Factors affecting the moisture permeability of porous materials and the calculation of moisture transport and condensation hazard in building structures

REPRINT 118

Faetors Affeeting the Moisture Permeability of Porous Materials and the Caleulation of Moisture Transport and Condensation Hazard in Building Struetures

By ANNANlAS TVEIT

Norwegian Building Research Institute

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NORWEGIAN BUILDING RESEARCH INSTITUTE

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00000 o S L o 1966

2 - 29

F ACTORS AFFECTING THE MOISTURE PERMEABILITY OF POROUS MATERIALS AND THE CALCULATION OF MOISTURE TRANSPORT AND CONDENSATION HAZARD IN BlliLDING STRUCTURES

ANNANlAS TVEIT

Nonve g i a n Bu i l d i n g Re s e a r c h In s t i t u t e, T r o n d h e i m N onvay

The transfer of heat and moisture in building materials and building structures is a very complex proeess. The heat and the moistwe flow· ale bach individually dependent on a number of faclors of physical and strucrural nature and they are at the same time more or less interdependent. Due to the complexity of the proeess and our limited lmowledge of it the heat flew and the muss flow are usuaUy considered separately for calcularion purposes. Exisring equurions, taldng inta account the muroal influence of heat and moisture, are in general inappli­

cable for technical calculations in pracrice. The reasan is parcly chat a com­

prehensive calculation work is involved and particularly that the applicution of these equations in many cases presupposes knowledge of various physical data which have not yeat been established for more [han same fe", materials. As an example may be mentioned the influence of temperature and moisture content

0[1 the suction properties and the mois ture flow factors of the materials.

To get an idea of the condensation hazard in building structures same simpli­

fied calculation methods are aften applied. A characteristic fea ture of these methods is the assumption of an ane dimensianal s[3tionary flow. The tempera­

ture and vapour pressure pattern in the structure are calculated from cenain in­

and outside design temperutures and humidities using more or less average values of the heat conductivities and moisture permeabilities of the materials. For struc­

tures consisting of parallell layers of material, the thiclmess of the various layers is aften represenred in unirs of heat and vapour resistan.ce instead of in units of length wruch gives the temperature and vapour pressure curves as straight lines.

If dle cumputed vapour pressure does not exeed nor is equal to the saturation pressure of water vapour at any point in the structure, there is assumed ro be no or liule condensation hazard.

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---Spruce 410kg/mJ 11,5mm

----Pine SJO-'- 11.5 -_

-·-·-Teak 600-0- 11,5 __

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In spire of the faet [hat the basis of [his calculation method is somewhat doubtful, the applic3tion of it has shown to be acceptable for pracrical purposes.

as damages due to condensation have been fairly SC3rce. It must hete be imer­

posed [hat [here have certainly been quite a lot of condensation ttoubles in light­

weight Ioof constructions, bue [hese have mostly been caused by air leakages.

The reasan why [his calculation method "seems [O make good is [hat some of the simplifications being done rend to draw in a favourable direction. Whilst for insrance the moisrure permeahility of the materials are determined under tempera­

ture equilibrium the mois ture transport will in pracrice he influenced by a tem­

perature gradient. Another peint is [hat the calculation of the mois ture transport ' in structures in most cases is carried out under assumption of extremely unfavour­

able temperature and humidity conditians.

A third point is the influence of temperature and humidity on the mois ture transport ability of porous materials. Experience has sho",n that the mois ture per­

meability of certain materials depends very much on their moisture content, which within the hygroscopic range under equilibrium conditions is a functian af ambient temperature and humidity. This appears for instance in the ASTM Designation:

C355�54T as n'lo different methods of measuring the water vapour transmissian of materials at a 10\"1 and a high humidity level, i. e. the "desiccant method" and the "water method". In calculations concerning the condensation hazard in struc­

tures no or little attention is pa id to this effect of the mois ture content.

Measurements1) which have been carried out in the laboratory of the Norwegian Building Research Institute clearly shows the great influence of the moisture con­

tent on the moisture transport ability of cercain materials. Fig. 1· ^.. 7 give an abstraet of dle results of the moisture soeption and mois ture permeability measure­

ments on various common, porous building materials at a temperature of 25°C.

Fig. 1 shows the relationship between the mois ture peemeance normal to fibee direction and ambiem mean relative humidity for 11. 5 mm thick specimens of spruce, pine and teak. The permeability is fairly 10\'1 in the lower humidity range bur increases rapidly wllen the relative humidity is greater than approxi­

mately 70 0/0. As will be seen from the sorption curves in the diagram to the left, this increase corresponds well to the increase in moisture content.

Fig. 2 and 3 show the sorption and permeability CUfves of some boards of wood fibres and \"Iood particles. These materials have naturally much the same sorption properties as \Vood, but the moisture permeability is othen .... ise as the distribution of the fibre directions are at random an� compared with ''1ood the boards have not the same cominuous capillary system. This leads to an increase

l) These measmements are a part of a research work on mois ture transport in porous materials· ''1hich is being carried out as a cooperation of The Danish National Institute of Building Research, The State Institute for Technical Research in Finland, The Swedish National Council of Building Research and The Norwegian Building Research Institute.

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5 2 - 29

in vapour diffusion in the !ower humidity range and a decrease in liquid transport in the higher range. The slope of the curves of the heavier qualities of fibre boards as 5hO\..,0 in Fig. 3 are, ho,,,ever, much the same as for wood. With regard to the sorption the effect of tile impregnation is surprisingly small. The reduced permeability of the impregnated boards may be due to the impregnation. but it may also be attributed to the higher density of these boards.

The moisture sorption and permeance of an unimpregnated and an asphalt satu­

rated building felt are shown in Fig. 4. The sarurated felt is intended for ""iod protection in exterior wall5. As appears from the diagram. the moisture perme­

ance increases very much with increasing relative humidity in the higher hurriidity range. This is a very favourable pro pert y of a sheathing felt, as it in a frame

\'1all normally is placed just \'1here the eondensation hazard is greatest. If by some reason the relative humidity in the \'Iall cavity is raised, the building felt \'lill sorb moisture and increase its moisture permeability. This increased moisrure per�

meability \'1ill rend to lower the vapour presswe and a ne", equilibrium may be reaehed.

The change in permeability of various, mostly inorganic materials as cellular � concrete, lime-sandstone, brick, eemenr-asbestos boards etc. is given in Fig. 5 and 6. The hygroscopic moisture coment of bricl< is very 10\'1 and the permea­

bility is almost independent on the relative humidity. This means that the pure vapour diffus ion

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s predominating. Cellular eenerete has a high permeability in the emire humidity range, and the vapour diffusien is predominating at the lower humidities. The measurements sho""ed that the permeability of cellular concrere of the same density differed quite mueh from one quality to another depending on the pore stIucture. The permeability of lime-sandstone, cement-asbestos board and plaster board is also fairly high. The permeability of plasrer board is increas­

ing very much at a relative humidity of approx. 80 % eorresponding to a marlæd increase in mois ture cOntent.

The permeability of mortars and eonerete is also increasing ""ith increasing humidity. As ""ill be seen from Fig. 7, the permeability is a1so dependent on the relative content of binder, and ineteases ""hen the amount of cement and lime is deereased.

In addition to the materials ""hieh have already been menrioned, approximarely 30 othet pOtOUS materials ""ere also tested. Arnong these were different qualities of expanded or foamed plastics, eorl< materials, boards of mineral \'1001, other Idnds of ""ood, brick, light""eight conerere etc.

The results of the permeability measurements have sho\."n that the change in mois­

twe permeability with the relative humidity primarily depends on the hygroseopie properties of the materials. The permeability of a material ""ith no or little hygroseopicity is practically independent on the relative hurnidity. The mois ture flm'l is essentially a pure vapour diffusjon. If the material is very permeable to airflow, a molar vapour ""ill occur if any pressure differences exist.

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Experience from practice has also shown that the calculations concerning con­

densation in lightweight sttuctures easily becomes worthless because of air leakages in the structures.

The permeability of hygroscopic materials changes 'vith the relative humidity depending on the degree of hygroscopicity and on the ability of the material to transport moisture in the liquid state or in the vapour state. The capillary suc­

tion properties and permeability to air flO\v are thus important factors. The per­

meability of many hygroscopic materials increases very much in the higher humidity ranges because of capillary condensation. This may lead to a reduced condensation hazard in strucmres as the moistwe ttansport ability increases very much when the vapour pressure approaches the saturation pressure. Measurements

have shown that the moisture transport ability of various kinds of sheathing felts and sheathing papers is same hundred times higher of the mois! paper [han for the dry paper. This kind of building papets are very effective in \'lood ftame walls, as water condensed on [hem \'lill disappear very quickly under more favour­

able conditions. The increased permeability of the material \'Iith increasing rela­

tive humidity and moisture coment ''lill a150 lower the condensation hazard in compact waUs.

For non hygroscopic air permeable materials the water vapour permeability wi11 change with temperature in the same manner as the permeability of water vapour in air, that means propartional to the squ3re power of the absolute tem­

perature. Materials of [his kind are for instance mineral wools and certain kinds of plastic foams.

For a hygroscopic material which is very permeable to air flow the temperature effect is the same as mentioned above. because the vapour diffusion then is predomi­

nating. If on the other hand the liquid flow is predominating. the permeability decreases with increasing mean temperature, provided the mean relative humidity being kept constant. This is similar to the temperature effect on the sorption.

The influence of the temperature on the permeabiliry of the materials will tI1Us depend on the hygroscopicity. the air permeability and the capillary trans­

port ability of the materials, the latter also including the surface creeping of the water molecules ..

Paper 2 - 29; TVEIT. FACTORS AFFECTING THE MOIS TURE PERMEABIllTY OF POROUS MATERIALS AND THE CALCUlATION OF MOISTURE TRANSFER AND CONDENSA nON l-IAZARD IN BU/IDING STRUCTURES

Page Line from Printed Should be read

1 1 1 3

3 3 3 B

above beneath 2

B

10

2

15 3

20 10

transport yeat

transfer yer

cumputed computed

the moisture permeability the mois ture permeability of the materials are deter- of the materials is deter- mined

transport shows permability

higher of the moist paper

mined transfer show permeance

higher for the moist paper