Journal of Building Physics

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This paper introduces a generalized limit states (GLS) method for evaluating long-term performance of building envelopes. The limit states method was originally developed for the design of structures to satisfy performance require ments for safety and serviceability. However, it may be generalized to address durabi lity of building envelopes. The approach used in this paper involves selecting a re quired period of service (design life) for the component of the building envelope, and then to evaluate the envelope for durability by applying the GLS method.

The goal of this research project was to get more information about the influence of wind pressure on the heat transmission through timber frame con structions and to establish a recommended limit for air permeance of wind barriers.* The project was divided into three parts: wind pressure measurements on a rotatable test house, hot-box measurements on a wall, and calculations. The theoretical studies as well as the experimental investigations in the hot-box, have been restricted to one specific type of forced convection in the thermal insulation, the interchange of air be tween the insulation and the air gap between the wind barrier and the outer cladding. The results of the project show the importance of protecting the insulation layer with a wind barrier to achieve full effect of the insulation in wind exposed constructions. The measurements indicate that heat loss caused by this type of forced convection can be three to ten times higher than calculated for ideal constructions. Based on the measurements carried out in this project, Norwegian Building Research Institute, NBI, is recommending an upper limit for the air permeance of wind barriers, includ ing joints, of 0.05 m³/m² h Pa (1.4E-5 m³/m ² s Pa).

In Norway, it is common practice to use a polyethylene foil as a vapour barrier on the warm side of the thermal insulation in wood frame construc tions. The question has been raised whether such a vapour barrier is really necessary. It has been claimed that so-called "breathing constructions" would contribute to bet ter indoor climate conditions and have just as good moisture performance as tradi tional constructions, even in a cold climate.

The paper summarises the results from two laboratory test series with wood frame walls having barriers with different water vapour resistance on warm and cold sides of the heat insulation. The results are discussed and some preliminary requirements regarding the vapour resistance of the barriers are outlined. The subject will be fol lowed up in further research in the laboratory and in a test house.

This paper is concerned with the heat transfer through materials whose thermal conductivities vary with temperature. In particular, complete solu tions are given for the steady state, one dimensional conduction heat transfer through a material with a temperature dependent thermal conductivity by use of the Kirch hoff transform. To determine this property as a function of temperature and for an application temperature range, a series of tests should be performed at different aver age temperatures and temperature differences and then a suitable temperature func tion (polynomial, exponential, etc.) should be found from the definition of the effec tive thermal conductivity. Discussion and examples are given for the temperature dependent apparent thermal conductivity of thermal insulating materials.