In cooperation with UCD
Major energy savings can be realised with exterior insulation as the original structures in historic buildings are usually in bad condition. Besides energy savings, exterior insulation can also be the solution to fabric damage, mould growth or air tightness problems. However, many constraints arise in historic buildings, hence these buildings are usually exempt from energy regulations. The Energy Performance Building Directive, for example, allows Member States to decide not to apply the energy requirements to protected buildings or monuments, ‘where compliance would unacceptably alter their character or appearance’. Still, it is important to insulate as many structures as possible for the best results regarding both energy savings and fabric protection.
For external walls, exterior insulation is not generally feasible if the façade is protected, so in these cases interior insulation must be considered. The exterior insulation of other parts of the building envelope, the topmost floor, cellar floor etc. is usually possible nevertheless, even if, for example, the ceiling and door height, position of windows etc. constrain the number of options.
With a simplified approach, savings in heating energy can be calculated according to the improvement of the thermal transmittance (design U-values calculated on the basis of thermal conductivity, Table 2). In a more complex approach, side effects of the insulation should be considered too (see below). However, the envisaged savings will only be realised if the retrofit is comprehensive and the old heaters are changed or modernised at the same time.
A1-A2: non-combustible, B1: not readily flammable, B2: flammab
|
Insulation type |
l (W/mK) |
Fire resistance class |
|
Expanded polystyrene (EPS) |
0,03-0,04 |
B1 |
|
Mineral wool |
0,035-0,04 |
A1/A2 |
|
Cork |
0,045-0,05 |
B2 |
|
Wood wool boards |
0,09 |
B1 |
|
Foam glass |
0,045-0,05 |
A1 |
|
Polyurethane foam |
0,025-0,03 |
B1 |
|
Cellulose |
0,045 |
B1/B2 |
|
Wool, coconut fibres, flax |
0,04-0,045 |
B2 |
In a secondary, more detailed approach, side effects of thermal insulation can be considered too, as they directly or indirectly contribute to energy savings or the improvement of thermal comfort.
Exterior insulation, for example, reduces the additional heat losses caused by thermal bridges. Reduced heat losses result in the increase of the balance temperature of the building, in other words the length of the heating season will also be shorter [ZOL99].
Also, higher and uniform internal surface temperatures reduce the risk of mould growth. It is possible to reduce ventilation heat losses, since higher surface temperatures allow higher relative humidity, so a lower air change rate is sufficient.
Thermal comfort will improve: the mean radiant temperature of the surfaces in the room increases, hence the air temperature can be lower, which again contributes to saving heating energy. Also the thermal mass of the building will be preserved for the interior so the summer performance of the building improves.
Exterior finishes and the additional insulation protect the load-bearing structure against driving rain and frost, which may significantly extend the expected lifetime of the structure.
In historic buildings, energy efficiency measures should only be provided if they do not alter the character of the building or increase the risk of long-term deterioration to fabric or fittings. There are different types of problems that need attention like moisture movements, thermal bridging and the compatibility of materials:
Historic buildings were usually built using porous materials, without damp-proof courses, vapour barriers or membranes. They tend to be wetter than modern buildings as there is often some rising and penetrating moisture. However, this usually does not cause any damage, since the permeable traditional constructions and high ventilation rates from leaky windows allow the moisture to evaporate. Adding new materials with high vapour resistance will change the moisture mechanisms. In general, cementitious mixes for plasters, renders and impermeable plastic paints should be avoided. Ground floors should not be covered with linoleum or other impermeable materials if there is no damp-proof membrane underneath, or at least a perimeter band should be left to allow vapour to escape. Materials must always be chosen carefully and a vapour calculation might be necessary.
Thermal bridging can be particularly problematic. A thermal bridge is a component, or an assembly of components, in a building envelope through which heat is transferred at a substantially higher rate than through the surrounding envelope area. That is the case, for example, if the ceiling is insulated but it is not possible to insulate the head of the wall at the eaves. To avoid deterioration, careful detailing is necessary.
The applied materials should be chemically and physically compatible with existing materials. For example, cellulose insulation with aluminium or ammonium sulfate as fire retardant should be avoided, as sulfate can react with moisture in the air and form sulfuric acid which corrodes metals and causes building stones to slowly disintegrate [ENG04].
The first few centimetres of insulation have a dramatic effect on heat demand reduction, but the savings decrease with increasing thickness. However, the price curve for the additional centimetres is not linear: many items are independent from the thickness of the applied insulation, for example the costs of scaffolding, surface finishes, etc. The cost of the additional thickness is only a fraction of the total costs. If the refurbishment of the element is necessary anyway, e.g. the waterproofing is damaged, the rendering is in a bad condition, the real cost for the insulation itself is even lower [OSZ01].
The exterior insulation of walls is rarely possible in historic buildings. Since exterior insulation does not allow preserving the original complexity of a façade, it is only feasible for non-protected façades.
However, with a special technique, it is possible to insulate the façade after removing the historical features carefully. First the structure of the façade has to be documented with photos and drawings. All dimensions and the profiles of cornices, windowsills, decorations etc. have to be recorded. If the profiles are in very bad condition, they have to be reproduced. The façade is then insulated with a composite system and finally the profiles are put back according to their original arrangement. This technique is, however, rather complicated and requires special care.
Exterior insulation of the walls has a lot of advantages: the thermal mass will be preserved for the interior so the summer performance of the building improves, the thermal bridge effect is significantly reduced and so on. It can be very cost-effective if the façade needs refurbishment anyway, since the cost for the insulation material is only a small fraction of the total cost (scaffolding, finishes etc.). If possible, the additional insulation of the wall should be combined with the change or upgrade of windows. To reduce thermal bridges near the windows, the ideal position of the window is as close as possible to the insulation layer. However, this might change the appearance of the façade.
In composite systems, the insulation boards are glued and/or mechanically fixed to the structure and then rendered in a thin layer (Figure 22). The original plaster can be retained if its resistance is sufficient. The surface might need equalising if it is too uneven for the installation of the insulation boards.
Expanded polystyrene and mineral wool systems are the most commonly available on the market. If the original structure has a low diffusion resistance, vapour calculations are necessary for polystyrene systems. Mineral wool is non-combustible, improves the acoustic performance of the wall and the risk of vapour problems is low. Systems combined with wood wool surfaces are also available, these can be rendered with conventional materials and they are more resistant to mechanical interventions.
At window junctions, the insulation layer should cover the frame. The fixing of external shading devices, canopies and lamps has to be considered during planning.
Figure 22: External insulation with a composite system
The ventilated cavity is favourable both in winter and summer: this way vapour is allowed to escape in the winter as well as hot air in the summer (Figure 23 a). It is, however, relatively expensive.
The insulation should be non-combustible, which makes mineral wool suitable for this purpose. It also has good acoustic performance and insects do not like it. It is advisable to apply insulation boards with an integrated air tight, but diffusion open layer so that air cannot enter the material.
Various cladding types are available (e.g. weatherboard, fibre cement, metal, stone, ceramic, etc.). The construction type depends on the chosen material (usually timber rails or metal profiles). Efficient ventilation should be ensured with sufficiently sized opening vents.
Figure 23: a) Cladding with ventilated cavity b) insulation of a cavity wall
Cavity wall
In old buildings, cavities were usually built without any insulation. A possibility is to fill the cavity with loose, hydrophobic insulation if the outer layer is intact (e.g. granulated mineral insulation, cellulose or perlite) [HAU01]. This does not change the appearance from outside, but a vapour calculation is necessary (Figure 23 b). This can be supplemented with exterior composite system insulation or interior insulation if necessary. A detailed inspection prior to filling the cavity is necessary in order to check if obstructions (loose bricks, tablets, cement, etc.) will not cause cold bridges afterwards.
Transparent insulation considerably changes the external appearance of the building, hence this is not a feasible option for protected façades.
The transparent insulation – a cellular structure or granulate from polycarbonate, aerogel, PMMA, etc. – transmits a significant proportion of the incident solar radiation. The energy is absorbed on the external surface of the wall. A heat flow will start towards the lower resistance, i.e. towards the wall. The temperature of the surface between the wall and the insulation increases: in average winter conditions this results in an overall heat gain, but the heat losses are significantly reduced even in cloudy conditions. The thickness of the insulation has to be optimised, as increasing thickness improves the thermal resistance but less energy is transmitted to the wall surface.
The insulation is covered with special glazing with changing transmittance depending on the heat load (phototropic or thermotropic glazing). The other possibility to avoid overheating in the summer is to apply shading devices or a ventilated air gap. Translucent rendering instead of the glass covering results in a less conspicuous appearance, which is more suitable for retrofit. In this case, there is no need for extra solar protection in the summer, since at high sun altitudes less energy is transmitted through the rendering [ZOL99].
If the roof covering is in bad condition and needs replacement anyway, or if there is no secondary waterproofing and the water tightness of the structure is not sufficient, the roof can be insulated from outside. As roof covering is usually an important feature of historic buildings, the old stone, slate or tile should be replaced after placing the exterior insulation if possible. In other cases, the roof can be insulated from inside, which has many advantages (see Part II – Chapter 3 - Interior insulation).
An advantage of insulation on the outside is that it does not affect the interior. The additional insulation layer will considerably prolong the expected lifetime of the timber load bearing construction and improve summer heat protection as well as acoustic performance to some extent. However, the costs of the work are higher than for insulation from inside, scaffolding is necessary and the appearance of the roof might change, which has to be approved by the building authority. The increased thickness will affect all edge details, e.g. verge, eaves and chimney junctions. This might be problematic in terraced or semi-detached houses.
The insulation above the rafters can be a continuous layer of rigid material with tongue and groove boards (e.g. extruded or expanded polystyrene), in this case no framing is required and thermal bridges are avoided. Another possibility is to apply mineral felt insulation between horizontal railings. If there is an existing insulation between the rafters, its condition has to be checked. If the spaces have not been insulated before, they can be filled with mineral insulation or loose fill.
The secondary waterproofing under the roof covering should be air tight but vapour permeable (e.g. microperforated vinyl membrane).
If originally there was no internal vapour barrier, this can also be mounted from outside (Figure 24). If the interior lining is fixed with nails, first rigid insulation boards (e.g. EPS) should be laid between the rafters to protect the vapour barrier. The membrane is then installed between and around the rafters.
Figure 24: Insulation of a pitched roof from outside
The additional insulation of the topmost floor of an unused loft is the most cost-effective and easiest insulation measure. Here relatively cheap materials are also suitable and the insulation does not require any special protection provided the water tightness of the roof is adequate.
The insulation can be laid on top of the floor (Figure 25 a). The existing flooring should be retained only if its surface and load bearing capacity is suitable to support the insulation. It is worth applying relatively thick insulation. Mineral wool can be installed between timber railings and then covered with wooden boards. There are also rigid polystyrene or mineral wool boards combined with a wood wool walking surface available. Another possibility is to blow cellulose flakes on the floor. To minimise the geometric thermal bridge effect, the top of the masonry wall at the floor junction should also be covered with insulation. A vapour barrier is usually not necessary.
In case of a timber joist floor, the voids between the joists can also be filled with insulation material (Figure 25 b). The wooden floorboards must be removed but can be built back after the insulation if they are in a good condition. All timber elements should be treated with preservative. An additional insulation layer can be laid on the top, but here polystyrene is not recommended due to its high vapour resistance. It is advisable to install an airtight layer under this insulation layer. At the joist supports, it must be ensured that outside air cannot enter [RIC07].
The door heights, hatches, stairs etc. have to be adjusted to the new floor thickness. The condition of these structures also have to be checked, especially the airtightness of openings to prevent extra heat losses. Reduction of air infiltration from the building into the roofspace is also important, since the roofspace will be colder in winter due to the extra insulation of the topmost floor. This may increase the risk of condensation, particularly if ventilation is limited or poorly distributed [ENG04].
Figure 25: Insulation of the topmost floor a) solid floor b) timber joist floor
The insulation of the cellar floor is less effective than other insulation measures as the structure is adjacent to an unheated space and not the external environment. However, it should not be neglected, as it can significantly improve thermal comfort and prevent fabric damages. Also, relatively cheap materials can be applied.
The insulation, which can be mineral wool or expanded polystyrene, is mechanically fixed or glued to the soffit of the ground floor slab (Figure 26 a). For gluing, first a rendering may be necessary to create a smooth surface. The other possibility is to install a suspended ceiling, e.g. with gypsum boards. This is the only solution if the soffit is not plane, for instance for a vaulted or arched ceiling (Figure 26 b). This is also the most common way of insulating arcade slabs. For a suspended ceiling, lower density and hence cheaper mineral wool felts are suitable. Uncontrolled air convection between the insulation and the slab must be prevented.
Pipes and electric cables running under the soffit have to be taken into account when designing the insulation. Lighting fixtures may need to be moved. Cellar windows and door heights might also limit the insulation thickness. If underneath insulation is not possible, insulation on the top might be a solution. Insulation with vacuum insulation panels increases the floor thickness only minimally (see Part 2 – Chapter 3 - Interior insulation).
The insulation of the ground floor slab should be supplemented with a perimeter insulation to reduce the thermal bridge effect at the junction between the external wall and the floor. In historic buildings, however, this measure is often not feasible. The insulation should continue at least 50 cm below the soffit of the ground floor slab [RIC07]. Only non-absorbent, water resistant materials can be used.
Figure 26: Insulation of the cellar floor - left: plane surface - right: vaulted ceiling
Although flat roofs are not very typical in historic buildings, some related basic principles are included in this guide. The retrofit solution depends on the type and condition of the roof. The energetic retrofit is always worth carrying out (reduction of heating energy consumption, fabric protection) but obviously more cost-effective if the roof needs refurbishment anyway, such as retrofit of waterproofing barriers.
If the fall is adequate, the best solution is to create an “upside-down” roof by adding insulation on top of the existing layers (Figure 27). First the condition of the waterproofing has to be checked and it must be repaired or changed if necessary. Only extruded polystyrene is suitable, since it is a non-absorbent, frost-resistant material with high resistance to compression. The insulation should be laid in one layer with tongue and groove joints to avoid the formation of a water film layer. Fixing of the insulation is with gravel ballast or concrete slabs. The load bearing capacity of the slab has to be checked for the additional loads. For low capacity roofs, special polystyrene combined with cast stone elements can be applied. Upside-down roofs are vapour-technically advantageous, the execution is fast, the waterproofing is protected, the temperature fluctuations are much reduced, hence the lifetime is prolonged. This solution is, however, quite expensive.
If the fall is inadequate and the waterproofing is in bad condition, the refurbishment of the roof is necessary anyway. In this case, the cost of additional insulation is only 25-30 % of the total cost, but contributes to a significant energy reduction [OSZ01]. The original layers can be retained depending on their condition. The fall can be corrected with the additional insulation if the second layer of EPS is laid to falls. The new waterproofing is on top of the insulation and fixed with ballast. Another possibility is to create the fall from lightweight foam cement or similar material. The vapour conditions have to be checked, a steam pressure equalisation layer might also be needed.
After insulating, it might be necessary to raise the height of the parapet. To reduce the thermal bridge effect, both sides of the parapet should be insulated.
Rooftop planting has many advantages, e.g. it improves the summer performance, but due to monument protection, this is usually not a feasible option in historic buildings.
Figure 27: Insulation of the flat roof by creating an upside-down roof
If the ventilation cavity is retained, it has to be ensured that even with the additional insulation the ventilation requirements are fulfilled to avoid moisture problems in the cavity.
Filling the ventilation cavity with cellulose flakes, for example, is a very cost-effective measure. All the ventilation openings have to be closed. The vapour conditions need to be checked and the installation of a vapour barrier might be necessary.
[ENG04] English Heritage. Building Regulations and Historic Buildings, 2004. www.english-heritage.org.uk (01.08.2008)
[HAU01] Hauser G, Höttges K, Otto F and Stiegel H. Energieeinsparuing im Gebäudebestand. Gesellschaft für Rationelle Energieverwendung E.V., January 2001.
[OSZ01] Osztroluczky M and Medgyasszay P. Energy conscious construction and refurbishment (Energiatudatos építés és felújítás - in Hungarian), Labor5, 2001.
[RIC07] Richarz C, Schulz C and Zeitler F. Energy-Efficiency Upgrades. Edition DETAIL, Birkhaeuser, 2007.
[ZOL99] Zöld A. Energy conscious architecture (Energiatudatos építészet - in Hungarian). Műszaki Könyvkiadó, Budapest, 1999.