Experimental Verification of a Moisture and Heat Transfer Model in the Hygroscopic Regime.
Experimental Verification of a Moisture and Heat
Transfer Model in the Hygroscopic Regime.
Burch, D. M.; Zarr, R. R.; Fanney, A. H.
U.S. Department of Energy (DOE); Oak Ridge National
Laboratory (ORNL); American Society of Heating,
Refrigerating and Air Conditioning Engineers (ASHRAE);
Building Environment and Thermal Envelope Council
(BETEC). Thermal Performance of the Exterior Envelopes
of Building VI Conference. Proceedings. Thermal VI.
December 4-8, 1995, Clearwater Beach, FL, 273-282 pp,
moisture; heat transfer; buildings; building envelopes;
MOIST; moisture transfer; calibrated hot box;
manufactured housing; moisture analysis
The National Institute of Standards and Technology
(NIST) has developed a personal computer model, called
MOIST, for predicting the transient moisture and heat
transfer within building envelopes. This paper
summarizes selected results from a comprehensive
laboratory experiment conducted to verify the accuracy
of the computer model in the hygroscopic regime. This
paper discusses three different multilayer wall
specimens installed in a calibrated hot box. The
exterior surface of the wall specimens were first
exposed to both steady and time-dependent winter
conditions, while their interior surfaces were
maintained at 21DG (70DGF and 50% relative humidity.
These boundary conditions caused moisture from the
interior environment to permeate into the wall specimens
and accumulate in their exterior construction materials.
Subsequently, the exterior air temperature was elevated
to 32DGC (90DGF), and the exterior construction materials
lost moisture to the interior environment. The moisture
content within the exterior construction materials and
the heat transfer rate at the inside surface of the wall
specimens were measured and compared to computer
predictions. The moisture and heat transfer properties
for the construction materials comprising the wall
specimens were independently measured and used as input
to the computer model. The agreement between predicted
and measured moisture contents was within 1.1% moisture
content. Predicted and measured heat transfer rates
also were in close agreement. Accumulated moisture was
observed to have little effect on heat transfer because
moisture did not accumulate above the hygroscopic limit
(i.e., the so-called fiber saturation point) and
capillary water did not exist within the pore space of
the materials. The insulation remained relatively dry,
and the boundary conditions did not give rise to a
latent heat effect (i.e., water was not induced to
evaporate from one part of the construction and condense
in another part).