Microbiological growth on building materials – critical moisture levels [PDF]

Critical moisture content - today. Critical moisture content for wood and wood based materials. Reference. For new mater

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Microbiological growth on building materials – critical moisture levels. State of the art Pernilla Johansson, Ingemar Samuelson, Annika Ekstrand-Tobin, Kristina Mjörnell, Per Ingvar Sandberg, Eva Sikander SP report 2005:11

Extracts by Anker Nielsen for IEA annex 41 meeting Trondheim

SP Swedish National Testing and Research Institute, Borås

State of the art • Background Building Regulators (Boverket) wanted recommendations for critical moisture conditions in new regulation • The selection of literature based on relevance for critical levels • Full report in Swedish on www.sp.se • Appendix 1 – data related to materials in not translated • References

Critical moisture content - today Critical moisture content for wood and wood based materials

Reference

For new materials, not more than 0,18 kg moisture/kg material

(AMA, 2003)

No risk 0,20 kg/kg)*

(Nevander et al., 1994)

* at temperatures where the microbial growth happens

Model for critical conditions We can change this to an equivalent time, defined as:

tekv = ∑ f RF ⋅ ftemp⋅ Δt tid

This gives a more general result with a summation of relative humidity and temperature over time.

Recommendation for critical moisture content Material type Contaminated or soiled Wood and wood based Mineral wool insulation Expanded polystyrene (EPS) Concrete *SP experience/estimat

Critical moisture level [%RH] 75-80* 75-80 90-95 90-95 90-95

Expected relative humidity and temperatures (Swedish conditions) Cold ventilated roof - at the underside of roof consists of wood Winter

85 - 100 %

< 5°C

Summer

40 - 70 %

>15°C

Outer wall with bricks - on the outside of the wind barrier Winter

Summer

85 - 95 %

15°C

Wood floor on concrete slap with insulation between the concrete and the floor – conditions under the insulation at the concrete

At the perimeter of the plate

At a distance from the perimeter

Winter

80 - 95 %

5 - 10°C

Summer

80 - 95 %

ca 15°C

Winter

80 - 85 %

ca 15°C

Summer

80 - 85 %

15 - 18°C

Ventilated crawl spaces – conditions at the underside of the floor construction

Winter

70 - 85 %

10°C

The risk for the indoor environment Location of growth

Conditions

Example of cases

Risk

Roof ventilated with outdoor air

Overpressure indoors will increase the risk of damage but will prevent the air flow from the roof

Natural ventilation

Low

Roof ventilated with outdoor air

Underpressure inside against the roof will eliminate the risk of damage from air convection. But the pressure difference will increase the air flow from the roof to the indoor environment

With balanced ventilation or system with only extract ventilation, the Middle effect will be increased in windy conditions

The lower part of the outer wall

Underpressure inside against the roof will increase the air flow from the lower parts of the walls to the indoor environment

With balanced ventilation or system with only Middle extract ventilation, the High effect will be increased in windy conditions

Location

Conditions

Example of cases

Risk

Crawl spaces

Underpressure inside against the crawl space will increase the air flow from the ground to the indoor environment

With all ventilation systems, the effect will increase in windy conditions

Middle High

High relative humidity at he surfaces.

Wet rooms, cellars without heating in the summer, sleeping rooms without enough ventilation, with high moisture production, with low outdoor temperatures

High

Internal surfaces

Recommendation for critical moisture content Material type Contaminated or soiled Wood and wood based Mineral wool insulation Expanded polystyrene (EPS) Concrete *SP experience/estimat

Critical moisture level [%RH] 75-80* 75-80 90-95 90-95 90-95

Comments to table • • •

Values for long time exposure. Values for the moisture conditions in the materials surface layer. If the materials have been exposed to water for instance rain or leakage is it necessary to treat the material. 1. SPs experience is that the material must be dried out (to valued below the critical moisture condition) in a few days or weeks to prevent germination of spores and microbiological growth. For concrete will we recommend that the drying must be done in a few weeks or months. For gypsum boards gives the literature indication that the material must not be exposed to free water.



The values in the table are for critical moisture content at normal room temperature around 20C. For wood and wood based materials use the figure from Viitanen.

We have not found similar figures for other materials.



• •

The values in the table are for clean materials. If the material (for instance EPS) is contaminated will the mould resistance be lower and the values for contaminated materials must be used. The contamination or soiling can come from mishandling the material or from dust or particles in the air. If the moisture condition is above the critical level in the table is it not certain that we will get mould growth. But there will be a risk for mould growth. The literature study shows that there is not a definitive limit for growths for the listed materials. Different investigations on the same material can give different results. The testing methods are not the same and the materials in the same group can be different. A producer of a material can by investigations and testing give a higher level than in the table.







It is possible to treat materials in different ways to get a higher critical moisture level, for instance by using fungicide treatment or substances that will reduce the mould growth. Such materials are not found in the literature. If producers will use these methods must the results be tested. In that case must also the long time effect of the treatment be evaluated. It must still work after 10-30 years. An uncertainty is that material typically is tested at certain level as 75% (no growth) and 85% (here is growth). The difference in these levels is rather large. The uncertainty in the moisture measurement must also be evaluated. To be on the safe side that the critical values in the table is not exceeded in the building phase and the long time use it is recommended to put a safety factor on the critical moisture content. Evaluation of the risk in the building phase is important to reduce the risk for future damages.

Recommendation for critical moisture content Material type Contaminated or soiled Wood and wood based Mineral wool insulation Expanded polystyrene (EPS) Concrete *SP experience/estimat

Critical moisture level [%RH] 75-80* 75-80 90-95 90-95 90-95

Important factors for mould growth • • • •

Moisture and Temperature Time dependence Contamination or soiling Dry periods between periods high moisture levels

Model for critical conditions At studies of microbiological growth on materials is often found curves of the type: Level of mold growth

t0

time

Typical curve for microbiological growth on material

Model for critical conditions fRH

1

0 RH1 Low 60-70% RH and high 80-90%

RH2

100 % RH

Model for critical conditions ftemp

1

0

Cold

Hot

Testing methods for mould •



The material is exposed for a fixed climate (relative humidity and temperature) and exposed to the normal types of spores found naturally on the material. The growth of this mix of species can then be test. The material is exposed for spores from one or more known mould species. This is then placed in a fixed climate and the mould can growth. To prevent that spores on the material influence the result is the material sterilized before the testing.

Measuring methods 1. extension of mould on the surface (either based on what can be seen with the naked eye or by microscopic analysis), as. (Viitanen, 2001), (Johansson, 2002) 2. generation of CO2 (Pasanen et al., 1992) 3. time for spore germination, as (Grant et al., 1989) 4. diameter on colony 5. number of (CFU) colony forming units /g material 6. total number of spores 7. amount of ergosterol in fungal cells

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