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Heat and Moisture Transfer in Wood-Based Wall Construction: Measured Versus Predicted.


pdf icon 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.