Hygrothermal Performance and Soiling of Exterior Building Surfaces

Research output: Book/ReportPh.D. thesis

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The hypothesis of this thesis is: “That by changing the exterior surface properties the hygrothermal performance of the whole building envelope can be improved”. But it is only possible to preserve the improvements if the surface does not soil. To test this hypothesis decisive surface properties are described both as phenomena and by the physical chemistry of surfaces. That surface properties can be changed is unquestionable, but the aim here is to gain insight as to what mechanisms makes it possible to change surface properties. This knowledge should be helpful in the development and choice of surface treatments and towards more scientifically based decisions than the trial and error experiments of the past.

After an introduction to the subject the hygrothermal properties are described from a theoretical point of view with a focus on moisture transport and hydrophobic treatments plus heat, radiation and low emissivity/high absorptivity coatings.

Soiling affects surface properties and is the subject of Chapter 3. First a discussion of whether soiling is a failure, then a description of what soiling consists of, where it comes from, the impact on surfaces and how it adheres to surfaces. Soiling is differentiated in biological growth with a description of different genera of microorganisms and environmental dirt with its different compositions and sources. Possibilities in self-cleaning properties are outlined.

The consequences of changing surface properties are described in Chapter 4, which is seen from a more practical angle. Possible gains and risks in designing surface properties are described based on state of the art knowledge and simulations. Issues like application and cleaning methods are discussed with emphasis is on durability and energy consumption.

To test the hypothesis several experiments have been conducted, as described in
Chapter 5. Two types of roofing tiles were used for the experiments; ordinary tiles and tiles with Lotus Effect i.e. tiles with a surface treatment which is supposed to be self-cleaning by a combination of hydrophobicity and a specific surface roughness.

The experiments showed:
− After almost 2½ years with natural exposure both tile types were soiled by algae growth, however the tiles with Lotus Effect soiled at a lesser rate.
− The treatment on the tiles with Lotus Effect was not hydrophobic. The water uptake in the Lotus tiles when exposed to natural weather was up to ten times higher than in the ordinary tiles.
− The bulk material of the two tile types was not similar; the pore size distribution was different.
− With SEM (Scanning Electron Microscope) it was possible to see the changed surface of the Lotus tiles. But the penetration depth and chemical composition could not be determined with SEM combined with EDX (Energy Dispersive X-ray spectrometer).
− XPS (X-ray induced Photoemission Spectroscopy) showed fluorine at the surface of the tiles with Lotus Effect.
− Both tile types passed a freeze-thaw test.
− Thermographical investigations indicated that the emissivity of the Lotus tiles was a little lower than the ordinary tiles.

The results of the experiments are used as starting point for a discussion in Chapter 6 of which surface properties have the most influence and which are realistic to change. The practical implications of the findings are outlined. The main issues and conclusions are:
− Hydrophobic treatments decrease the moisture content but no single treatment can be used for all porous materials. Chemical composition and molecule size of the treatment must be compared to pore size and composition of the substrate. The application must also ensure a sufficient penetration depth.
− Low surface free energy and non-polarity of a surface treatment reduces the soiling.
− Low emissivity increases the surface temperature and thereby inhibits algae growth. The effect on energy savings in the winter is negligible but the enhanced need for cooling in the summer could be a problem.
− Service life prediction of surfaces by the factor method is inadequate.
− Self-cleaning properties for porous surfaces have not been obtained yet.

Although not all the experiments had the expected outcome the main hypothesis
still stands: The hygrothermal performance of the building envelope can be improved by changing the properties of the exterior surface. However, further research is needed. The most promising fields are:
− Fluorinated polymers for hydrophobic and self-cleaning treatment
− Low emissivity surfaces for porous materials
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages229
ISBN (Print)87-7877-130-7
Publication statusPublished - Mar 2004
SeriesByg Rapport


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