An Investigation of Water Transport from Building Walls
Objective was to find out how do Hitex Diamond-clad building walls dry?
What are the drying mechanisms, rates of drying, optimum size of Diamond grooves for drying, test building papers for suitability, benchmark RH and vapor pressures
Drying research complex, but measurable
Results showed there was complex drying mechanisms but drying effective, 10mm grooves best, drying best with no wall insulation, RH can nominally be maintained below 55\% with certain building papers, overlap of Diamond down slab affects drying, drying found: mean 0.19h/gm(lineal)
Key facts
- Tests performed by: Bryan Holyoake supervised by Prof. Geoff Duffy
- Tests performed at University of Auckland
- Research conducted as a 4th Year undergraduate research program at the University of Auckland School of Chemical and Materials and Materials Engineering
- Accosiated Research Bulletin: RB 310 Diffusion Chamber
Extended Abstract
Most building walls leak, and let in moisture. Walls rot in the presence of excess moisture due to the cultivation of mould. This has recently been occurring in New Zealand, and has been labelled the Leaky Building Crisis. Currently walls are made airtight and any moisture that leaks into a wall cavity is trapped there, creating a high humidity environment. If left like this, the area is perfect for the establishment and growth of rot-causing mould. To prevent this occurring, there is a need for an escape path for moisture from a wall enclosure.
Specific aims of research
This study looks to determine the moisture loss rate from a building wall system clad in polystyrene-based HITEX Diamond cavity system cladding and investigates possible mechanisms affecting the rate of moisture egress out of a building wall. This study is not definitive. Previous studies by Professor Austin from the Physics Department of the University of Auckland have shown that moisture is lost from a HITEX-clad wall at approximately 2g/hr. Other studies on the Leaky Building Crisis have focussed around computer modelling of phenomenon and the prevention of water from penetrating the wall cavity. However practicality and research shows that most buildings will let moisture into the walls at some point in their life, and so must be addressed. A dry wall is considered to be a wall that does not support the growth of mould. This is equivalent to a timber moisture content of 14\% and a relative humidity in the air of 65\%. If wetted, a wall needs to be dried out within 30 days to prevent mould growth. Three groups of experiments were conducted: one central experiment supported by two auxiliary ones. The central experiment simulated the drying of a wetted wall section to watch the total system response. The two auxiliary experiments were conducted to analyse individual variables affecting the central experiment total system: namely the effect of cladding cavity dimensions on drying rate and the relationship between moisture and various building materials. It was found that a heated wall simulation could dry timber from 30\% moisture content to 14\% in 14 days. A large vapour pressure was observed between the chamber and laboratory environment. The drying rates for unheated runs were significantly smaller, and were not likely to dry to 14\% within 30 days. The major mass transfer mechanisms identified in the drying process were condensation on building wrap, wicking through it, evaporation and egress by natural or forced convection out the bottom of the cladding cavity. Diffusion did not play a significant role in the egress of moisture. A similar experiment that was not heated had much slower rates of drying, and did not look to dry to 14\% moisture content within 30 days. High sections of timber dried faster than others caused by gravity redistribution to the base plate. In order for building walls to dry out within 30 days, a temperature gradient must be present to allow condensation and wicking through building paper that supports this mechanism. The egress of moisture from wet building walls clad with HITEX Diamond cladding is controlled by the degree of air movement around the bottom section of the cladding. Of the auxiliary experiments conducted, it was shown that the dimensions of the cladding cavity at the bottom of the chamber significantly affected the rate of drying from a wall section. A deeper and shorter cavity drop gave increased drying. It was also found that increasing the level of air agitation around this area also caused increased drying rate. The capacity of building materials to carry water when immersed or exposed to a high humidity environment was investigated. It was found considerable amounts of water could be stored by building materials, especially batts (1622\% of dry weight). Four building wraps investigated to determine their effect on the drying rate of wetted timber samples. Cover-Up from Tyvek slowed the drying rate of wrapped wetted timber the by the greatest amount. The others slowed down the rate of drying by a similar amount. The initial moisture loss mechanism by most building wraps excluding Tyvek was wicking.
