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3i infill panel systems





Tradition meets high-tech. Oakwrights patented 3i System offers a modern solution to an age-old problem, out performing all similar solutions in the market. The 3i System works as a floating panel within the oak frame. Highly insulated and with advanced weather resistance, it delivers enhanced thermal performance.
The 3i system is dry fitted and manufactured off-site, reducing on-site time for you and our dedicated site team ensuring the highest level of quality control throughout the process.

3i oak house panel system


0.19W/m2K through centre of panel
0.25W/m2K through overall wall structure





When Robert and Lesley Todd were building their traditional style farmhouse they were looking for a panel solution that gave them a traditional look externally but gave a slightly more contemporary look internally. With these demands though came a pre-requisite, the look had to be achieved with the assurances of the best insulation. The solution was achieved with the Oakwrights 3i panel system.

Oakwrights have spent many years researching and developing the type 3i panel and are confident that it is the most advanced system currently available on the market. The beauty of the 3i panel is that it maintains the look of a traditional ‘half-timbered’ house externally, but is far more energy efficient and weatherproof than any other panel system available today. Just to be extra sure the 3i panel system has been thoroughly tested and approved by TRADA (the Timber Research and Development Association) and is accepted by the NHBC (the National House-Building Council).
Here are just some of the innovative features of our panel system that persuaded Robert & Lesley that 3i was the route to take:
Rather than have every timber visible both internally and externally (which by passing through the whole width of the wall causes cold-bridging) many of the timbers in the 3i system are half-width. This means that only the large structural beams and posts are visible both inside and out, all secondary timbers are only visible on one side which means an almost uninterrupted layer of insulation can be run around the walls. It is this principle that allows the 3i panel system to achieve a U-Value of 0.25 for the whole wall not just through the centre of the panel. This figure has been verified by both Kingspan and TRADA. This gives the best possible insulation value in the industry for this type of construction.As per our initial conversations with Robert and Lesley, we find a general comment when talking to clients is that too many visible timbers internally can lead to problems in arranging furniture and hanging pictures. The 3i system creates a much more flexible and contemporary internal environment allowing the interior to be more open and light.

In order to create an air tight seal around every panel and allow our frames to pass air pressure tests, a groove is cut into the oak members all the way around. Set into this groove is the panel, the main body of which is composed of a 12mm ply board; the ply is secured with gaskets to allow the ply to move with the oak frame rather than fight against it. From there, butyl tape is run along every junction where the ply meets the oak.
For additional household protection, an intumescent strip is located on the inside of the panel where the plasterboard abuts the oak. This strip when heated forms a seal around every panel, thus in the case of a fire it would serve to prevent it spreading to the insulation or to other panels around the property.

With the 3i panel system Robert and Lesley have created a fantastic looking traditional home.

oakwrights award winning 3i infil panel system
oakwrights award winning infill panel system





There is a U-value of 0.19W/m2K achieved through the centre of the panel and 0.25W/m2K through the overall wall structure. This gives the best possible insulation value for a traditional infill panel system.


Offsite manufacture increasing speed and reducing waste on site.

Integral floating panel that allows for shrinkage of green oak frame.

Detailed weather proofing system that gives you peace of mind.

Traditional external look allowing for render or brick slip finish.

More contemporary clear internal look.

oakwrights award winning 3i infill panel system



What are U-values
A U-value is a measurement that describes how much heat is lost from a particular building material, such as a wall, window or roof. A low U-value means that the material is good at retaining heat, a high U-value means that it loses a lot of heat. As far as your home is concerned, the lower the U-value, the better.

What are Psi-values
In a similar way to a U-value, a Psi-value is a measurement that describes how much heat is lost along a linear junction between two surfaces, for example, two walls, the wall and roof or floor and all around windows and doors. The same principal is true of Psi-values in that the lower the value, the better the heat retention.

The scientific bit
U-value is measured in watts per metre-squared kelvin, or W/m2K. In other words, it is the amount of energy (in Watts, or W) that travels through one square metre of a material (m2) per every 1 degree of temperature difference between inside and outside (K).

U-values can apply to single materials (e.g. a metal sheet) or a composite building element (e.g. a softwood encapsulation system).

Psi-values are measured in watts per metre kelvin, or W/mK. In other words, it is the amount of energy (in Watts, or W) that travels through one linear metre of a junction (m) per every 1 degree of temperature difference between inside and outside (K).

Building regulations
The government have set out minimum U-values that are allowed when constructing a new building. Different U-values are required for different building codes. Although to get the final U-value calculation for a house is a very complex set of equations and is dependent on a lot of different factors.

We have had our junctions for both WrightWall Light and WrightWall Natural modelled to calculate the Psi-values which in turn can be inputted into a SAP calculation which is required on all new properties to predict the energy requirements and CO2 production of the proposed building. This will give a significant energy saving instead of using the default values which otherwise have to be used.

U-values of the different Wrightwall Systems.
The table below shows the typical U-values of Oakwrights Wrightwall Systems with different external finishes. Oakwrights Wrightwall Light can be used to build up to and over Code 4 & our Natural System can be used to build up to Passivhaus Standards.



Lightwall - Weatherboarding


U-Value 0.17 W/m2K

 Lightwall - Rendered

 258mm (depends on exact render) 

 U-Value 0.17 W/m2K

 Lightwall - Brickwork


 U-Value 0.16 W/m2K

 Lightwall - Stonework

388mm (150mm stone) 

 U-Value 0.16 W/m2K

 Lightwall - Roof 

 283mm (plus battens and slates/tiles)

 U-Value 0.14 W/m2K

 Natural - Weatherboarding


 U-Value 0.15 W/m2K

 Natural - Rendered

 398mm (depends on exact render detail)

 U-Value 0.15 W/m2K

Natural - Brickwork 


 U-Value 0.15 W/m2K

 Natural - Stonework


 U-Value 0.15 W/m2K

 Natural - Roof

421mm (plus battens and slates/tiles)

 U-Value 0.12 W/m2K


The Passivhaus standard was developed in Germany in the early 1990s and, supported by the European Commission, is rapidly becoming a pan-European standard for low carbon dwellings. The standard is performance based and at its heart are requirements that annual space heating demand does not exceed 15 kWh/m2/yr and that primary energy use (for all purposes) does not exceed 120 kWh/m2/yr.

The standard also requires:

  • Fabric U values not exceeding 0.15 W/m2K.

  • Window U values not exceeding 0.8 W/m2K.

  • Air permeability not exceeding 0.6 air changes per hour at 50 Pa (demonstrated by a pressure test of the completed building).

  • Advanced whole-house mechanical ventilation with heat recovery with at least 75% heat recovery efficiency and electricity use no greater than 0.4 W/m3 of supply air.

Breathability in an Air Tight House!
Breathability or Vapour Permeability – using the term breathable - has caused a lot of misunderstanding across the industry.

The wall construction should be vapour permeable, but can still be made airtight. On Wrightwall Natural the inner Airtightness Membrane is sealed between the panel junctions as the panels slot together, this layer also doubles as vapour control were required (kitchens, bathrooms etc.).
The most important factor in the design of the panel is that the outer layers (cold side) are five times more vapour permeable than the inner layer (warm side of the wall). This means that any moisture that gets into the envelope either during manufacture or on site will be able to move from the inner layers of the walls out through the outer layers and evaporate into the outside environment or not stay trapped in the wall panel itself.

Airtightness is important in such constructions, and is achieved by sealing the joints between panels, as air leakage will allow moisture into the structure, increasing relative humidity and the risk of rot. An advantage of vapour permeable construction is that moisture settling within the structure under certain conditions will usually escape later on, as temperature and relative humidity change.
So just call it vapour permeable, and make it airtight and it should be fine in a pressure test.

What is air tightness?
Air tightness refers to the infiltration of cold/hot air into the building and/or the loss of heated/cooled air from inside through gaps, cracks, holes, etc in the building fabric.

By limiting the leakage of heated/conditioned air from buildings, it is possible to reduce energy consumption and costs.

Too much air leakage leads to unnecessary heat loss and discomfort for the occupants. As the Government strives to reduce CO2 emissions from new buildings, building regulations now place greater emphasis on the quality of the fabric of the building.
At Oakwrights we have developed our Wrightwall Systems for a high level of airtightness. Our systems differ in that we allow for the movement of the oak frame & also sealing the hard to get to areas of the encapsulation system behind the oak frame! We have had test results back for our Wrightwall Natural as low as 0.3 air changes per hour at 50 Pa.

How is the Air Tightness Testing done?
In a nutshell a fan or a number of fans are installed to a suitable external opening and the entire building is pressurised over a range of pressure difference.

The testing is measured in air flow m3 over an average hour period at an average of 50Pascal for every m2 of building fabric. A typical large detached house would have around 400m2 of exposed fabric on the floor, walls and roof. A test figure of 10 would give 4000m3 of leakage at 50Pa over an hour period or as an equation 10 m3.hr.m2@50Pa.Quite a lot of leakage we think you will agree!!
Before the test is carried out, passive ventilation must be temporarily sealed. HVAC plant is switched of and temporarily sealed. The exterior envelope and all its openings are closed.

When is the Air Tightness Testing done?
Testing is carried out when the envelope is complete. If possible, it is wise to test twice - once before the covering-up of the membrane when remedial work can easily be carried out, and again at completion.

Decrement delay
Anyone familiar with spending a hot summer's day in a caravan and then another in a stone house with closed shutters will understand decrement delay. The inside of the caravan closely maps the rise and fall in external temperature to provide the familiar stifling effect on the occupants. In the stone house, the internal air temperature, staying well below the midday heat, barely varies throughout the day and so provides relative comfort to those sheltering from the sun.

Different materials, allow the passage of heat at different rates. The time it takes the peak temperature on the outside of a material, such as a wall or a roof, to make its way to a peak temperature on the inside face, is called 'time lag' or, more commonly, 'decrement delay'.

By controlling decrement delay it is often possible to control and prevent the overheating of a building. Denser Insulations have greater ‘thermal mass’ and heat takes longer to travel through such insulation. Therefore a building wrapped in a relative dense insulation will have a longer delay in the transfer of the heat through the wall. In the summer this keeps the inside of the house cooler for longer & in the winter keeps the heat in the house for longer!

By using Wrightwall Natural with Weatherboarding the system has a Decrement Delay of 6 hours, this is in comparison to a SIP’s panel of 5 hours and a non-insulated wall of 1 hour!

encapsulation systems

wrightwall natural

wrightwall light

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