INTRODUCTION
1800 Ft2 4 BEDROOM
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Nuess, R. M. and Prill R. J.
Washington State Energy Office
Energy Extension Service
N 1212 Washington, Suite 106
Spokane WA 99201
ABSTRACT
The normal operation of a continuous mechanical ventilation system, incorporated into a relatively airtight house, and designed to control pressure-differences, has been demonstrated to provide sufficient control of radon entry in a two story residential building.
This was accomplished via a "two-cell barrier-enhanced pressure-difference control system."
Ventilation rates, energy usage, moisture levels, pressure-differences, and radon concentrations were monitored. Changes in radon concentrations in several building locations, as a function of distinct pressure-difference configurations, have been measured.
Indications are that this design offers the new residential construction industry an opportunity to realize affordable control of radon entry, while simultaneously optimizing potentials for moisture control, energy efficiency, and control of other indoor air pollutants.
This project explores the use of an airtight building envelope (and separately isolated airtight crawlspace) integrated with a continuously operating mechanical ventilation system, to enhance pressure-difference control strategies for minimizing soil-air entry into the indoor air.
This approach seeks to obtain robust control of radon entry, while concurrently optimizing potentials for several building design goals including: moisture control, energy efficiency, control of other indoor air pollutants.
There are several primary design goals for the environment control system "house:" safety, comfort, durability, healthy indoor air, and energy efficiency. These goals are not only increasingly achievable, but can be mutually advantaged in a manner that can reduce net system cost.
A systems approach that seeks to optimize a building's performance with regard to several desirable performance qualities might well include several very successful radon solutions.
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 (1). Hence, the degree of soil-air entry control required is neither constant nor predictable.
Two conditions are necessary for soil-air entry:
The tight building envelope, coupled with a properly designed mechanical ventilation system, can play a central role in a systems approach that incorporates pressure-difference control to limit soil gas entry. The tighter the air barriers of the system, the more effective the pressure-difference control for a given amount of fan power. This dovetails nicely with the desirable advantages of a tight building envelope for several other building performance purposes, including:
The normal operation of a commercially available continuous mechanical ventilation system incorporated into a tight house, and designed to control pressure-differences, can provide sufficient day-to-day control of those pressure-differences (induced by weather, internal household activities, and mechanical systems) to prevent entry of radon and other soil-air pollutants. This can be reasonably accomplished by developing a "two-cell, barrier-enhanced pressure-difference control system." (4).
In 1988, a tightly sealed and energy efficient two-story residential building was constructed with the intent to exceed any energy performance standards currently in place in the U.S. The building was among those instrumented and continuously monitored for one year as part of the Residential Construction Demonstration Program (RCDP), a multipurpose research and development effort of the Bonneville Power Administration and the Washington State Energy Office. As an RCDP Cycle II Future House, the expected energy performance of the building was designed to exceed that required by the Northwest Power Planning Council's Model Conservation Standards by 30%.
The building was constructed in Spokane, WA. Spokane has a winter outdoor design temperature of 4°F (-15°C), 6882 normal heating degree days, and 411 normal cooling degree days. Spokane weather has the characteristics of a mild arid climate in the summer and a cold coastal climate in winter. Winter solar potentials are limited by both the climate and the site. The building was calculated to have an annual need of 2.5 kWh/ft2 (97 MJ/m2) for space heating.
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. 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. One year later it was tested at 1.4 ACH at 50 pascals. The measured Pacific Northwest average is 9.3 ACH at 50 pascals (5). The vapor retarder was established on the interior surface of the drywall with a rated paint. The glue in the laminated subflooring provided the floor vapor retarder.
The building is divided into two distinct cells, that are atmospherically decoupled from both each other and the outdoor air . The tightness and isolation of these two "cells" enables pressure-difference control with the mechanical ventilation system (and prevents contamination of air in cell 1 by air in cell 2. Cell 1 contains all occupied space, so that the breathable indoor air is contained in cell 1. The volume of cell 1 is 16,500 ft3 (467 m3). Cell 2 is a plenum by which stale air from cell 1 is removed. Though atmospherically decoupled, it is thermally coupled to cell 1, so it provides warm floors. Cell 2 adds another 3000 ft3 (85 m3) to the conditioned volume.
The first floor subfloor was the selected air barrier between cell 1 and cell 2. All joints in the tongue and groove exterior grade plywood were sealed with urethane sealant during installation. Special care was taken to identify and seal any holes created in this barrier by the construction process (eg; temporary nailing for wall bracing, sawhorses, measuring and cutting tables). A tracer gas was injected into cell 2 prior to carpet installation and two small air leaks were located using a detection instrument.
A small (5000 to 7000 btuh) commercially available integrated residential heat recovery ventilation system (HPV) provides continuous ventilation, partial space heating, space cooling, water heating, as well as the desired pressure-differences (6). The unit consists of a water heating tank and a space conditioning module (SCM). The SCM contains 2 constant speed fans, 2 coils, and utilizes a reversible refrigeration cycle to provide heating or cooling via the same ductwork. During the winter heating cycle, heat is extracted from stale exhaust air and delivered to either the domestic hot water tank or the mixed air supply. In summer, heat is extracted from the mixed air supply and either exhausted outside or used to heat domestic water.
On the exhaust air side, stale indoor air is removed from kitchen and bathrooms by fan F1 and ducted to cell 2. From this point it remains isolated from cell 1. The stale air travels across cell 2, then exits via a sealed duct which leads to the SCM. Continuous operation of fan F2 (SCM exhaust fan) is necessary to maintain a lower pressure inside the SCM than in the mechanical room, so that no leakage back into the indoor air occurs. After passing through the SCM the air is exhausted above the roof line.
TWO-CELL,
BARRIER ENHANCED PRESSURE-DIFFERENCE CONTROL
Cells 1 and 2 are isolated from each other, from the outdoor air, and from the indoor air; by accessible and maintainable air barriers. Sealed ducts allow controlled air passage. Continuous mechanical ventilation removes stale air from cell 1 and delivers it outside via cell 2. Depending on which fans are selected to operate, cell 2 can be either pressurized or depressurized relative to cell 1 and/or the soil-air. This project incorporated four fans in the ventilation system in order to enable comparison between these two approaches, as well as other possible ventilation and pressure-difference configurations:
ENERGY
PERFORMANCE
Limited data is available at this time and results should be considered
preliminary.
The building was completed and occupied during January of 1989.
Shortly thereafter extensive energy performance monitoring was begun by
the Washington State Energy Office, via a subcontract with W.S. Fleming
Inc. Selected air and water temperatures, air and water flows, relative
humidities, and electrical energy usage have been monitored and recorded
(six second averages) on a multi-channel datalogger. Data collection
for the first year has been completed. Once the data are analysed
a more complete energy performance profile will become available.
Zoned electric resistance heaters were separately submetered. The integrated heat recovery ventilation system, which provides continuous ventilation, partial space heating, space cooling and water heating was also submetered. Electrical main and submeter data were recorded by the author. For the one year period between March 4, 1989 and March 3, 1990, electric resistance heating used 1.4 kWh/ft2 (54 MJ/m2). The HPV unit used 3.5 kWh/ft2 (135 MJ/m2) for continuous ventilation, space heating and cooling, water heating, and pressure-difference control. The HPV system is estimated to provide 44% of the space heating load. This brings the total cost of space heating to $216/year. The measured average Kwh consumption for heating conventional electrically heated homes in the Pacific Northwest is 12,420 Kwh, which amounts to $596 (5).
Humidity sensors (7) were calibrated and placed inside structural wood framing in six locations prior to completion of construction. Two sensors were placed in the attic, two in the walls, and two in the floor of cell 1. An attempt was made to select locations with the greatest moisture potential; generally downwind from the prevailing wind direction, shaded areas on the north side, and (for walls) high in the building:
VENTILATION
PERFORMANCE DYNAMICS
The clock timer on the HPV unit is set to provide continuous exhaust ventilation, so the unit's exhaust fan (F2) drawing stale air from cell 2 is always activated. If there is a demand for water heating the compressor also operates. If there is a demand for space heat or cooling the supply fan (F3) also activates. Both fans operate at a constant speed and flows must be adjusted by dampers.
The baseline mode of operation has been to adjust fan F1 to maintain a slightly lower pressure at the ceiling of cell 1 than that outside (thus also pressurizing cell 2). This typically resulted in a 2-13 Pa lower pressure at the ceiling of cell 1 relative to outdoors during space heating. The neutral pressure plane was maintained above the ceiling of cell 1, there was no exfiltration, and therefore all air exchange was induced by the HPV unit. The resultant pressure in cell 2 was generally 3 to 7 pascals greater than the pressure in cell 1, during space heating mode of operation. The supply air fan (F3) operates during space heating and cooling, and tends to pressurize the building, by increasing the flow of outside air through the earth tubes. However, when it is off (during periods of ventilation and water heating), fans F1 and F2 remain on, so the cell 1/cell 2 pressure-difference increases (10 to 20 Pa). Additionally, a manual timer switch in the bathrooms enables short pulses of greatly increased ventilation by boosting fan F1 to full power, and the cell 1/cell 2 pressure-differences become even larger (45 to 60 Pa).
Intermittent measurements by the author indicate that the mechanically induced air exchange rate for the first year has been roughly .6 ACH, or equivalent outdoor air supply for 11 persons at 15 cfm (7 L/s) per person. Since the pressure in cell 1 was lower than the pressure outside (therefore no exfiltration), all the air leaving cell 1 had to pass through fan F1. A Kurz Model 435 Linear Air Velocity Transducer was used to measure the mass flow of air in the duct downstream of fan F1.
The purpose of fan F4 is to pull outdoor air through the earth tubes and provide adequate outdoor air supply. It was found to be unnecessary and was not operated. The negative pressure of cell 1 induced sufficient flow in the earth tubes. When the unit's supply fan (F3) did not operate (ventilation and water heating modes) the earth tube flow averaged 27 cfm (13 L/s). When the supply fan operated the average earth tube flow was 57 cfm (27 L/s). Approximately one third of the outside air supply was via the earth tube. Envelope infiltration, due to the induced negative pressure of cell 1, provided the remaining outside air.
The pressure-difference control under these conditions appears to have been very robust. Though pressure-differences were not continuously monitored, they were frequently observed during cold and windy periods. No reversals of the desired pressure-difference directions were observed.
RADON PHASE ONE
Five continuous radon monitors (CRMs) were placed in the same location for five days to establish a comparison baseline. The monitors were then placed in five different locations in the building for a thirteen day period between November 11, 1989 and November 23, 1989. Fan F1 was off during the first part of of this period, so that fan F2 depressurized both cell 1 and cell 2 relative to outside.
After the first 112 hours, Fan F1 was activated and adjusted to maintain a slightly lower pressure at the ceiling of cell 1 relative to outside during the space heating mode. This resulted in a 10 to 60 Pa greater pressure in cell 2 (depending on the HPV system's operating mode at time of read).
Cumulative CRM data were recorded intermittently and averaged over each
elapsed time period. Thirty-two readings were recorded. Average
radon levels in cell 2 decreased dramatically when fan F2 was activated;
average radon levels in cell 1 also showed a tendency to decrease (Figure
5).
Figure 5. Radon levels in five locations.
RADON PHASE TWO
Two CRMs were placed in separate locations within cell 1 (CRMs 1a and 1b) and two CRMs were placed in separate locations within cell 2 (CRMs 2a and 2b). Hourly radon averages were recorded for the three month period between 17 February and 21 May 1990. During this period, the baseline system configuration (continuously pressurize and flush cell 2 while depressurizing cell 1) was held constant for the first 23 days. Then 9 deliberate alterations in system configuration were made. After each alteration the system was returned to the baseline configuration, before the next alteration was initiated. Upon completion of the 9 alterations the system was returned to the baseline configuration for 22 days.
The alterations had powerful effects upon Cell 2 radon levels, and clear effects upon radon levels in Cell 1. Effects of wind, rainfall, and temperature did not appear to have noticeable influence on radon levels, except that Cell 2 radon levels may have showed some indication of response to temperature. Nonetheless the effect was subtle relative to the effects of system operation.
The baseline system configuration and the alterations to it are discussed below, and referenced in the following graphs.
Alteration 1. Fan F1 was turned off on 3/11 at 10:20 am. The pressure in cell 2 had been greater than the pressure in cell 1, but now shifted to about 30 pascals lower than cell 1, since F2 now pulled air through the cell 2 plenum. In two hours radon levels in cell 2 had increased by a factor of three. Radon levels in cell 2 averaged 29 pCi/l. Radon levels in cell 1 increased also to an average of about 2.5 pCi/l. On 3/17, six days later, fan F1 was turned back on. Pressure in cell 2 returned to 3-4 pascals greater than cell 1. Radon levels in cell 2 dropped by a factor of three in three hours and returned to baseline levels.
Alteration 3. On 4/4 fan F1 was turned off for 62 hours. Also F2 and F3 were turned off. There was no ventilation. The ductwork joining the two cells was sealed to atmospherically decouple the cells from each other. Radon levels in cell 2 rose gradually (whereas in the previous alterations they had risen abruptly) to a peak of 66 pCi/l. Then F1, F2, and F3 were reactivated and radon levels in both cells returned abruptly to baseline levels. The pattern of a more gradual rise in radon levels also seemed to occur in the cell 1 radon levels which rose to over 5 pCi/l. The gradual rise is assumed to be attributed to soil recharging and the slower response related to the lesser stack pressures.
The three fans were activated 62 hours after the initial alteration. However, cell 2 remained atmospherically decoupled from the ventilation cycle (F2 drew exhaust air from cell 1), as well as isolated from cell 1 by the sealed ductwork. The condition was that cell 2 was pressurized by fan F1 but there was no flushing of the air in cell 2. The resultant pressure in cell 2 was 45 pascals greater than that in cell 1. Radon levels returned to baseline in about 6 hours, hence were already at baseline levels when the ductwork was reconnected and the system configuration returned to baseline.
Baseline. The baseline operating condition was reestablished for 22 days, from 4/29 to 5/21. During this period the average radon concentration in cell 1 was less than one pCi/l. The average radon concentration in cell 2 was about 2.5 pCi/l.
There is some uncertainty associated with the radon measurements. The CRMs were research instruments that were not calibrated immediately prior to these measurements. However, they were compared to each other by operating them in the same location for six days at the beginning of this project. All 4 CRMs tracked radon levels consistently (Figure 9).
CRMs 1a and 2a did respond in a consistent manner throughout the entire measurement period. Fortunately one was located in cell 1 and the other in cell 2. It was also fortunate that CRM 1a -- which was recording the lowest and least variable radon levels -- was also the CRM that continued to have a correctly operating pump and was compared to the calibrated Pylon instrument after the study. Both the consistent response of these two remaining CRMs to the repetitive nature of alteration #4, and the similarity of their ending baseline responses to their starting baseline responses, suggest that these two CRMs responded with acceptable accuracy to radon fluctuations throughout the study.
_________________
* This study was not funded and was conducted as time allowed.
It is not known when the the pump on CRM 2a began to fail. The
data set suggests that CRM 2a was operating correctly during the study
period and likely failed after the study period.
It is very difficult to assign costs to the radon prevention and mitigation features of this building, since virtually all the features that control radon also enhance energy performance, durability, comfort, and the control of other pollutants. The simple energy-only payback for these features is 10 to 20 years. The building's useful life has also been extended due to such features as the vented rain screen designed to extend siding life, and the elimination of air transported moisture into the exterior walls. Since these features primarily address energy, and there is a clear energy payback for them, it can be argued that there is no incremental cost for the control of radon entry. The building cost $80,824, approximately $45 to $48 per square foot.
REFERENCES
1. U.S. EPA & New York State Energy Office, Reducing Radon in Structures/Training Manual, unit 2, page 4.
2. Brennan T (1990) Evaluation of Radon Resistant New Construction Techniques, Preprints of The 1990 International Symposium on Radon and Radon Reduction Technology, Vol. 5, p.1.
3. White J H (1988) Radon--Just Another Soil Gas Pollutant? Presented at 81st Annual Meeting of the Air Pollution Control Authority, Dallas TX, USA, June 1988, p.5.
4. This concept was built upon conversations with Bo Adamson, Lund Institute of Technology, Sweden. Also discussions by Sven-Olov Ericson and Hannes Schmied (1987) Modified Design in New Construction Prevents Infiltration of Soil Gas That Carries Radon, American Chemical Society, 0097-6156/87/0331-0526.
5. Sensible Living for the 90's, Bonneville Power Administration, BPA DOE/BP-23821-4 10/90.
6. HPVAC-80 Envirovent, DEC International Inc., Therma-Stor Products Group, Madison, WI.
7. HS-1 Humidity Sensors and Model J-3 Moisture Meter, Delmhorst Instrument Co., Boonton, New Jersey, U.S.A.