit of the pressurized crawlspace is that it robustly prevents entry of radon, moisture, molds, fungi and other pollutants generally in the soil beneath the home. This exhaust air, containing pollutants and moisture generated in the home, is released outdoors after passing through the crawlspace. The exit duct is deliberately sized to maintain the desired pressure in the crawlspace zone. This polluted air contains one desirable thing: heat. So as the air exits the crawlspace, much of the heat it contains is extracted by a small heat pump and transferred to the incoming fresh air.

This pressurize-with-ventilation strategy is contributing to successful solution of all the design goals listed above except aesthetics, to which it does no harm.

There are several important elements necessary to the successful use of this strategy.  For example, a good air-barrier must be created between the two pressure zones to prevent the transfer of crawlspace air back into the occupied zone.  Also, it is important to prevent the transfer of moisture from the crawlspace to the insulated perimeter of the floor.

Finally, there are several elements that can be added to further enhance the benefits to be gained. For example, in some climates the addition of earth tubes to precondition the incoming fresh air may be considered.

Energy

The building is of double-wall construction and the wall thermal resistance is approximately R45.  This insulation level extends from the ceiling to the concrete footing, except as interrupted by doors and windows.  The windows are R4. Fifty-five percent of the window area faces south. The ceiling is insulated to R60. The continuous thermal envelope is completed by R25 fiberglass batt insulation laid directly upon the ground (over a gravel capillary break).

A continuous air barrier was established with the interior drywall by gasketing the drywall to the wood framing and sealing any penetrations through the drywall barrier.  Upon completion of construction, the building had a tested air leakage rate of 1.2  air changes per hour (ACH) at an induced indoor/outdoor pressure-difference of 50 pascals. The measured Pacific Northwest average is 9.3 ACH at 50 pascals.

The home was occupied in January 1989. Utility billing data for the first year shows the total electricity usage for all purposes was 13,666 Kwh for 1780 square feet of conditioned floor area. This amounts to an average of $656 for lighting, appliances, water heating, space heating, space cooling, continuous ventilation, and pressure-difference control.

The building was constructed to the specifications of the Residential Construction Demonstration Program (RCDP), a program in the Northwest of the Bonneville Power Administration (BPA).  As an RCDP Future House, it was extensively monitored for one year.  Zonal electric-resistance space heating was separately submetered. It required 2511 Kwh ($120 @ $.048/Kwh) for a one year period.

However, the heat recovery ventilation (HRV) system supplies a significant portion of the heat load, as well as significant water heating, space cooling, ventilation, and pressure-difference control.  The space heating contribution had to be indirectly measured, in this manner:

The measured energy use for space heating in 158 RCDP homes built in the  Northwest in the prior year averaged 33% of the average total home energy use.  If the assumption is made that this RCDP Future home would likely use the same percentage for space heating, then we can conclude that this home used 1/3($656) = $216/year for space heat.

Structural Moisture

Transfer of moisture from indoors to the building shell can reduce insulation values, result in mold and mildew growth, damage paint, and in extreme cases cause structural wood to rot.  In this home the occupied zone was optimally protected by depressurization. However the pressurized crawlspace would be particularly vulnerable to moisture problems if the installed air and vapor barriers were not adequate.  A moisture monitoring system was installed to verify and validate proper performance.

Humidity sensors were calibrated and placed inside structural wood framing in six locations.   An attempt was made to select locations with the greatest potential vulnerability to moisture;  generally downwind from the prevailing wind direction, north shaded areas, and (for walls) high in the building.

• Attic top chord, north side, near center of building.
• Attic bottom chord, north side, near center of building.
• East wall exterior framing stud, north side on upper level.
• West wall exterior framing stud, north side on upper level and near electrical outlet.
• Warm joist in heated crawlspace, center of building.
• Cold joist in crawlspace, next to north rim joist, cold side of air and vapor barriers.

The radon source of concern is soil-air. The primary goal with regard to control of indoor radon is the prevention of soil-air entry into the indoor air.  Radon is a given component of soil-air though its concentration both varies from one site to another and is not readily predictable.  In one case, measurements of soil radon within a distance of 9 meters varied by a factor of 250.  Hence, the degree of soil-air entry control required is neither constant nor predictable.

Two conditions are necessary for soil-air entry:

• There must be openings in the building envelope that couple the soil-air to the indoor air.
• There must be a driving force, a pressure-difference that results in a flow from the soil-air zone into the indoor air zone.

This home was able to demonstrate very robust control of radon concentrations.  When the mechanical ventilation system operated normally, the indoor radon concentration was very low. A continuous radon measurement (alpha track monitor) for the first year indicated an annual average radon level of .7 PCi/l, reasonably close to the average outdoor level. The U.S. Environmental Protection Agency recommends homeowners take action to reduce indoor radon levels at 4 PCi/l.

Deliberate experimental manipulations of the crawlspace pressures demonstrated that crawlspace radon levels could be radically altered.  For example depressurizing the crawlspace could generate crawlspace radon levels of 50 or 100 pCi/l.

Ventilation

A sophisticated energy reclaiming ventilation system was installed. A commercially available integrated residential hvac system provides ventilating, partial space heating, space cooling, and water heating (HPVAC-80 Envirovent™ from Therma-stor, Inc.).  The unit consists of a water heating tank and a space conditioning module (SCM).  The SCM contains 2 fans and 2 coils, and utilizes a reversible refrigeration cycle to provide heating or cooling via the same ductwork.  During the winter heating cycle, heat is taken from exhausting stale air and delivered to either the hot water tank or the mixed air supply.  In summer, heat is taken from the mixed air supply and either exhausted outside or used to heat water.

On the fresh air supply side, two 150 feet long by 4 inch diameter earth tubes provide pre heated (or cooled if summer) outdoor air which is mixed with recirculating air before it passes over the SCM's supply side coil and is further heated (or coolde in summer), then distributed to living areas.  The earth tubes are buried approximately about 4 feet below grade and serve to temper both winter and summer air.

Fan (1) is one of the SCM's fans. It draws the fresh outdoor air through the earth tubes, across the heat pump coil, then distributes this conditioned air to the house. Fan (1) operates only when the SCM is in space heating or space cooling mode.

On the stale air exhaust side, stale indoor air is removed from kitchen and bathrooms by fan (2) and ducted to the crawlspace.  From this point it remains isolated from the occupied zone.  The stale air travel across the insulated crawlspace keeps it warm. THe air then exits via a sealed duct which leads to the SCM.  Continuous operation of fan (3) in the SCM is necessary to maintain a lower pressure inside the SCM than in the mechanical room, so that no contaminated exhaust air can leak back into the indoor air.  As the air passes through the SCM heat is extracted from it by the SCM's heat pump coil (if needed to either heat water or air).  After passing through the SCM the air exits outside above the roof.

Fan (2) operates continuously. It depressurizes the occupied zone and removes stale air. It pressurizes the crawlspace. It is adjusted to operate at a higher flow than fan (3) in order to maintain pressure in the crawlspace.

Fan (3) is the SCM's other fan. It draws stale air from the crawlspace across the heat pump coil, then exhausts it outdoors.  It must operate continuously to maintain a lower pressure in the SCM’s housing than in the house.

The building cost $80,824 in 1988, approximately $45 to $48 per square foot. Several of the design elements serve to advantage multiple design goals.  This should be recognized when attempting to allocate costs.  For example, soil-air may also contain other pollutants of concern, such as garbage gasses (methane), herbicides, fungicides, pesticides, spores of soil fungi, etc.  The cost of preventing soil-air entry should be life-cycled against the delivery of several health benefits.  Also, in this particular project, the cost of mechanical ventilation and the tight envelope must also be apportioned to comfort, energy performance, radon control, moisture control, and control of other pollutants.

The total extra cost of the installed elements that contributed to energy savings, improved indoor air quality, comfort, and durability was $8,177. Energy savings only would achieve a simple payback in 21 years at $0.05/kWh, in 7 years at $0.15/kWh.  Reasonable allocation of these costs to the other benefits achieved may reduuce the payback by half.