During recent years, energy efficiency in buildings has become one of the main priorities of European energy policy, playing a major role in the so-called “20-20-20 targets” (20% reduction of greenhouse gas emissions, increasing renewable energy production to 20%, and a 20% improvement in the EUs overall energy efficiency). Great effort has to be made not only in the construction of new buildings, but also in the refurbishment of existing ones, according to high energy-efficiency standards. In particular, concerning the building envelope, adequate design and materials have to be employed to ensure high quality and damage-free constructions. Avoiding detailed description of well-known standard solutions, this work focuses on problems that continue to represent a challenge from the technical point of view, such as internal insulation systems. Indeed, critical moisture conditions may occur in cases where internal thermal insulation is applied. In the long term, vapor diffusion from the inside of the building may lead to high levels of moisture content behind the insulation, and even to the damage of components made of organic materials, such as timber beams. A prerequisite for the correct planning of an insulation system is a deep knowledge of vapor and liquid water transfer in the employed materials. A lot of work on both numerical modeling and experimental research has been recently gone into this topic; the purpose of this work is to extend the resultant knowledge, focusing on both the material and the component scale. Concerning the material scale, a semi-empirical function for mathematical description of the liquid water diffusivity is proposed by giving a physical interpretation of both phenomena contributing to the liquid transfer, means surface diffusion and capillary suction. The second task of this work is the development of simple experimental procedures for the determination of the liquid water diffusivity in materials used for building insulation (for example, calcium silicate board). Two alternative procedures are proposed: the first one based on desorption tests and the second one on heat flux measurements carried out with a hot plate apparatus. In particular, the second proposed procedure differs substantially from the other methods generally used to the same aim. The main advantages of the proposed method are the limited required equipment (only a guarded hot plate and a balance are used), and the fact that the dependence of the liquid water diffusivity on the temperature can be determined. The experimental results are employed to validate a numerical model, including coupled heat and moisture transfer in porous building materials introduced in the first part of the thesis. On the component scale, great effort is made in the modeling of convection in air gaps and for evaluating its effect on moisture risk in constructions. Models based on computational fluid dynamics (CFD), as well as simplified models, suitable for long time simulations, are proposed. The plausibility of the result is analyzed by means of numerical tests and comparison with measured data from the available literature. In the last part of the thesis, solutions for practical building physics problems such as mold germination behind internal insulation and at timer beam ends, are introduced. Considerations on the air-tightness of the building envelope are made by taking into account the impact of air leakages on moisture distribution. Different sealing systems for the wall-beam junction are analyzed. Furthermore, the behavior of building components sensitive to moisture damage is simulated by means of both 2D and 3D models, with the results compared. This work aims to be a brief but complete overview of the hygrothermal behavior of building materials and components, presenting an innovative and detailed characterization of the physical background, but also proposing solutions for practical building physics problems.