Heat and Moisture Transfer in Wood-Based Wall Construction: Measured Versus Predicted.
Heat and Moisture Transfer in Wood-Based Wall
Construction: Measured Versus Predicted.
(4040 K)
Zarr, R. R.; Burch, D. M.; Fanney, A. H.
NIST BSS 173; 83 p. February 1995.
Available from:
National Technical Information Service
Order number: PB95-200655
Keywords:
heat transfer; mass transfer; moisture transfer; walls;
wood; apparatus; building science; building technology;
calibrated hot box; computer models; experiments; MOIST;
relative humidity; temperature; thermal resistance;
validation; thermal analysis
Abstract:
The National Institute of Standards and Technology has
developed a personal computer program, MOIST, that
predicts the transient one-dimensional heat and moisture
transfer in building envelopes. MOIST allows the user
to vary building materials, their relative placement
within the building envelope, and the geographic
location of the building. For a given geometry and
location, it predicts the resulting moisture
accumulation and transfer across each construction layer
as a function of time. This report describes a
comprehensive laboratory study to verify the accuracy of
MOIST for 12 different wall specimens. The rate of heat
transfer through each of the 12 wall specimens was
measured. The moisture content of the exterior
construction materials were measured for eight of the
twelve wall specimens. For the remaining four walls,
the relative humidity level was measured at the interior
side of the exterior sheathing. The measured heat
transfer rates, moisture content levels and relative
humidities were compared to the predictions of MOIST.
In general, the agreement between MOIST and the
experimental measurements was good. The moisture
content predicted by MOIST was within one percent of the
measured values for seven of the eight walls that
contained moisture content sensors. The measured
relative humidities for two of the remaining four walls
agreed well with the MOIST predictions. The relative
humidity measurements from the ohter two walls could not
be compared to MOIST since the walls were constructed
with vapor retarder defects that introduced
two-dimensional effects. The heat flux predicted by
MOIST was within ten percent of the values measured
under steady-state conditions. When the walls were
subjected to a series of diurnal ambient temperature
cycles, the root-mean-square difference between the
measured and predicted heat flux values ranged from four
to fifteen percent. MOIST heat flux predictions were
also in close agreement with the values predicted by the
Thermal Analysis Research Program (TARP). A comparison
was made between measured steady-state thermal
resistances and corresponding calculated values using
procedures recommended by The American Society of
Heating, Refrigerating, and Air Conditioning Engineers
(ASHRAE). The ASHRAE calculations agreed with the
measured values within thirteen percent.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899