Contaminant transport by air infiltration from crawl space to occupant area: Numerical simulations and field measurements in Swedish schools

Some Swedish school buildings built in the 1960’s and 1970’s have indoor air quality problems. Many of these buildings have a crawl space from which contaminants are suspected to originate. The poor indoor air quality cause discomfort among pupils and teachers and a solution to the problem is not always found. This thesis summarizes the work done on investigating contaminant transport driven by air leakage from the crawl space to the classroom in such buildings. Field measurements of temperature, wind, and pressure difference across the floor construction between classroom and crawl space has been\ua0 conducted in two school buildings. A method in which frozen carbon dioxide is used to determine if air leakage to the classroom originates from the crawl space is also successfully tested. Also, a numerical infiltration model is developed in MATLAB and used to investigate how temperature, wind and air permeability distribution affect the pressure difference across the floor construction and contaminant concentrations. The numerical model is also used with the Monte Carlo method to investigate, for example, correlations between model parameters, such as air permeability and temperature, and to analyze measures, such as increased ventilation or use of an exhaust fan in the crawl space.Results presented in this thesis shows that outdoor temperature and wind has astronger influence on the concentration levels indoors and the pressure differenceacross the floor than for example the building airtightness. For buildings with an imbalanced ventilation system, where the exhaust airflow is larger than the supply airflow the most critical weather case, in terms of high concentrations of contaminants indoors, is during mild and calm days. Numerical simulations also show that the pressure difference across the floor construction is positive (so that air leaks from the crawl space to the classroom) for most weather cases and building configurations

Moisture impact on dimensional changes and air leakage in wooden buildings

Wood is a hygroscopic material, it has the ability to adsorb or desorb water in response to the ambient relative humidity. Thus, the ambient air will affect the moisture content of the wood, and in turn, the dimension of the wood. If the wood itself is part of the air barrier in a construction, the shrinking and expansion can create gaps in the construction, for example in the window sill. In case of an air barrier consisting of a foil, the joints in the foil can be clamped by wooden joists, or the foil can be taped to wooden part. In both cases, the movement (shrinking, expanding) can cause air leakages, in particular since wood is an anisotropic material and will behave differently in different directions.This project investigates the air leakages that occur in wooden buildings from seasonal variations in climate and from finished construction (first airtightness measurement) to equilibrium (when inbuilt moisture has dried out). Measurements are made in real buildings (full scale) and in laboratory (climate chamber). The full-scale measurements on buildings with wooden construction have shown magnitudes of up to 10% increase in air leakage (at 50 Pa) for the seasonal variations and the increase for reaching equilibrium in a newly built construction is\ua020%

Pressure distribution around the thermal envelope - a parametric study of the impact from wind and temperature on contaminant transport within a building

Several school buildings in Sweden have indoor air quality problems. The contaminant source is often\ua0 assumed\ua0 to\ua0 come\ua0 from\ua0 within\ua0 the\ua0 construction,\ua0 for\ua0 example\ua0 from\ua0 the\ua0 crawl\ua0 space\ua0 or\ua0 attic\ua0 space.\ua0 Contaminants,\ua0 in\ua0 these\ua0 cases,\ua0 are\ua0 transported\ua0 by\ua0 air\ua0 leaking\ua0 between\ua0 compartments\ua0 in\ua0 the\ua0 building.\ua0 Here,\ua0 the\ua0 driving\ua0 force\ua0 for\ua0 the\ua0 air\ua0 leakage\ua0 is\ua0 difference\ua0 in\ua0 pressure\ua0 and,\ua0 therefore,\ua0 determining\ua0 pressure\ua0 also\ua0 determines\ua0 the\ua0 direction\ua0 of\ua0 contaminant\ua0 transport.\ua0 In\ua0 many\ua0 cases,\ua0 measures\ua0 to\ua0 improve\ua0 the\ua0 air\ua0 quality\ua0 are\ua0 taken without a thorough understanding of how it might affect the pressure distribution in the building. In this\ua0 paper\ua0 a\ua0 numerical\ua0 model\ua0 is\ua0 used\ua0 to\ua0 examine\ua0 how\ua0 different\ua0 climate\ua0 scenarios\ua0 and\ua0 different\ua0 building\ua0 configurations affect the leakage and contaminant transport in a building with a crawl space. Results show that\ua0 for\ua0 leaky\ua0 buildings\ua0 the\ua0 ventilation\ua0 rate\ua0 increases\ua0 with\ua0 increased\ua0 wind\ua0 and\ua0 therefore\ua0 the\ua0 contaminant\ua0 concentration\ua0 decreases.\ua0 The\ua0 worst\ua0 scenario\ua0 in\ua0 terms\ua0 of\ua0 high\ua0 contaminant\ua0 concentration\ua0 is\ua0 mild\ua0 days\ua0 with\ua0 little\ua0 wind.\ua0 Also,\ua0 when\ua0 installing\ua0 an\ua0 exhaust\ua0 fan\ua0 in\ua0 the\ua0 crawl\ua0 space\ua0 with\ua0 the\ua0 purpose\ua0 to\ua0 prevent\ua0 air\ua0 from\ua0 leaking\ua0 from\ua0 the\ua0 crawl\ua0 space\ua0 to\ua0 the\ua0 classroom\ua0 it\ua0 is\ua0 advisable\ua0 to\ua0 also\ua0 consider\ua0 the\ua0 airtightness\ua0 and\ua0 the\ua0 climate, not only the pressure difference across the floor

Modelling VOC levels in a new office building using passive sampling, humidity, temperature, and ventilation measurements

New buildings often have high initial concentrations of VOCs that, although not necessarily harmful, may be disturbing and cause discomfort among occupants. In new buildings, running the ventilation system continuously and at full rate during the first year is common practice to reduce VOC levels. However, the drawback of such an arbitrary strategy is the risk of over-ventilating with unnecessary heat losses as a consequence. In this article, a new approach, a VOC-passport, is developed where early measurements of VOCs together with a calculation model are used to find an optimized ventilation strategy. The proposed calculation model is tested on two newly built office rooms where VOCs were measured using passive samplers, together with temperature, humidity and ventilation rates, and it shows good agreement with measurements. An example of how a daily ventilation schedule may look like if optimized with the prosed model is presented. The example illustrates that in buildings where VOC levels are allowed to increase periodically, VOC levels can be kept at acceptable levels during occupancy hours if the effective storage capacity is known. The proposed method has a potential to improve the indoor air quality in new buildings without compromising energy efficiency

Impact of weather conditions and building design on contaminant infiltration from crawl spaces in Swedish schools--Numerical modeling using Monte Carlo method

Some Swedish school buildings built in the 1960s and 1970s experience indoor air quality problems, where the contaminants are suspected to come from the crawl space underneath the building. The poor indoor air quality causes discomfort among pupils and teachers. Installing an exhaust fan to maintain a negative pressure difference in the crawl space relative to indoors or increasing the ventilation in the classroom are two examples of common measures taken to improve the indoor air quality. However, these measures are not always effective, and sometimes the school building has to be demolished. The relation between pressure distribution, contaminant concentration in the classroom, outdoor temperature, wind, mechanical ventilation, and air leakage distribution is complex. A better understanding of these relations is crucial for making decisions on the most efficient measure to improve the indoor air quality. In this paper, a model for contaminant infiltration from the crawl space is used together with the Monte Carlo method to study these relations. Simulations are performed for several cases where different building shapes, building orientations, shielding conditions, and geographical locations are simulated. Results show, for example, that for a building with an imbalanced ventilation system, air is leaking from the crawl space to the classroom for the majority of cases and that concentration levels in the classroom are usually the highest during mild and calm days

Glazed spaces for a resource efficient, social and healthy living, Part 1. Inventory of geometries and functions by study visits and interviews

The main aim of the Spaces project is to support architects in the design of well-functioning glazed geometries, such as atria and rooftops, in residential buildings. The studied geometries are primarily spaces for communication and leisure in residential buildings. These spaces may have a varying indoor climate, which is governed by the construction of the building, as well as residents’ activities, rather than by building services. The project contributes with examples of geometries and usages, methods to evaluate the performance early in the design process and to provide guidance for architectural design and increased social interaction. The project also investigates obstacles that exist in current practice and Swedish legislation for glazed geometries.In this report, the first part of the project “Inventory of geometries and functions by study visits and interviews” is presented. Methods used in this first part is literature studies, interviews and case studies. The topics investigated are social and human aspects, technical aspects such as thermal comfort, energy, air quality, humidity, acoustics and to some extent urban farming. From the literature and by contacting architects and consultants in the building industry, eight case study buildings were found, located. The buildings were either housing cooperatives or rental buildings, and the glazed spaces in the buildings were either atria, glazed balconies or glazed rooftops.For the case studies, information was gathered from databases, through interviews and during study visits. The opinions of the residents were captured during structured interviews and through quantifiable surveys, and the results were analysed by the project group with input from the reference group.For social interaction, the investigations show that even with a developed design for social interaction (such as common areas, kindergarden, private areas in connection with glazed space), the interaction might fail. Social activities are highly dependent on individuals and thus, engaged persons are very valuable to obtain a social environment. In addition, a clear purpose of the space and a sense of ownership is beneficial to the social environment. In this study, the projects that worked well socially were the four housing cooperatives and one rental building. The three projects that worked less well were all rental.There are slightly different opinions of the optimal size to achieve social interaction. The architect of one of the projects that works well socially suggests a maximum of 60 persons (25-30 families) and at another socially successful building, there are 48 apartments. Two out of the three largest buildings (71 apartments and 126 apartments) did not function well socially.Daylight levels are usually considered good in the glazed spaces. However, there are darker areas, in particular under access balconies, and this also affects the daylight levels in the apartments. The air quality is usually perceived as good in the glazed spaces and the most common problem connected to air quality is a high level of moisture in the air, which can result in condensation on windows.For thermal comfort, the expected level of comfort is important for the experience of the space. If the space looks like it is indoor, the expectation is room temperature in wintertime and, consequently, people are disappointed if it’s much colder. Both studied rooftops have problems with high temperatures in summertime. The temperature in the glazed space depends to a large degree on shading and ventilation, but also on the thermal mass of the materials in the glazed space. This is further investigated in part 2 of the project (Wahlgren et al. 2021), where also evaluation tools and design guidelines are presented

Consequences of Varying Airtightness in Wooden Buildings

Previous research has shown that the airtightness in wooden buildings can vary over the year and that buildings can be less airtight during wintertime, likely caused by variations in humidity. In order to investigate possible causes and consequences, such as increased energy use and moisture damage, a number of numerical simulations and laboratory measurements have been performed. In a wooden guesthouse, without a moisture barrier, the indoor relative humidity is kept at 90 % during 8 days and then decreased to 25 % during 7 days. The airtightness is measured frequently during both periods and shows a change in airtightness from 0.74 l/sm2 to 1.21 l/sm2 at 50 Pa pressure difference. The moisture scenario can be related to common levels of inbuilt moisture as well as moisture loads from indoor activities. In order to investigate the consequences of varying airtightness, a numerical model is set-up to resemble a typical wooden detached house. Airflows and pressure profiles are then calculated for different values of airtightness. Simulations show that the total increase in exfiltration is dependent not only on airtightness value but also on leakage distribution. For example, exfiltration will increase with 1.9 % per percentage point of decrease in airtightness, if the distribution of leakages are concentrated to the lower parts of the building, compared to 1.6 % if leakages are more evenly distributed. Interestingly, results also show that if the cold attic is accessible through an attic hatch that is not airtight, air is likely to leak from the indoor environment to the attic during some periods of the year and downward to the indoor environment during other periods. Results show that variations in airtightness in wooden houses is affected by surrounding humidity and that the consequences of varying airtightness will affect both moisture and energy performance

Air leakage variations due to changes in moisture content in wooden construction- magnitudes and consequences

The airtightness of buildings is important for several reasons, such as being a prerequisite for low-energy buildings and for a healthy indoor air quality (without i.e. mold or radon). The airtightness of buildings can vary over time and investigations are made on these variations due to moisture induced movements in wooden constructions, and subsequent consequences, using both measurements and numerical simulations. Measurements were performed in a wooden guest house that was built in a laboratory hall with an approximate relative humidity of 30 %. The guest house was designed with a wind barrier but without a moisture barrier, and with a majority of the leakages situated in the wall/floor connection. The relative humidity in the guest house was varied so that the indoor relative humidity was kept at 90 % during 8 days and then decreased to 25 % during 7 days. This variation in moisture content in the wooden part can also illustrate the built in moisture in construction timber (starting at a moisture content of 16%). The air permeability was measured frequently during both periods and showed a change in air permeability from 0.74 l/sm2 to 1.21 l/sm2 at 50 Pa pressure difference. Consequently, for a wooden construction with a moisture dependent air permeability, it is easier to fulfill airtightness demands (checked by measurements), when the building is just erected, compared to a couple of months later. Numerical simulations on the moisture induced leakage variations and the impact of the resulting variation in air permeability are performed in Simulink and MATLAB. Air leakage is calculated using a set of object oriented functions within the MATLAB environment. These functions follow the same mathematical principle as presented in the airflow simulation software CONTAM. The simulations are made for the climate of Gothenburg in south-west part of Sweden

Influence of oxidation on radiative heat transfer in polyurethane insulation used for district heating pipes

Thermal conductivity of cellular rigid polyurethane foam (PUR) increases by time which leads to higher heat energy losses in district heating pipe networks. The main reason for increased thermal conductivity is diffusion of low conductive gases out of the PUR and diffusion of surrounding air into the PUR. However, oxidation of the PUR occurs during the service-life of the PUR and is accelerated by the higher temperatures close the fluid pipe. The effect that oxidation has on the thermal conductivity is not yet fully understood and existing models for prediction of long-term thermal performance of PUR insulation in district heating applications does not take oxidation processes into account. It is possible that the radiative heat transfer is affected by the oxidation and changes over the service-life of the PUR. In order to investigate the influence of oxidation on thermal conductivity, the extinction coefficient was therefore calculated for samples subjected to different levels of ageing. The input data for the calculations were measured by FTIR. The extinction coefficients were then used to calculate the overall thermal conductivity of the PUR with typical gas compositions. Results indicated that the extinction coefficient was 22 % higher in the samples exposed to lower temperatures. However, the effect on the overall thermal conductivity of the same samples was an increase of about 1.8 %. Since the comparison was made between two samples subjected to different levels of ageing, the increase in total thermal conductivity should be interpreted as a minimum if considering the total service lifetime of the PUR insulation