Moisture behavior of internal insulation with fiber insulation materials

© Fraunhofer IBP

Calculated change in relative moisture on the separating layer of lime mortar / internal insulation at the Holzkirchen location compared to the range of fluctuations reported in the Passivhaus Research Group Vol. -> with rain, absorbent mortar

© Fraunhofer IBP

-> with rain, water-repellent mortar

© Fraunhofer IBP

-> no rain; red = cellulose fiber 1; yellow = cellulose fiber 2; green = cellulose fiber 3; gray = mineral wool fiber 1; blue = mineral wool fiber 2 Range of fluctuations reported by the Research Group Vol.: gray stripes = for mineral wool; green stripes = for cellulose fiber.

Installing internal insulation is often the only way to improve heat conservation when renovating older buildings. From a building physics perspective, however, the Central European climate is such that internal insulation is usually less desirable than external as far as thermal bridges (heat loss), condensation and drying potential are concerned. This makes careful planning and execution a prerequisite for anyone wanting to install internal insulation that is going to last. An article in the WTA Journal 4/04 about renovating older buildings to be more energy efficient using internal insulation examines the hygrothermal behavior of various internal insulation materials without built-in vapor barriers. The study compares moisture buildup on the reverse side of the insulating layer. Where structural protection against driving rain was already in place, conditions conducive to condensation were achieved in fiber insulation (e.g. cellulose fiber or mineral wool) over varying lengths of time during the winter. The author therefore advises against using these materials in the absence of a vapor barrier.

However, an article on moisture protection solutions that appeared in Passivhaus’s Research Group Volume No. 32 (2005) comes to a completely different conclusion. This article states that, in every case, the variants of cellulose fiber insulation without vapor barrier that were tested achieved a maximum relative moisture of 80 percent on the cold side of the insulation. In contrast, the maximum values for mineral wool insulation featuring a variable vapor barrier were markedly higher, with relative moisture levels of up to 98 percent. According to these results, the moisture behavior of cellulose fiber insulation without vapor barrier would be far better than internal insulation using mineral wool and an additional variable vapor barrier. Since these contradictory findings consistently cause construction companies and planners to seek clarification, the various designs and recommendations shall now be examined in more detail.


In the Passivhaus publication, different internal insulation systems were modeled using the DELPHIN simulation program and analyzed in terms of the relative moisture on the original interior surface of the existing wall.

The following tests were conducted using hygrothermal simulations run with WUFI®, a model developed at Fraunhofer IBP. In keeping with the examples cited in the Passivhaus publication, the results were calculated for a 30-cm thick solid brick wall featuring lime‑cement mortar on the outside and lime mortar on the inside. The following configurations were tested as internal insulation: three samples of cellulose fiber insulation (each 8 cm thick) without additional vapor barrier, and three samples of mineral wool insulation (each 8 cm thick) with a moisture-variable vapor barrier.

The tests were performed on an exterior westward-facing wall at the Holzkirchen location. Exterior mortar was applied, once in an absorbent form, and once as a water-repellent version (w = 0.5 kg/m²√h), in compliance with DIN 4108-3. To reflect the conditions found in areas not exposed to driving rain, results were also calculated without including driving rain as a factor. The calculations start in October and cover a five-year period.


The Passivhaus publication cites the range of fluctuations of relative moisture between the lime mortar and internal insulation as reported for the fifth year of the simulation (striped areas in diagrams 1-3). The results of the new tests were charted to show fluctuations in relative moisture at the same position over the entire test period. Diagram 1 shows the results of the calculations with rain and absorbent mortar. At the end of the test period, all six insulation variants exhibited very high relative moisture levels between 97 and 99 percent. While the Passivhaus publication forecast similarly high relative moisture for mineral wool insulation under the same conditions, their reported values for cellulose fiber insulation were too low (maximum value was too low by 19 percent, minimum by 42 percent).

Diagram 2 displays the results for the calculation based on water-repellent exterior mortar. Relative moisture climbed over the five-year period, slowly but steadily. The cellulose fiber samples exhibited a maximum value of 93 percent in the fifth year, 15 percent higher than the maximum reported in the Passivhaus publication. The calculated minimum value of 77 percent is 24 percent above the value given by Passivhaus. Relative moisture in the mineral wool samples also rose consistently, reaching values of between 87 and 90 percent in the fifth year. In this case, the calculated relative moisture was found to lie within the range of fluctuations reported by the Passivhaus publication, although the calculated amplitude was much less.

In the calculation without rain (diagram 3), the cellulose fiber insulation fluctuated between 68 and 92 percent – that means both the maximum and minimum values are above what was reported in the Passivhaus publication (14 percent and 15 percent higher, respectively) for the scenario with rain and water-repellent mortar. The range of fluctuations in variants with mineral wool and vapor barrier was recorded as being between 73 and 80 percent, much narrower than the range given in the Passivhaus publication (53 to 78 percent).


In contrast to the major difference between the two kinds of fiber insulation as reported in the Passivhaus publication, the tests presented here reveal that, overall, walls internally insulated with cellulose fiber insulation (without moisture-variable vapor barrier) and mineral wool insulation (with moisture-variable vapor barrier) exhibit similar hygrothermal behavior. When large volumes of moisture from precipitation penetrate the interior, sorptive cellulose can help curtail spikes in moisture. However, other risks may arise if protection against driving rain is inadequate, such as durability problems, frost damage or increased heat loss in the damp materials. If the moisture enters the material primarily through diffusion from the interior, the variants with mineral wool fiber and vapor barrier are more effective.

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