3 Interior insulation
In cooperation with UCD
3.1 General principles of interior wall insulation
In historic buildings, external insulation of walls is generally not possible as the façade is usually under monument protection and the original appearance – stuccos, sculptures – has to be preserved. In this case there are two possibilities when retrofitting the building: applying interior insulation or not insulating the wall at all. If no insulation is applied but the windows are changed or sealed, or if the heating system is modernised, there is a risk of mould growth on the cold surfaces. Even driving rains might cause mould problems. Interior insulation has advantages, but careful planning and execution is necessary as its installation is not risk-free (see below).
Obviously, the most important advantage of interior insulation is that the appearance of the building does not change from the outside. However, if the interior of the rooms is also of high value – covered with frescoes, for example – insulation must be abandoned. For interior insulation no external scaffolding is necessary, hence the costs are lower. In addition, it can be applied only in certain rooms, or parts of the façade, if for some reasons the building is to be retrofitted only partly. The potential energy saving is lower than with external insulation, but still significant: the results of Csoknyai [CSO98] showed that with 4 cm interior EPS insulation of a historic building in Hungary 27 % heating energy saving could be realised.
The problems and risks connected to interior insulation are the following:
§ Vapour condensation: in the layer of the interior insulation the temperature drops rapidly, and the load-bearing structure becomes colder than without insulation. This might lead to vapour condensation on the surface between the insulation and the structure. If interior insulation is applied, the vapour conditions in the structure must always be checked. Construction failures might also lead to similar problems.
§ Thermal bridges: the thermal bridge effect will be of greater extent than with external insulation.
§ Risk of freezing: due to the temperature drop in the structure, water in pipes buried in the external wall might freeze.
§ Thermal mass will be reduced: historic buildings have large thermal mass in general. Interior insulation isolates the heavy structure from the internal space, so that the possible daily heat storage will be considerably cut down. Conversely, this can be advantageous if intermittent heating is applied.
Vapour calculation
If interior insulation is applied, a vapour calculation is always necessary to check against surface and interstitial condensation. It can be done manually based on stationary conditions or with computer programmes.
If a stationary calculation proves that there is vapour condensation in the structure, it does not necessarily mean that the structure is problematic. Two more conditions must be checked:
§ The acceptable moisture content depends on the material type: in certain cases a moisture content corresponding to the sorption saturation or a higher value is still acceptable, but for sensitive materials, e.g. wood and some fibrous materials, even capillary condensation should be avoided.
§ Structures are dryer at the end of the summer period than at the end of the winter period. A certain time period is necessary so that stationary conditions typical for winter can develop. The length of this period depends on the vapour flow and the allowable moisture content of the materials. If this period is longer than the heating season, the structure can be regarded as acceptable even if the stationary calculation shows condensation.
3.2 Practical implementation of interior insulation
Walls and flat roofs
The insulation material can be either directly glued to the surface or placed between timber battens depending whether it is a rigid or non-rigid material.
Installing a vapour barrier close to the internal surface can eliminate moisture-related problems. Insulation products with an integrated vapour barrier are also available on the market. In the absence of a vapour barrier, moisture infiltrating from the room will leave the insulation layer very slowly due to the relatively high diffusion resistance of the rendered masonry wall. This will, however, result in a very different moisture behaviour than originally, which is not always favourable.
The continuity and tightness of the vapour barrier is essential. Special attention has to be paid to the penetrations. To protect the layer against mechanical damages, it should be mounted a few centimetres from the surface, ideally behind an “installation layer” where all the cables and plugs are located. However, the junctions of the external and internal walls and floors where the barrier ends are still problematic (Figure 64).
Figure
64: The risk of
condensation is high, especially at the junctions where the barrier is
interrupted
In certain cases – based on the results of the vapour calculation – the vapour barrier can be omitted. This might be the case if the insulation material has a high vapour diffusion resistance (e.g. cellular glass), or if the calculation shows that the moisture condensing on the surface between the insulation and the masonry does not harm the materials. In the latter case, it has to be ensured that the wall can dry out during dry periods and moisture resistance materials should be preferred (mineral fibre, rigid PUR foam or calcium silicate boards) [RIC07].
Due to the placing of interior insulation thermal bridges occur. The effect of thermal bridges should be minimised at the wall and floor junctions. Here, the interior insulation should be extended by approx. 50 cm, as shown in Figure 65 a). Also the wall-window junction should be covered with insulation (Figure 66), behind radiators not only the backside but also the sidewalls should be insulated and special attention must be paid to timber joist floors supported in pockets in the external wall. With interior insulation applied, the temperature of the external wall drops and it might be around or below the dew point at the supports. Timber is especially sensitive to moisture and rotting may develop at the joist ends. A solution is to uncover the joist ends and fill all the voids with PUR-foam to minimise the air convection around the joist ends [RIC07].
The interior insulation obviously affects the internal space: it might be necessary to move the radiators, for example. The insulation of kitchens and bathrooms is especially problematic due to the built-in furniture and sanitary equipments. If these places are neglected, the risk of mould growth increases as these surfaces are colder than the insulated parts. This has to be considered during planning.
a) b)
Figure 65: Junction of external and internal walls a) with internal insulation and vapour barrier; b) calcium-silicate wedge-shaped profile
Figure 66: Window junction
A special technique is to install an insulated suspended (false) ceiling in high rooms. This way the heated volume of the room is reduced, which results in a reduced heating energy consumption. This measure is only feasible in insignificant rooms, since it significantly changes the character of the room.
Insulation of pitched roofs
The insulation of pitched roofs is necessary if the existing insulation of the built-in attic is insufficient or if the loft is converted into a usable space for the first time. Preconditions of interior insulation are that the existing roof covering is in a good condition and that there is an existing functioning secondary waterproofing membrane. In historic buildings, however, this layer is usually missing except if it had already been mounted during a previous retrofit or conversion. Only in special cases - if the roof slope is appropriate and the water tightness of the covering is sufficient - can the waterproofing be omitted and the space between the rafters filled with a hydrophobic insulation material [HAU01].
If interior roof insulation is applied that does not change the external appearance, then no new roof covering or scaffolding will be necessary. In addition, the roof is waterproof also during the construction works. The drawback of the measure is that the internal space will be to some extent smaller and the works might temporarily obstruct the use of the attic space.
The most common measure is to apply insulation between the rafters. However, as the rafter height is limited, it might be necessary to mount an additional board on each rafter or to double them. The application of battens perpendicular to the rafters is a good solution. This way, thermal bridges are reduced to “points” at the junction of rafters and battens. Cables etc. can run in this additional insulation layer. Wedge-felt materials made from natural and mineral fibres or cellulose flakes blown into the cavities are suitable for filling the space inside the frame. If the space between the rafters was originally insulated, it might be worth retaining the insulation provided its insulating capacity is adequate (l < 0,6 W/mK) and that thermal bridges are limited [HAU01].
If the existing secondary waterproofing is vapour tight (e.g. bituminous felt, PE sheeting), the infiltrating moisture can leave the insulation only very slowly. In this case, an additional ventilation cavity below the secondary waterproofing might be necessary, which is also favourable for the summer conditions. A vapour calculation is always necessary. To reduce the diffusion permeability and to increase the air tightness of the structure, a continuous vapour barrier should be applied. The air tightness also contributes to the reduction of heat losses in the winter. Careful execution is necessary at joints and junctions. The best is not to interrupt the layer and mount it below the purlins, collar beams etc. It might happen that temporary condensation occurs on the internal surface of the membrane, which makes it advisable to include an air layer between the vapour barrier and the internal cladding. Vapour barriers with variable vapour diffusion resistance depending on the ambient moisture level are also available: their diffusion resistance is high in winter and only very small quantities of moisture can infiltrate the insulation. In summer, the resistance decreases allowing the insulation to dry out towards the inside [RIC07].
Figure 67: Internal insulation of the roof
3.3 Special materials
Two materials are described in detail. Calcium-silicate insulating boards are suitable for handling moisture-related problems without installing a vapour barrier, which would entirely change the moisture mechanisms. Innovative vacuum insulation panels are a good choice if space is scarce.
Calcium-silicate insulating boards
Calcium-silicate insulation is a material open to diffusion and with high capillary suction ability (Table 7), which can be applied successfully on the interior surface of walls or flat roofs to handle mould growth problems. It can absorb humidity of approx. 3.5 times of its own weight without significant deformation or reduction of its insulating capacity: 1 m2 board of 2.5 cm thickness can absorb 21 l water. If the air humidity decreases later, the stored moisture will be released again, hence controlling the relative humidity of the room. The material is alkaline, which also contributes to prevent mould growth.
Table 7: Physical characteristics of calcium-silicate insulating boards
|
Thermal conductivity, l |
0.045 – 0.07 W/mK |
|
Density, r |
115 – 300 kg/m3 |
|
Vapour diffusion coefficient, m |
2 / 20 |
|
Combustion category |
Non-combustible |
Insulation boards can be applied locally, for example for the insulation of end walls or at window junctions where mould growth issues are the most critical. For the minimisation of the thermal bridge effect at the junction of external walls and partitions or floors, a special wedge-shaped profile of approx. 60 cm length has been developed to provide an aesthetic transition (Figure 65 b).
The calculations and measurements of Csoknyai for calcium-silicate boards showed that in these structures the partial vapour pressure might reach the saturation point if stationary conditions are assumed, but the time period necessary for the development of these conditions is significantly longer than the heating period [CSO03]. Hence there is no risk of material damage or mould growth. According to his suggestions, at least 5 cm thickness is necessary to stop mould growth depending on the original structure. If energy saving is the primary goal, larger thicknesses might be necessary.
Vacuum insulation panels
The use of vacuum insulation panels (VIP) is an innovative solution if the available space is limited. The thermal resistance of these elements is higher by a factor of 5-10 compared to conventional insulation materials, the thermal conductivity is around 0.004 W/mK. In other words, 2 cm VIP equals 15 - 20 cm of mineral wool or polystyrene insulation.
VIP consists of a core material with a good compressive strength laminated with gastight composite foils (e.g. aluminium or synthetic foils) in a vacuum chamber [HEG06]. Fibres, open-cell foams or pyrogenic silicic acid can be the filling material, for example. The initial gas pressure is 1-5 mbar, this increases by about 2 mbar every year. The gas pressure influences the thermal conductivity. The expected lifetime of the panel is around 30-50 years. The panels are virtually vapourtight.
VIP with plasterboard facing can be applied internally in walls, roofs or floors. It is also a good solution for the insulation of floors over an unheated basement: if it is not possible to mount insulation on the bottom side of the slab, VIP can be integrated in the floor construction without changing the original floor level. In this case, floor finishes have to be removed and VIP is installed under the new screed. Lifting of floor covering will result in a changed appearance as the surface cannot be exactly re-created.
VIP is sensitive to mechanical damages and has to be protected before, during and after the installation. Normally prefabricated sandwich elements are applied where VIP is combined with other insulation materials, such as EPS or XPS. The panels obviously cannot be cut and special sizes are costly.
3.4 References
[CSO98] Csoknyai T. Energy conscious retrofit of traditional buildings (Hagyományos építésű lakóépületek energiatudatos felújításának lehetőségei - in Hungarian), research report, BUTE, 1998.
[CSO03] Csoknyai T. Thermal and vapour analysis of Masterclima insulation boards (A Masterclima hőszigetelőlapok páratechnikai és hőtechnikai elemzése - in Hungarian), research report, BUTE, 2003.
[HAU01] Hauser G, Höttges K, Otto F and Stiegel H. Energieeinsparuing im Gebäudebestand. Gesellschaft für Rationelle Energieverwendung E.V., January 2001.
[HEG06] Hegger, Auch-Schwelk, Fuchs and Rosenkranz. Construction Materials Manual, Birkhaeuser, 2006.
[RIC07] Richarz C, Schulz C and Zeitler F. Energy-Efficiency Upgrades. Edition DETAIL, Birkhaeuser, 2007.