Best Way to Insulate a Timber Frame House

Insulation in Timber-framed Buildings

Robert Demaus


	The large original verge overhangs of this historic timber-framed firm protect the wall below, while an angled 'pentice' board higher up the ground flooring window sheds water away from the wall below. (All photos: Robert Demaus)

Thermally, timber-framed walls generally perform badly compared with other traditional construction, and struggle to meet modern expectations. This article addresses the problems and risks associated with retrofitting insulation to upgrade their thermal operation. Information technology focusses on cases where the timbers are exposed externally, which are commonly the nigh problematic, but besides considers timber frames which are concealed behind cladding (either of the same period or later).

There are circumstances in which retrofitting insulation to a timber-framed wall is adequate and benign, but other measures might show more than cost-effective and less damaging. To determine the best manner frontwards, survey and assay should exist carried out past an independent consultant rather than by a materials supplier or contractor. As well as comfort, cost-saving and environmental gain, many other factors must besides exist considered, including:

The historic significance of the building equally a whole, too equally the relative significance of individual elements, and the degree to which retrofitted insulation will alter it
The condition of the building cloth and the nature and extent of whatsoever interventions (other than thermal insulation) that might be necessary
The causes of any existing degradation and how these might best exist remedied
The current hygrothermal performance of the timber-framed walls and the building equally a whole
The 'landscape value' of the building and the potential bear upon of any change to its external appearance
The operation of heating and hot water systems and the cost benefit of upgrading
The condition and efficiency of existing insulation, for example in roof spaces and floors, and the cost benefit of upgrading
The potential for introducing cost-effective and reversible new elements such as secondary glazing that practice not involve significant harm to historic textile
The building's electric current use and the occupants' expectations.

The accented and relative importance of these and other factors will vary profoundly, not just between buildings, merely between areas of the same building.

The thermal performance of a timber-framed wall is non only controlled past its component materials. Condition, orientation and exposure will take a far greater effect on a 100mm thick timber-framed wall than on a 225-350mm brick wall. Moisture memory within the wall is also critical to its thermal operation.

Timber frames are total of joints and cracks through which air (and h2o) can penetrate. The most effective improvement that can be made to the overall hygrothermal performance is to fill up these gaps. A sensitive thermographic camera is the all-time way of locating them, provided there is a reasonable temperature difference (5-10°C) betwixt the within and outside, and preferably when the wind is bravado. Information technology is as important to survey the outside of the building to place where estrus is escaping, equally it is internally to place where cold air is entering. The survey should be repeated when the remedial work has been completed, but is almost meaningless unless carried out in the same weather conditions. Most timber-framed buildings are too air-porous for standard air pressure level tests to be meaningful.

Optimum methods and materials for gap filling will vary depending on the size and location of the gaps, but should always be flexible and breathable: sheep'southward wool pushed into the gap with a thin blade and finished with haired lime plaster tin can be very effective. Proprietary sealants, mastics and cementitious mortars should not be used.


	This timber-framed house has retained its original eaves and verges, both beautiful and practical. The house must always have been tiled.

	The depth of thatch shelters the wall below. If the thatch were replaced with tile or slate (as then much was) the wall becomes very vulnerable.

	These rafters have been restored to their original overhang, profoundly improving protection of the wall below.

There is a much greater variety of constructional materials and details in timber-framed walls than might be found in brick or rock walls, with correspondingly complex, variable and unpredictable concrete properties and interactions. Moreover, the original broad palette of materials is often further complicated by subsequent alterations and additions, such as changes from wattle and daub to brick infill, from lime to cement return and from permeable to impermeable finishes. Many of these variations can occur within a single elevation. By comparison, brick and stone walls are relatively homogeneous and predictable.

As a result, desktop heat-loss calculations using standard formulae and computer modelling are less reliable where walls are timber-framed. On-site detailed physical investigation is required. Sensitive infrared thermographic cameras may exist used to locate concealed timbers, identify the brand-upwards of infill panels and assess oestrus loss and damp penetration. Disuse-detecting micro-drills are also very useful for these investigations.

For such assessments to exist of value, the assessor must take a adept working knowledge of how the building was constructed, what changes might have occurred since, and the causes and extent of any deposition.

Typically, timber-framed buildings were built using freshly felled timber that shrank and moved significantly, particularly over the offset 30 years. The timbers were usually left exposed externally and internally and the spaces between the frame were filled with clay-based daub, often finished with limewash. Gaps that formed between the frame and infill equally the materials settled and shrank were regularly filled and additional coats of limewash applied.

The entire textile was therefore very breathable, allowing whatsoever moisture that entered to readily evaporate, and moisture levels in the wall generally remained below the indicate at which the various materials would degrade. The walls were protected by large roof overhangs and pentice boards (run into title analogy), but over the following centuries, these were lost, causing the walls to be wetter more often and for longer periods. As a consequence, the wattle and daub began to dethrone rapidly and the timber more slowly.

In the 18th and 19th centuries timber-frames were often concealed behind facades of weatherboard, brick, tile or lime render. Early renders were lime-based and breathable: later renders were oft much less breathable, such as Parker'due south Roman cement which was patented in 1796.

Where frames remained exposed into the 19th century, the degraded wattle and daub was oftentimes replaced with brick, which tended to exacerbate deposition. Increasing use of cementitious renders, impermeable paints, damp-proof membranes and mastic sealants in the 20th century tended to reduce breathability and trap h2o, increasing deposition and oestrus loss.

More than recently, economic and environmental pressures to meliorate thermal performance have become increasingly important, only frequently poor detailing and inappropriate materials take exacerbated decay.

In the 21st century there has been a growing understanding of the demand for buildings to breathe and a consequent move to more permeable materials. The crucial point is that impermeable mod finishes and sealants not only cause meaning and continuing damage to the timber frame and other historic fabric, they also profoundly diminish the thermal functioning of the wall.

The status of the wall and its hygrothermal behaviour are intimately linked. Unless faults are remedied, the introduction of insulation may be of relatively little do good and can profoundly increase the gamble of further deterioration. Just when the detailed survey has been completed can the advisability of retrofitting insulation be evaluated and the best method selected.

There are essentially three options for retrofitting insulation to an exposed timber-framed wall; externally, internally or within the depth of the frame.

Within THE FRAME

Given that timber-framed walls are often less than 100mm thick, insulating within the depth of the frame well-nigh inevitably involves loss of the existing infill material.

Original wattle and daub should exist retained and repaired if possible, only where there is a later brick infill, its historic and aesthetic significance and its condition may affect the decision. Where there is prove of significant degradation, a adept case can be made for its replacement with a more sympathetic and better performing material. Where the timber frame requires repair that involves removal of the infill, there is an opportunity to innovate more sympathetic and meliorate performing infill.

It is at present generally accepted that infill panels should be breathable and vapour permeable throughout their thickness, only in that location are many theories nigh the best materials and techniques. Many recommended systems involve complex combinations of materials including synthetic border seals, breather membranes and vapour barriers, stainless steel mesh, wood-wool substrates and softwood sub-frames. Systems such as these may work better in theory than in the variable atmospheric condition found on site, where quality command may exist difficult, particularly when the timber frame is neither direct nor in perfect condition.

As a dominion, the simpler the method and materials, the more likely they are to function predictably and reliably. There is slap-up merit in using methods and materials equally close to the original wattle and daub as possible. The theoretically poorer U-value may not exist as bad in practice and the greatest reduction in rut-loss is oft achieved simply by creating a dry, draught-free construction. A modern material similar in concept to daub, only with more immovability and better U-value, is a hydraulic lime/hemp mix that tin can be cast in-situ to class a homogenous breathable infill.

If the frame and/or the panels are in poor condition and repairs would involve the loss of a high proportion of historically significant fabric, there would be a stiff case for protecting the wall behind a shelter coat of lime render or other regionally appropriate material. This is usually preferable to creating a crude modern replica of the wall in band-sawn timber, and may provide the opportunity to insulate outside the wall line.

Within THE WALL LINE

If the timber frame and infill are in sufficiently adept condition, and are robust plenty to cope with continuing exposure with limited interventions, insulation can exist fitted to the inside face up, either directly to the wall or with an air gap. Withal, this will have a serious impact on the appearance of the room, obscuring features such as window surrounds, skirtings and adjacent ceiling mouldings, and it will reduce the internal flooring surface area. More than significantly, there is an increased risk that moisture entering the wall will go trapped, fifty-fifty if all the materials used in the new lining (insulation, plaster and pigment finish) are vapour permeable. If problems do occur, they are unlikely to become apparent until pregnant damage has occurred.

The gamble of driven rain penetration can exist reduced by careful gap-stopping and the reinstatement of overhangs, merely any intervention that restricts the passage of water vapour through the wall significantly increases the risk of condensation and/or water entrapment. For this reason, non-breathable rigid insulation such as PIR (polyisocyanurate) boards should not be used, even though they tin achieve better U-values at relatively small thicknesses.

Insulating inside the wall line besides greatly increases the adventure of condensation due to cold-bridging in those areas which, for various reasons, cannot exist insulated. In particular, the ends of floor beams and joists built into the external wall are at greater risk of increased degradation.

Exterior THE WALL LINE

For many reasons, plumbing fixtures insulation to the outside face of a timber-framed wall is often the best solution, both in terms of hygrothermal operation and building conservation.


	Historically, many timber-framed buildings were rendered to improve their weather-tightness. Some of the visible return is lime-based and probably early 19th century, other sections have been replaced with a cementitious render in the 20th century.

The wall is fully protected (assuming materials and detailing are correct)
Necessary repairs can be kept to the minimum structurally required, and can ordinarily have the form of additional surface-stock-still straps, etc. These repairs are reversible and involve no loss of celebrated fabric.
Air penetration through the wall can be fully controlled
Insulation can be continuous with all original fabric on the warm side, reducing the risk of common cold-bridging and condensation
Keeping what thermal mass there is in the wall on the warm side as well helps to balance diurnal variations
The historic significance and advent of the interior is not compromised
The intervention is reversible.

External insulation will alter the external appearance: the additional thickness requires changes to window reveals and other features, and conceals the timber frame. This often meets with resistance, both professional and public. All the same, in that location is a strong historical precedent and the benefits are considerable.

Historically, render was usually applied directly to lath nailed to the frame, and it is widely held that this must offer good protection to the frame, simply because it is breathable. However, it is quite common to find widespread active Deathwatch beetle attack in timbers immediately behind lime renders, but rare to find it in exposed external timbers, suggesting that sometimes moisture content of a lime-rendered frame can be high enough to sustain fungal and beetle assail. When applying new or replacing sometime return, a vapour permeable membrane should exist used and the lath set off the frame on counter-battens if possible.

The recent evolution of relatively high-functioning breathable multi-layer insulation quilts, effectively insulated breather membranes, has corking potential equally they increase wall thickness far less than most other breathable insulation materials. Although designed for use in roofs, these quilts take been successfully used to insulate timber-framed walls behind return or weatherboard. New materials demand to be used cautiously until their long-term performance is better understood, simply equally, they should non exist dismissed out of hand. Furthermore, imported materials that perform well in cold dry out climates may not work in wetter Britain weather. Peradventure the all-time advice is to question everything.

In a surprising number of cases, what appears to be a timber frame is actually an agglomeration of paint, mastic and cementitious return repair concealing a severely degraded and structurally compromised frame. Sooner or later this will crave such extensive repair/ replacement that protection with a lime render or other cladding would almost certainly provide a more effective and conservative solution while avoiding further loss. If the appearance of a timber-framed edifice is deemed desirable, this can e'er be practical to the face of the new render – at that place is a long tradition of what many now consider 'fakery'. At least what remains of the frame and surrounding fabric is retained for future generations.

RELATED REPAIRS

If the timber frame is to remain exposed, the essential starting time step in improving the thermal performance is to ensure that the frame and surrounding fabric are in practiced condition, and consist of materials that allow the wall to breathe. A disharmonize arises where an alteration regarded equally office of the building's history is demonstrably causing damage. Brick infill for example, does not always crusade problems, but tin can significantly increase the rate of degradation of the frame, peculiarly when bedded in cementitious mortar, where frames are relatively light, poorly synthetic or weakened by decay, or where the bricks project exterior the face of the frame, creating ledges that trap water.


	The severely degraded timber framed end wall has been strengthened and protected behind weatherboarding, with a layer of breathable multi-layer insulation included.

The apply of inappropriate materials is non the only problem. The introduction of impermeable materials was usually prompted by the failure of earlier or original wattle and daub infill, which unremarkably began to fail once the protection of large overhangs was lost. Although impermeable materials are by and large damaging, if permeable materials are reintroduced without reinstating the original protection (such as overhangs), their exposure to extensive and persistent wetting will pb to fungal deposition, loss of cohesion and frost damage.

Furthermore, rut loss through persistently wet daub, render or brick is much greater. Contempo changes in weather patterns may also create greater bug for poorly protected buildings. It is therefore an essential element of whatever edifice upgrade (specially for timber-framed buildings) that adequate overhangs and other protective measures are re-introduced, even where the evidence for them is inconclusive.

Another important issue is that moisture content is critical and often finely balanced. Typical ambient wet content of timber in a well maintained edifice is effectually 16 per cent (lower if heated). This tends to rise to around 18-20 per cent in well-maintained external walls. Many fungi volition germinate at around 27 per cent, simply tin can survive down to 23-24 per cent. Deathwatch beetle thrive where there is or has been fungal activity and can survive in timber down to 16 per cent moisture content or lower.

Controlling water penetration, condensation and evaporation are therefore critically important, and using the wrong materials or details might raise the wet content by merely a few per cent and risk starting or re-starting degradation. Equally, reintroducing the right materials and detailing should lower the moisture content past just a few per cent into the safe zone.

SUMMARY

1	The decision whether to retrofit insulation, and if so, which approach to adopt, cannot be taken in isolation. A detailed appraisement of the building, including the historic significance of the timber frame, infill panels and other features, besides as an accurate condition assessment, must exist carried out.
2	Virtually traditional timber-framed buildings will exist listed. There should be give-and-take at an early stage with the local conservation officeholder nigh the bug identified and proposed remedies.
3	Upgrading the hygrothermal functioning of timber-framed walls by retrofitting insulation is very difficult and can rarely be achieved without significantly compromising the historic significance and/or appearance of the edifice. Any potential benefits in terms of cost saving, comfort and reduced carbon emissions demand to exist weighed against the initial cost, loss of historic fabric and potential for further degradation of celebrated textile.
4	Where timber-framed walls retain a high proportion of original or historically meaning fabric, retrofitting insulation should exist considered a last resort and but used when other potential improvements have been explored.
5	Estrus loss through the various materials that make up a relatively thin timber-framed wall is often compounded by air leakage effectually the edges of panels and through joints in the frame. Minimising uncontrolled air move is critical and will frequently bear witness more constructive and less damaging.
6	Culling measures to upgrade the overall operation of the complete building should exist considered. These might include reinstatement of roof overhangs and plumbing fixtures of pentice boards, removal of impermeable materials and finishes, and measures to reduce current of air exposure.
7	Timber-framed walls mostly have depression thermal mass and high uncontrolled air penetration. Heating systems that make use of large internal masonry stacks or stone floors every bit estrus stores are often more effective than systems that heat the air via conventional radiators. Radiators should never be placed against external timber-framed walls.

Heritage Retrofit, 2017

Writer

ROBERT DEMAUS BEng MSc (Timber Conservation) specialises in the location, assessment and conservation of structural timber in historic buildings, combining advanced technologies with historic noesis and practical feel, working for national institutions and private clients throughout Britain and abroad.

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Conservation science

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Best Way to Insulate a Timber Frame House

Insulation in Timber-framed Buildings

Robert Demaus

Within THE FRAME

Within THE WALL LINE

Exterior THE WALL LINE

RELATED REPAIRS

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