Technology Behind Closed Panel Timber Frame Construction Construction Essay

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This primary objective of this chapter is to provide a technological overview of the research topic. The first Section of the chapter provides an overview of the Closed Panel Timber Frame system. The next section explains Passive House Design and the requirements of a dwelling to meet Passive House Standards. Section three will give an overview of the various elements that are used in Closed Panel Timber Frame construction and how they meet these standards. Finally section four will offer concluding comments.

The Closed Panel system is based on the traditional stud frame but with slight differences. The main difference with this system is that the insulation material is installed into the panel at the factory stage and this is then retained with some other layer of material to 'close' the panel. A controlled manufacturing process eliminates issues with workmanship and quality. This ensures increased thermal performance with lower u-values and rating along with guaranteeing draft proofing and air tightness. Another advantage of the system is in the off-site fitting of windows, doors and electrical conduits. This ensures that the air barrier is not broken on-site ensuring air-tightness is maintained throughout the structure.

The thermal performance of a plane building element within a particular construction type is described by its U-Value (W/m2K). It is a measure of the heat transmission through the element per degree of temperature difference (degrees Celsius denoted as degrees Kelvin to signal temperature difference) between the internal and external environments. The U-Value is the inverse of the total resistivity of the components of a building element. The resistivity of each component is calculated by dividing the thickness of the element by its thermal conductivity.

Thermal bridging refers to a material, or assembly of materials, in a building envelope through which heat is transferred at a substantially higher rate (due to higher thermal conductivity) than through the surrounding materials. Junctions between window or door and wall, wall and floor, and wall and roof should be designed carefully to avoid thermal bridging. A thermal bridge increases heat loss through the structure, and in some extreme cases may cause surface condensation or interstitial condensation into the construction. Surface mould growth or wood rot may be the consequences of a thermal bridge.

A thermal bridge defines an area of increased heat loss in comparison to the surrounding elements. There are two distinct types of thermal bridge - 'repeat' and 'non-repeat'. Repeat Thermal Bridging is where a thermal bridge occurs at regular intervals, for example in a closed panel system at the I-beams or studs and is the type that will concern us in this study. Repeat thermal bridging is accounted for in U-value calculations. Non-repeat Thermal Bridging usually occurs at junctions of building elements such as a wall/floor, wall/roof or wall/window junction, etc.

The quantity which describes the heat loss associated with a thermal bridge is its' linear thermal transmittance (Ψ) value and is expressed in W/mK. This is a property of a thermal bridge and is the rate of heat flow per degree per unit length of bridge that is not accounted for in the U-values of the plane building elements containing the thermal bridge.

According to the SEI Guidelines for Passive House design on Thermal Bridges the Linear heat Coefficient Ψ should be less than 0.01 W/mK.

6.2.3 Air-Tightness:

The air-tightness of a dwelling, or its air permeability, is expressed in terms of air leakage in cubic metres per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals. Air-leakage (or infiltration) is the uncontrolled penetration of outside air into a building. It takes place through openings, primarily through inadequate and imperfect sealing between window frames and walls, between the opening sections of the window and along the joints of the building envelope. The dwelling envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the dwelling, including elements adjoining other heated or unheated spaces. Air leakage is defined as the flow of air through gaps and cracks in the building fabric. Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside. Air leakage of warm damp air through the building structure can also lead to condensation within the building, which reduces insulation performance and causes fabric deterioration.

The SEI Guidelines for Passive House design states that the Structural Air Tightness of a building element should be n50 < 0.6/ air changes per hour. This means at a pressure of 50 Pascals the amount of air that escapes through the buildings fabric should be less than 0.6 of the total internal volume of the building.

6.3 Key Components of Closed Panel System to meet Passive Standard:

To achieve the desired U-Values, minimised thermal bridging and levels of air tightness several components are essential. While various systems are available on the market some of them vary as regards the type of insulation used and how thermal bridging is minimised or eliminated.

6.3.1 Insulation:

The U-Values for a Passive House are achieved through super insulating the building envelope and retaining all internal gains from the occupants, their activities, use of appliances and most importantly the sun. It is achieved by two measures, the first being the insulation used, and secondly the depth of this insulation. There are several types of insulation used in Closed Panel construction and the type chosen depends on the demands of the designer. Generally there are three types of insulation currently being used on the market in closed panel construction. The first type of insulation investigated is Mineral Wool and it usually comprises of either rock or glass.

Mineral Wool: Rock-Wool is manufactured from volcanic rock, typically basalt or dolomite with an increasing proportion of recycled material in the form of briquettes. The molten material is spun into wool and small quantities of resin binder and mineral oil are added to lock the strands together and make them water repellent. The wool is formed into a mat, which is then carried through ovens where it is cured and compressed giving it good structural strength. Glass Wool is a mixture of natural sand and recycled glass at fused together at a temperature of 1,450 °C and the glass that is produced is converted into fibers. A resin is added to give the desired cohesion and mechanical strength.

Fig 6.4 shows mineral wool compressed between Panels

Mineral Wool is the preferred insulation type used by Scandinavian Homes Ltd in Co Galway. One of the primary reasons they use this type of insulation is that the thermal conductivity of mineral wool does not deteriorate over time, and remains in and around 0.04 W/mK. This ensures that the U-Value of the wall structure will remain up to Passive House Standards for the whole of its life time. Other reasons for choosing this type of insulation are because it is fire-proof, non-toxic and non-biodegradable.

Cellulose Insulation is the second type of thermal insulation that will be examined and is the type favoured by Eco-Timber Frame in Cork. It is usually used when Green Products are required. The cellulose insulation is from recycled newsprint and is densely packed between the panels and stud work. The material is treated with Borate, a naturally occurring mineral compound which greatly increases fire, moisture, mould and vermin resistance.

Stephen Spillane of Eco Timber Frame says that the main advantages for choosing this type of insulation are its low thermal conductivity of 0.040W/mK and good specific heat capacity which helps to smooth the heating and cooling cycle. Another major advantage of this type is Cellulose fills walls and stops air infiltration better than mineral wool as the fibres of cellulose insulation are much finer. When cellulose is pneumatically installed it takes on almost liquid-like properties that let it flow into cavities and around obstructions to completely fill walls and seal every crack and seam. No fibreglass or rock wool material duplicates this action. Fig 6.5 shows the insertion of cellulose insulation

Polyurethane:

Polyurethane is an organic polymer formed through step-growth polymerization by reaction of a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl (alcohol) groups in the presence of a catalyst. The main advantage of this type of Insulation is its low thermal conductivity of 0.02W/mK which allows efficient retention of heat.

Polyurethane foam has been specially developed to increase structural strength, water vapour resistance and durability. Each of the voids in each of the panels is injected with a special mixture of proprietary chemical so that foam is produced with both excellent insulation properties and also excellent environmental credentials.

Fig 6.6 shows the mechanical insertion of Expanded Polyurethane into panel .

6.3.2 Minimising thermal bridging:

Designing and building a passive house in Ireland requires the development of construction details that go far beyond guidance provided (to avoid excessive heat losses and local condensation) in Building Regulations Technical Guidance Document Part L, Conservation of Fuel and Energy 2005. In Closed Panel Timber Frame walls several techniques reduce this cold bridging.

The first method is the addition of a layer of thermal insulation on the cold side of the wall panel. This method can be used where a cavity is in place between the panel and external cladding and ensures a continuous layer of insulation without any interruptions, thus eliminating any thermal bridging.

Figure 6.7 shows 50mm layer of cavity insulation

An additional method is in the design of the stud work and common practice sees the use of I-Shaped stud. The fraction of repeat thermal bridging is considerably reduced due to the narrow web of the beam. While bridging is not completely removed it is reduced below the passive house standards.

Fig 6.8 shows the I-shaped stud which reduces the transmittance of heat

Fig 6.9 shows a thermal image of A Closed panel with I-Beam studs. The green indicates any cold paths and as can be seen from the image these bridges have been minimized.

Fig 6.9

The most contemporary method of reducing the transmittance of heat is the use of a twin wall structural frame. Twin frame systems are based on utilizing two standard timber studs engineered to allow a deeper timber frame which can be filled with a greater quantity of insulation. The twin frame system ensures that no thermal bridging occurs within the construction helping maximize insulation performance.

Fig 6.10 shows a Twin wall structural timber frame

6.3.3 Ensuring Air-Tightness:

In a Closed Panel Timber Frame Wall several components ensure that it meets the Passive House requirements on air-tightness of n50 < 0.6/ air changes per hour. The main components are generally the sheathing and/or vapour-barriers. It should be noted that while the additional elements of plaster, cladding insulation etc also assist in keeping the structure air-tight they are not part of the air-tightness layer. The air-tightness layer is usually located at the internal face of the panel as seen in Fig.

Fig 6.11

While the sheathing is generally considered to be just a structural element, once gaps at the joints are kept to a minimum it can also provides excellent ait-tightness qualities. The purpose of the vapour barrier is to keep heat in while it keeps draughts out.

.

Fig 6.12

The primary area where air leakage can occur is at the joints of the sheathing and the vapour barriers. The key method of preventing this is the use of special air retention adhesive tape to cover the joints. Once this is applied correctly air-leakage is kept to a minimum. There are several brands of tapes on the market but they all work on the same principle.

Fig 6.13 shows the application of a sealant tape to a vapour barrier.

Testing for Air-tightness is done on site towards the end of a build but generally before second fix carpentry such as the fitting of window or skirting boards. This is because if there are any defects in the structure they can still be accessed for repair. Blower door testing or air pressure testing is the preferred method of calibrating air-flow in a dwelling. This allows the accurate measurement of air movements through the fabric of your building. Using calibrated equipment in a standardized test manner allows a result which can then be compared against other properties, or earlier tests to the same property.

Fig 6.14 shows the powerful fan suspended in a canvas used in the testing process

6.3 Conclusion:

After examining the technological aspect of the research topic several conclusions arose. The first is that it is clear that the closed panels are made to a very high standard which is vital for Passive House design. It can also be concluded that Passive House certification is not achieved by several requirements acting independently but in conjunction with each other. While the wall panels may be made to Passive House standard with the rest of the components in tune the dwelling will not be certified as a whole. Finally it can be observed that the majority of components are designed to have several purposes. For example one may presume that insulation is there solely for its thermal properties but it can also assist with the air-tightness of the building. To conclude it is the development of the various components mentioned that are responsible for the improvement of Low Energy design, and as the changes in legislation, as mentioned in chapter 5, demand more from buildings technology is improving to coincide with these demands.

Chapter 7

Case Study: Study of Eco-Build by Passive House Builders, Athenry

7.0 Introduction:

The purpose of this chapter is study the Eco-Home constructed by Passive House Builders and show that Closed Panel Timber Frame Construction is a suitable form of construction for passive house design. The reasons for choosing a house constructed by Passive House Builders was because they do not manufacture or use their own building elements, instead send their builds out to tender, and therefore would not be biased as towards any building system. To do this the chapter is divided into three main sections. The first section gives an overall description of the house including its orientation

7.1 Overview of Dwelling:

The construction of the Eco-House in Athenry proved to be a major challenge for Cyrill Manion of Passive House Builders. The house took five months to design and a further five to construct. The site chosen by the client was ideal for Passive Design. The back elevation was 20 degrees off full south and although there are houses on either side of the dwelling over-shadowing was not a problem.

The construction phase of the house began in August 2009 with the installation of the insulated foundations using EPS and low carbon concrete. The system was provided by Viking House and the foundation raft consisted of two separate densities of Polystyrene. Even though polystyrene is an oil based product the energy used in its manufacture is a fraction of conventional insulation products. For example the temperature required in the manufacture of rock or glass wool is roughly around 950 degrees while polystyrene requires only 110 degrees. It should also be noted that and for every 1 litre of oil used in manufacture around 300 litres of oil can be saved in heating. Manion says that ''the system was chosen for two primary reasons, one because it provides a completely cold bridge free raft, and two because of its ease of connection with the closed panel wall system''.

The clients Scott and Ann Cook gave Manion free role as to what type of construction type to use for the envelope so Manion decided on his desired U-Values and sent the contract out to tender to both masonry and timber frame contractors. In this tender he included the U-Values he wanted to achieve and that the house needed minimal thermal bridging and extremely low levels of air-tightness. He also stated that he wanted all values to be well inside the standards specified by the passive institute. Another major issue that Manion had was leaving the building exposed to the elements while under construction leading to concrete floor absorbing moisture. Therefore he wanted a build that would be extremely quick and protected from the elements in the shortest time possible. Incorporation of a service cavity was the last request that he made. This was not only for the typical services such as electrical cables but also for the Mechanical Ventilation Heat Recovery System (MVHR). The MVHR System is the only technology that is vital for a Passive House and it was important that it is could be incorporated within the Air-Tight Layer.

7.2 The System:

After running it by the clients Manion decided on a Closed Panel Timber Frame structure to be supplied by Eco-TimberFrame. The panels were manufactured at their Cork based factory and transported to the site by lorry. The chosen system comprised of a twin-wall structural frame. The benefits of this were that sufficient thickness was provided to super insulate the building and thermal bridging was limited to the head and base of the walls. Stephen Spillane, lead designer with Eco-TimberFrame explained that "It's a twin wall construction, 350mm thick with densely packed cellulose insulation. The cellulose utilised was Warmcell 500 from green insulation providers Warmcell in the United Kingdom''. The full wall build up from outside to inside is as follows:

13mm Cement Board

25mm Air-Cavity

12mm Breathable Wood Panel

350mm Densely Packed Cellulose

12mm SmartPly OSB Sheathing

50mm Service Cavity Filled with Rockwool

12mm Internal Plaster Board

Spillane said that they were looking for advanced techniques to achieve a high quality building envelope that would meet Cyril's demands of being well insulated and extremely air-tight. Eco-TimberFrame studied what was done in Germany and Austria and eventually developed this wall construction. The OSB board on the inside is taped for airtightness and the wood fibre board on the outside allows the house to breathe. It is the OSB board that provides the airtightness layer and it vital that great care was taken when taping across all joints with Siga tape. Manion said that '' it was the first time they had used Ordinated Strand Board as the air-tight layer but thinking about it makes sense from a structural point of view''. He explained that with a regular membrane if you ever want to fix even a picture to the wall it is very difficult to do so without puncturing the barrier, however with an OSB you can fix a screw directly into it and it will maintain its air-tightness.

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