Mould Growth Risk for Internal Retrofit Insulation of Heritage-Protected Timber Plank Frame Walls
Abstract
1. Introduction
1.1. Climate Goals and Energy Consumption
1.2. Heritage Protection
1.3. Historical Timber Plank Frame Buildings in Norway
1.4. Internal Insulation and Moisture Damage
1.5. Mould Growth Risk
1.6. Objective and Scope
- 1.
- What is the range of water vapour resistance for commonly used building papers in heritage-worthy buildings?
- 2.
- What are the effects of the water vapour resistance of building paper on the moisture performance of internally insulated timber plank frame walls?
- 3.
- What is the effect of moisture-adaptive vapour barriers on the possibilities for internal insulated timber plank frame walls?
2. Materials and Methods
2.1. Laboratory Measurements
2.1.1. Selection of Products
2.1.2. Preparation of Samples
2.1.3. Test Procedure
2.2. WUFI® Pro Simulations
2.2.1. Geometry and Materials of the Original Wall
2.2.2. Geometry and Materials of Internally Retrofitted Wall
2.2.3. Simulation Input Data
2.2.4. Assessment of Mould Growth Risk
3. Results
3.1. Values of Building Paper
3.2. WUFI® Pro Simulations
- The exterior building paper was found to significantly affect the risk of mould growth for both the original wall and the internally retrofitted wall.
- Cases 1–4 were simulations of the original wall in different climates and with different types of exterior building paper. Cases 2 and 4, with vapour-permeable building paper (two layers of E1; value: 0.040 m) on the exterior side, revealed a reduced risk of mould growth compared to Cases 1 and 3, in which a more vapour-tight building paper was used (sample E3; value: 5.293 m).
- Cases 5–7 comprised different thicknesses of insulating mineral wool, with PE foil and building paper E3 ( value: 5.293 m) on the exterior side. From these simulations, it is clear that the thickness of the insulation has a large impact on the mould growth risk. An insulation thickness of 50 mm with a PE foil vapour barrier resulted in a low mould growth risk measure.
- Cases 8–9 demonstrate the effects of different thicknesses when a vapour-open exterior building paper (E1 with sd value 0.040 m) was applied. Such a vapour-open exterior building paper, together with a PE foil vapour barrier, allowed for an internal retrofit thickness of 100 mm.
- Case 10 demonstrated that the outdoor climate affects mould growth risk, as the location in Hamar had a lower mould growth risk than the location in Kristiansund (Case 7).
- Cases 11–13 had no vapour barrier on the interior side of the internally retrofit insulation. The simulation showed that the mould growth risk was high in all of these cases, independent of the climate and the vapour impermeability of the exterior building paper.
- Cases 14–16 included different thicknesses of wood fibre insulation in combination with exterior building paper E3 ( value: 5.293 m). This construction configuration allowed for an internal insulation thickness of 50 mm provided by a PE foil vapour barrier.
- Cases 17–19 revealed that the mould growth risk was high for all cases with MAVB, independent of the thickness of the insulation.
- Case 20 included a shortwave radiation absorptivity of 0.8, which matched the dark-coloured exterior cladding. This case revealed that the shortwave radiation absorptivity rate had a low impact on the mould growth risk of the wall, which was also the case when MAVB was applied. A shortwave radiation absorptivity of 0.8 did not contribute to a lower risk of mould growth compared to Case 17, with a shortwave radiation absorptivity rate of 0.4.
- Case 21 shows that an indoor climate with a lower humidity class (i.e., lowering from class 3 to class 2) significantly reduced the mould growth risk.
4. Discussion
4.1. Vapour Resistance of Types of Building Paper
4.2. Moisture Performance of Internally Insulated Timber Plank Frame Walls
4.3. Moisture Performance of the Internally Retrofitted Wall with MAVB
4.4. Assumptions for the Simulations
4.5. Limitations and Future Studies
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
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Material | Time ca. | Use | Data |
---|---|---|---|
Tar- or asphalt-impregnated sheathing paper and cellulose board | 1870– | Outside | Asphalt paper is available in both sanded and unsanded variants [14]. Sheathing paper is a thin type of paper produced as felt paper with added wood pulp, which is then impregnated. It weighs 250–300 g per m2. |
Newspaper | 1850–1920 | Inside | Around 50 to 55 g per m2, according to a specialist in paper materials. |
Felt boards | 1870–1920 | Inside | These weigh around 350 g per m2 [14]. |
Unimpregnated cellulose boards | 1870–1950 | Inside | These are made from cellulose pulp [14] and used only in dry areas, as they absorb moisture. |
Product | Picture | Year | Location | Description | |
---|---|---|---|---|---|
Exterior paper | E1 | 1908 | Orkanger train station | Brown paper. Taken from “the knee walls” on the outside. | |
E2 | 1938 | Belsnes | Asphalt-impregnated paper. Black. Not grainy. House built in 1938 and renovated in the 1950s. | ||
E3 | 1938 | Belsnes | Asphalt-impregnated paper. Black. More grainy than E2. House built in 1938 and renovated in the 1950s. | ||
E4 | 1938 | Vanvikan, Indre Fosen | Black paper, probably asphalt-impregnated. Unknown origin and use. One roll found in a barn from 1938. The associated house was renovated in the 1950s–1960s. | ||
Interior paper | I1 | 1908 | Orkanger train station | Brown-grey wool paper. Taken from “the knee walls” on the inside. | |
I2 | 1908 | Orkanger train station | Brown-grey wool paper. Taken from “the knee walls” on the inside. | ||
I3 | 1932 | “Orkmannen” newpaper | Newspaper from Orkdal in the south of Trøndelag in Norway [53]. A local newspaper from 1926 to 1945. | ||
I4 | 1933 | “Orkmannen” newspaper | Same as above. | ||
I5 | 1936 | “Orkmannen” newspaper | Same as above. | ||
I6 | 1880 | Northern Trøndelag, Norway | Brown paper. One smoother side and one rougher side. Found on the interior side of exterior walls in an 1880s log house. Assembly date unknown. | ||
I7 | No date | Oslo | Haakon paper. Brown. The entire roll was stored in a basement and had not been used. | ||
I8 | 1880 | Northern Trøndelag, Norway | Impregnated wool paper. Black-brown colour on one side, grey on the other side. Found inside an 1880s log house. Assembly date unknown. |
No. | E1 | E2 | E3 | E4 | I1 | I2 | I3 | I4 | I5 | I6 | I7 | I8 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Thickness [mm] | 1 | 0.52 | 0.68 | 0.71 | 0.58 | 0.45 | 0.98 | 0.10 | 0.10 | 0.12 | 0.50 | 0.46 | 1.40 |
2 | 0.48 | 0.70 | 0.72 | 0.59 | 0.52 | 0.96 | 0.10 | 0.10 | 0.12 | 0.50 | 0.46 | 1.24 | |
3 | 0.50 | 0.75 | 0.72 | 0.59 | 0.54 | 0.96 | 0.10 | 0.10 | 0.12 | 0.50 | 0.43 | 1.24 | |
4 | 0.47 | 0.73 | 0.60 | 0.48 | 0.10 | 0.10 | 0.12 | 0.50 | 0.43 | ||||
5 | 0.48 | 0.75 | 0.59 | 0.50 | 0.10 | 0.10 | 0.12 | 0.50 | 0.44 | ||||
Average [mm] | 0.49 | 0.72 | 0.72 | 0.59 | 0.50 | 0.97 | 0.10 | 0.10 | 0.12 | 0.50 | 0.44 | 1.29 | |
Grammage | |||||||||||||
[g/m2] | 322 | 541 | 541 | 574 | - | - | 57 | 58 | 57 | 377 | 261 | 767 |
Sample No. | Orientation |
---|---|
E1–E4, I1–I5, I7 | Identical on both sides. The orientation had no impact. |
I6 | The smooth surface facing down to the salt solution. |
I8 | The impregnated side facing down to the salt solution. |
Time Interval | Samples | Test Chamber |
---|---|---|
Every 120 min | I1 and I2 | Gintronic GraviTest |
Every 24 h | E1, E2, E3, E4, I6, I7, and I8 | Test enclosure with shelves |
Every morning and afternoon | I3, I4, and I5 | Test enclosure with shelves |
Parameter | Input | Comment |
---|---|---|
Air exchanges in cavity | 0 h−1 | See the Data Availability Statement. |
Initial temperature | 21 °C [61,62] | |
Initial RH | 0.7 | |
Wall inclination | 90° | |
Exterior surface heat resistance, Rse | 0.0588 (m2K/W) | Standard value in WUFI® Pro [63]. |
Interior surface heat resistance, Rsi | 0.125 (m2K/W) | Standard value in WUFI® Pro [63]. |
Ground shortwave reflectivity | 0.2 | Standard value in WUFI® Pro [63]. |
Simulation start | October | Beginning of wet season |
Simulation duration | 5 years | |
Building height | <10 m | Short building |
Driving rain coefficients | R1 = 0 R2 = 0.07 | Standard values in WUFI® Pro for short buildings [63]. |
Surface treatment of interior and exterior cladding | Oil painting value: 1.35 m | Oil paint has the highest level of vapour resistance, as measured in [64,65]. Three layers of paint. |
Indoor temperature | 21 °C [61,62] | |
Orientation wall | South | |
Timber cladding and timber planks | Scandinavian spruce, transversal direction II | Material in WUFI® Pro, taken from the NTNU database |
Paper on interior side | To layers of I6 ( value: 0.046 m) | As all types of interior paper were vapour-permeable, the least vapour-permeable type was chosen. |
Parameter | Basic Input | Variable (s) |
---|---|---|
Outdoor climate | Kristiansund | Hamar |
Insulation thickness | 50 mm | 100 mm 250 mm None |
Insulation material | Mineral wool ( = 0.034 W/(mK)) | Wood fibre insulation ( = 0.038 W/(mK)) |
Vapour barrier | PE foil 0.2 mm ( value: 87 m) | MAVB No vapour barrier |
Shortwave radiation absorptivity | 0.4 (white house) | 0.8 (dark house) |
Paper on the exterior | E3 ( value: 5.293 m) | Two layers of E1 ( value: 0.040 m) |
Humidity class inside | 3 | 2 |
Colour | Risk |
---|---|
Red | High mould growth risk: RH over 80% and temperature between 5 and 30 °C for a continuous time period of minimum one day continuous. |
Yellow | Medium mould growth risk: RH under 80% but exceeding 80 % for a short time period (<one day continuous); alternatively, RH increasing over the simulated 5-year time period (this needs to be investigated further over a longer time period); alternatively, RH over 80% at the beginning of the simulation but decreasing over the simulated 5-year time period. |
Green | Low mould growth risk: RH under 80 %, and RH does not increase during the simulated 5-year time period. |
No. | Description | Value [m] |
---|---|---|
E1 | Brown paper. | 0.020 |
E2 | Asphalt-impregnated paper; not grainy. | 0.881 |
E3 | Asphalt-impregnated paper; grainy. | 5.293 |
E4 | Black; impregnated paper. | 0.435 |
I1 | Wool paper. | 0.018 |
I2 | Wool paper. | 0.021 |
I3 | Newspaper from 1932. | 0.013 |
I4 | Newspaper from 1933. | 0.014 |
I5 | Newspaper from 1936. | 0.008 |
I6 | Brown paper. | 0.023 |
I7 | Haakon paper; brown. | 0.017 |
I8 | Impregnated wool paper. | 0.022 |
Case | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Input Parameters | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | |
Insulation thickness | 0 mm (original wall) | x | x | x | x | |||||||
50 mm | x | x | ||||||||||
100 mm | x | x | ||||||||||
250 mm | x | x | x | |||||||||
Outdoor climate | Kristiansund | x | x | x | x | x | x | x | x | |||
Hamar | x | x | x | |||||||||
Insulation material | Galva mineral wool | x | x | x | x | x | x | x | ||||
Wood fibre insulation | ||||||||||||
Vapour barrier | Normal | x | x | x | x | x | x | |||||
MAVB | ||||||||||||
None | x | x | x | x | x | |||||||
Shortwave radiation | 0.4 | x | x | x | x | x | x | x | x | x | x | x |
absorptivity | 0.8 | |||||||||||
Paper exterior side | E3 | x | x | x | x | x | x | |||||
Two layers of E1 | x | x | x | x | x | |||||||
Humidity class | 3 | x | x | x | x | x | x | x | x | x | x | x |
inside | 2 | |||||||||||
Mould growth risk |
Case | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Input Parameters | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | |
Insulation thickness | 0 mm (original wall) | x | |||||||||
50 mm | x | x | x | x | x | ||||||
100 mm | x | x | |||||||||
250 mm | x | x | |||||||||
Outdoor climate | Kristiansund | x | x | x | x | x | x | x | x | ||
Hamar | x | x | |||||||||
Insulation material | Mineral wool | x | x | x | x | x | x | ||||
Wood fibre insulation | x | x | x | ||||||||
Vapour barrier | Normal | x | x | x | |||||||
MAVB | x | x | x | x | |||||||
None | x | x | x | ||||||||
Shortwave radiation | 0.4 | x | x | x | x | x | x | x | x | x | |
absorptivity | 0.8 | x | |||||||||
Paper exterior side | E3 | x | x | x | x | x | x | x | x | x | |
Two layers of E1 | x | ||||||||||
Humidity class | 3 | x | x | x | x | x | x | x | x | x | |
inside | 2 | x | |||||||||
Mould growth risk |
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© 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Share and Cite
Harberg, M.E.; Asphaug, S.K.; Kvande, T. Mould Growth Risk for Internal Retrofit Insulation of Heritage-Protected Timber Plank Frame Walls. Heritage 2025, 8, 278. https://doi.org/10.3390/heritage8070278
Harberg ME, Asphaug SK, Kvande T. Mould Growth Risk for Internal Retrofit Insulation of Heritage-Protected Timber Plank Frame Walls. Heritage. 2025; 8(7):278. https://doi.org/10.3390/heritage8070278
Chicago/Turabian StyleHarberg, Martha Eilertsen, Silje Kathrin Asphaug, and Tore Kvande. 2025. "Mould Growth Risk for Internal Retrofit Insulation of Heritage-Protected Timber Plank Frame Walls" Heritage 8, no. 7: 278. https://doi.org/10.3390/heritage8070278
APA StyleHarberg, M. E., Asphaug, S. K., & Kvande, T. (2025). Mould Growth Risk for Internal Retrofit Insulation of Heritage-Protected Timber Plank Frame Walls. Heritage, 8(7), 278. https://doi.org/10.3390/heritage8070278