Moisture and temperature conditions in cold lofts and risk of mould growth

Transcription

1 Moisture and temperature conditions in cold lofts and risk of mould growth Sivert Uvsløkk, M. Sc. Norges byggforskningsinstitutt; KEYWORDS: roof, loft, attic, moisture, temperature, mould growth SUMMARY: The paper presents some results from a theoretical study of moisture and temperature conditions in cold lofts and the risk of mould growth on the roof in various climates in Norway. The computational model developed for this study calculates the heat, air, and moisture balances within the loft considering parameters describing the building, building materials, outdoor climate as well as parameters influenced by the occupiers. Both stack effect and air leakage distribution are included in the model as air leakages play a key roll both for the moisture content of the indoor air and for the moisture transport in and out of the loft. In order to perform quantified parameter studies a mould growth potential is introduced. By using simplified relations between the relative rate of mould growth and temperature and relative humidity, respectively, a total mould growth potential is summarized for successive calculation periods. Traditionally cold lofts in Norway have been vented, by openings at the eaves, gables and the ridge, in order to prevent snow melting and to give the roof drying out capability. Closed lofts, without openings to the exterior, however, have several advantages like improved protection against spread of fire reduced risk of snow drifting into the loft and reduced heat loss due to reduced air flow through the roof insulation. The study shows that closed cold lofts may be constructed with low risk of mould growth. The roof however, has to be correctly designed using roof with sufficiently low vapour and a ventilated air space between the roofing and the roof. The work has ben carried out within the Norwegian research programme Climate 2 Building constructions in a more severe climate, initiated and managed by the Norwegian Building Research Institute. 1. Pitched roofs in cold climate Pitched timber frame roofs in cold climate have to be ventilated of two reasons: 1. To give the roof materials ability to dry out to prevent mould growth 2. To keep the roofing cold to prevent snow melting and damage caused by ice on the eaves The design and location of air gaps for ventilation varies and depends mainly on the position of the thermal insulation and the properties of the material. 1.1 Pitched roofs with thermal insulation between the rafters Pitched roofs with thermal insulation between the rafters have to be ventilated by use of one or two air gaps between the insulation and the roofing Roofs with one ventilated air gap If the have low vapour, s d <,5 m, no ventilation gap is needed between the and the thermal insulation. Roofs with roofing on battens are ventilated by airflow through the air gap between the and the roofing, the batten space. This is roof design is the most common roof design for detached houses in Norway to day. The thermal insulation has to be mounted in contact with the and the must be mounted with airtight joints, as a wind barrier. If the and the joints are not sufficiently air tight, external cold air will flow into and parallel to the insulation layer due to the wind pressure gradient in the air gap. This airflow takes place when it is windy, even if the air-/vapour barrier at the warm side of the insulation is perfect airtight.

2 1.1.2 Roofs with two ventilated air gaps If the have a vapour, s d >,5 m there should be a second ventilated air gap, between the insulation layer and the, to give the roof sufficiently drying out capability. The insulation must then be protected by a wind barrier at the cold surface to prevent cold air from flowing into and through the insulation layer causing extra heat loss. 1.2 Pitched roofs with horizontal thermal insulation and cold loft Roofs with cold lofts can be ventilated in two alternative ways: 1. Cold, ventilated loft. The loft is ventilated directly through openings to the loft at the eaves, at the ridge or in the gables. 2. Cold, non-ventilated loft. By using an with low vapour, s d <,5 m, and ventilating the air gap between the roofing and the, the batten space, no ventilating openings into the cold loft are needed. The moisture will dry out by diffusion through the vapour permeable and further out by airflow through the ventilated batten space Cold, non-ventilated loft The later roof alternative, non-ventilated loft, has been introduced the latest years and is recommended for undetached houses and other multifamily houses to reduce the risk of fire spreading. There have been several large fires through the years where the fire has spread from windows, through the ventilating openings at the eaves and into the cold loft and further to the neighbouring dwellings. Other advantages of cold, non-ventilated lofts are reduced risk of snow drifting into the loft and reduced heat loss due to reduced airflow through the roof insulation. FIG. 1: Pitched roof with cold, non-ventilated loft. To improve the against spread of fire there are no openings to the loft at the eaves or at the ridge. The necessary ventilation is achieved by air flowing through the air gap between the roofing and the, the batten space. The theoretical study (Uvsløkk, 25) referred in this paper show the advantage of using roof with as low vapour as possible. The last years several thin products with very low vapour, s d <,2 m, has been introduced to the building market. The good drying out capability of roofs using these types of materials has been demonstrated by several studies (Ojanen 1999). To achieve sufficient fire a thin roof has to be supported by a roof sheeting of timber boards or similar. This supporting sheeting will give an additional vapour to the. On the basis of this study and the practical limit set by the vapour of a supporting sheet we recommend that the total vapour of the sheeting and the should not exceed a s d value of.5 m.

3 A roof sheeting of 15 mm thick timber boards has a s d -value of approximately.3 m (RH=85 %). By using an with a s d -value <.2 m the total vapour is within the recommended limit. 2. Computational Model The computational model developed for this study (Uvsløkk, 25) calculates the heat, air, and moisture balances in the loft considering parameters describing the building, building materials, outdoor climate and parameters influenced by the occupiers. The model includes sorption by the timber frame members and the timber boards supporting the. An example of calculated conditions in the loft is shown in fig Air leakages plays a key roll Air leakages play a key roll for the hygrothermal conditions in the loft. To treat air leakages in an appropriative way stack effect, total air tightness and air leakage distribution are taken into account by the model. The total air tightness of the exterior floor, walls and ceiling, is given by the leakage number of the building, n 5 [m³/m³h at 5Pa]. An with airtight joints represents an extra flow and will reduce the airflow from indoor through the loft. This extra flow is also taken into account by the model. Air leakages contribute to the total ventilation of the building and thereby to the reduction of the vapour content of the indoor air. As shown in fig. 4 air leakages have a positive effect and may reduce the risk of mould growth in the loft. 2.2 Mould growth potential In order to perform quantified parameter studies for comparing alternative roof designs and materials a mould growth potential is introduced. By using simplified relations between the relative rate of mould growth and temperature and relative humidity respectively a total mould growth potential is summarized for successive calculation periods. See fig. 2, which is a simplified version of a relation in (Geving 22). As an example: five days with a mould growth rate of 2 % of maximum possible growth rate is equal to one day with maximum grow rate, both giving a summarised mould growth potential of one max-day. The total yearly mould growth potential presented in this paper are the sum of the mould growth potential calculated for each of the twelve months of the year using monthly average values Temperature, C RH, % Relative mould growth rate, E/Emax. FIG. 2: Relative mould growth rate as simplified function of relative humidity, RH, and temperature.

4 RH, %, Air temperature, C RH loft RH outdoor Temp. loft Mould growth pot. roof Mould growth potential, max-days pr. months September October November December January February March April May June July August Oslo, 2 floors, total floor area: 15 m², leakage number: 4 m³/(m³h5pa), sd-value of roof :.5 m, moistureproduktion: kg/d, basic ventilation:.2 m³/(m³h), mold growth pot.: 16 max days FIG. 3: The figure shows an example of how calculated temperatures and RH in the loft and mould growth potential on the roof varies through a year. The mould growth rate is highest in the spring as the temperature in the loft is increasing and low in late autumn and winter when the temperature is low. 3. Results Some results from the study are presented graphically in the figures 4 to 9. In all these figures the curves show calculated mould growth potential for alternative vapor s. Mould growth potential, max-days/year sd = m sd =.7 m sd =.5 m sd =.4 m sd =.3 m sd =.2 m Leakage number, n5, m³/m³h5pa sd =.1 m sd =.2 m Oslo, 2 floors, total floor area: 15 m², moisture production: kg/d, basic ventilation:.2 m³/(m³h) FIG. 4: Leakage number, n 5 The figure shows how calculated mould growth potential of the varies with the leakage number, n 5, of the building and the vapour, s d -value,. The main input parameters are given in the bottom of the figure.

5 Mould growth potential, max-days/year sd = m sd =,7 m sd =,5 m sd =,4 m sd =,3 m sd =,2 m Leakage number, n5, m³/m³h5pa sd =,1 m sd =,2 m Røros, 2 floors, total floor area: 15 m², moisture production: kg/d, basic ventilation:.2 m³/(m³h) FIG. 5: House in cold inland climate, Røros The figure shows how calculated mould growth potential of the varies with the leakage number, n 5, of the building and the vapour, s d -value,. The moisture content of the supporting sheeting of the may be high wintertime, but the low temperature limits the mould growth potential. Mould growth potential, max-days/year sd = m sd =.7 m sd =.5 m sd =.4 m sd =.3 m sd =.2 m Moisture produktion, kg/d sd =.1 m sd =.2 m Oslo, 2 floors, total floor area: 15 m², leakage number: 4 m³/(m³h5pa), basic ventilation:.2 m³/(m³h) FIG. 6: Moisture production. The figure shows how calculated mould growth potential of the varies with the moisture production in the building and the vapour, s d -value,. The main input parameters are given in the bottom of the figure.

6 Mould growth potential, max-days/year Basic ventilation, m³/m³h sd = m sd =.7 m sd =.5 m sd =.4 m sd =.3 m sd =.2 m sd =.1 m sd =.2 m Oslo, 2 flors, total floor area: 15 m², leakage number: 4 m³/(m³h5pa), moisture produktion: kg/d FIG. 7: Basic ventilation. The figure shows how calculated mould growth potential of the varies with basic ventilation of the building and the vapour, s d -value,. A recommended basic ventilation of.5 air changes an hour gives a low moisture supply and reduces the risk of mould growth effectively. Mould growth potential, max-days/year ,1,, 1,, Air permeance of roof, m³/m²hpa sd = m sd =.7 m sd =.5 m sd =.4 m sd =.3 m sd =.2 m sd =.1 m sd =.2 m Oslo, 2 floors, total floor area: 15 m², leakage number: 4 m³/(m³h5pa), air permeance of the ceiling:,9 m³/m²hpa, moisture production: kg/d, basic ventilation:,2 m³/(m³h) FIG. 8: Air permeance. The figure shows how calculated mould growth potential of the varies with the air permeance and the vapour, s d -value,.

7 Mould growth potential, max-days/year resistiance of the ceiling, sd, m sd = m sd =.7 m sd =.5 m sd =.4 m sd =.3 m sd =.2 m sd =.1 m sd =.2 m Oslo, 2 floors, total floor area: 15 m², leakage number: 4 m³/(m³h5pa), moisture production: kg/d, basic ventilation:.2 m³/(m³h) FIG. 9:, s d -value, of the ceiling. The figure shows how calculated mould growth potential at the varies with the vapour of the ceiling and the vapour, s d -value,. Moisture transfer to the loft by diffusion is normally not a problem, even when the vapour of the ceiling is relatively low. 4. Discussion 4.1 Simplifications and limits of the computational model The hygrothermal conditions at a roof depend on a lot of parameters and various heat, air and moisture transfer mechanisms. Performing accurate calculations of the risk of mould growth in a loft is therefore a complicated and heavy task. The mould growth potential calculated by use of our simplified model can only to a limited degree be used to predict the risk of mould growth for a specific building. Calculations on daily or hourly basis would have given more correct absolute values than the monthly mean values used. The main objective of our study however was to compare various combinations of materials, workmanship and indoor and outdoor climate conditions in order to give guidance to designers, contractors and occupiers on how to minimize the risk of mould growth. For this purpose relative values from simplified calculations on monthly basis give valuable information Wind pressure neglected Wind pressure as driving force for air leakages is neglected of two reasons, firstly to simplify the computational model and secondly because the wind speed is rather low most of the year. At only one of the nine cites we have performed calculation for the average wind speed exceeds 4 m/s. Wind pressure differences gives air leakages and thereby an extra drying out mechanism for non ventilated lofts most of the time. Neglecting wind pressure therefore gives results on the safe side Sun radiation When the roof is exposed to sun radiation for several hours at high air temperatures in summertime the temperature may rise to more than 5 C and most of the fungi will die. This effect is neglected in the model, but play probably an important roll in practise limiting the mould growth in most pitched roofs.

8 4.2 Main parameters influencing the risk of mould growth of the As seen from the figures vapour is a very important property of a roof for non-ventilated cold lofts. Choosing products with as low vapour as possible may keep the risk of mould growth at a low level even for detached houses with relatively high moisture production, (fig. 6) low basic ventilation (fig. 7) and high moisture supply Air leakages The figures 4 and 5 illustrates that air leakages influence the risk of mould growth in several, partly contradictory ways. If the, ceiling or the building is very air tight, n 5 < 1 [m³/m³h at 5Pa], the transport of moisture from indoor to the loft by air leakages will be low and thereby give low risk of mould growth on the roof. In a very leaky house the total ventilation will be high in wintertime giving low moisture content in the indoor air. As the air permeance of the is kept constant, independent of the leakage number of the building n 5, the net moisture flow from indoor into the loft decreases Moisture supply The occupier of the building have off cause a great influence on the production of moisture in the house, the ventilating rate and thereby the moisture supply. A very efficient measure to prevent mould growth in the roof in Nordic climate is to secure low moisture supply by installing mechanically balanced ventilation with heat exchanger with high efficiency. Then the occupier will keep a high ventilating rate also in wintertime as the ventilating system provides good indoor thermal comfort at low energy costs Climate In inland regions with low temperatures the moisture content of the supporting sheeting of the may be high wintertime, but the low temperature limit the mould growth potential as seen in fig Experience We have limited experience with cold, non-ventilated lofts, but so fare we have got only a few reports of moisture problems in this type of roofs. The problems reported are caused by high level of build in moisture or air leakage up through the ceiling. 5. Acknowledgement This paper has been written within the ongoing NBI research & development programme Climate 2 - Building Constructions in a More Severe Climate (2-26), strategic institute project Weather Protection in the Construction Process. The author gratefully acknowledges all the construction industry partners and the Research Council of Norway. 6. References Uvsløkk, S. (25). Tak med kaldt loft risiko for soppvekst. Rapport, Norges byggforskningsinstitutt, Oslo, Norway Byggdetaljer (1997).Skrå tretak med kaldt loft. Byggforskserien, Norges byggforskningsinstitutt, Oslo, Norway Geving, S. and Thue, J.V. (22). Fukt i bygninger. Håndbok 5, Norges byggforskningsinstitutt, Oslo, Norway. Ojanen, T. (1999). Moisture Performance of Sealed Roof System with Permeable Underlay, Proceedings of the 5 th Symposium of Building Physics in the Nordic Countries, CTH, Göteborg, Sweden, p