Specifying Airtight OSB for Passive House and Low Energy Buildings - MEDITE SMARTPLY


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About Medite Smartply

Medite Smartply is the market leading manufacturer of sustainable timber construction panels. Our brands Medite, MDF medium density fibreboard and Smartply OSB, oriented strand board, are renown for delivering quality products, customer led innovation and industry leading customer service. Our manufacturing sites in Ireland featured the latest production technology to deliver straighter, flatter and more consistent boards than ever before in a range of sizes and thicknesses. The raw materials used come from our own forests.


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Constant progression and investment has allowed Medite Smartply to enter new diverse markets and sectors meaning there are always new products to address market demands. Our products are as environmentally conscious in their makeup as they are in their application. If you would like to know more about the products, please visit www.mdfosb.com.


Medite smartply is part of the coillte group an Irish state owned commercial company operating in forestry, land based businesses, renewable energy and panel products manufacturer. Coillte owns over 445 thousand hectares of FC and PEFC certified forest, which is approximately 7% of the land mass of Ireland. Coillte plant seeds, grow saplings, maintain the forest, harvested timber and from their supply chain, we manufacture wood based panel products cradle to gate certified environmental profile.


Learning Aims

Learning Aims for this CPD are:

  • The standards required for passive house and low energy construction;
  • Achieving air tightness in construction;
  • Airtight products OSB;
  • Vapour control in timber frame buildings.


About Passive House Standards.

Passive House is a low energy performance standard that can be applied to many building types from domestic dwellings and care homes to schools, hotels and supermarkets. It is intended to reduce the energy demand for space heating and cooling, while creating excellent indoor comfort levels. This is primarily achieved by adopting a fabric first approach to the design, with very high levels of air tightness, increased levels of insulation and reduced thermal bridging and the use of mechanical ventilation with heat recovery.


Air tightness is an essential part of creating an energy efficient building and is a building regulations requirement. According to the national house building council home energy use is responsible for approximately 27% of the UKs carbon dioxide emissions.


A 2002 report from the BRE found that air leakage can be responsible for up to 40% of heat loss from buildings. Buildings designed to Passive House standard achieve a 75% reduction in space heating requirements, compared to standard practice for new buildings in the UK. The Passive House standard provides a robust method to help the construction industry meet the government target of an 80% reduction in carbon emissions of the 1990 levels by 2050.


Achieving airtightness

The key principle for achieving air tightness is to create a single continuous robust airtight layer which, surrounds the heated volume of the building. This is typically located on the warm side of the insulation, fulfilling the requirements of the vapour control layer. In order to achieve Passive House certification, a building must achieve air tightness of less than or equal to 0.6 air changes per hour under test conditions. This is expressed as in the slide where N50 is defined as the number of air changes per hour at a reference pressure differential of 50 pascals. The result is calculated using the buildings internal air volume, cubic metres rather than its envelope area, square meters, so the units are expressed as in the slide.


The airtightness figure achieved must be the average of pressurisation and depressurization undertaken using a blower door test as shown here. Best level of air tightness is challenging but possible with a clear design strategy. The final result is sensitive to the quality of workmanship onsite.


Current building regulations require air permeability at 50 pascals to be 7 to 10 cubic metres per hour per square metre, depending on whether a project is in England, Wales, Scotland or Northern Ireland. The Passive House limit is approximately 5 times less than the maximum permitted.


It is not possible to make a standardised conversion between air permeability values and N50 air change rate values, because there is no direct relationship, and each uses different testing and measurement protocols.


To put this in context the passive house air tightness limit as expressed in this slide is roughly equivalent to a hole in the envelope of the building that is less than the size of a 5p coin for every five square metres. In comparison a building that complies with the limited figure for air tightness in the building regulations part L 2013, would have a hole the size of a 20 pence coin for every one square metre of envelope.


Airtight products

Standard OSB and other types of wood panels such as plywood, particle board and MDF are not suitable as an air and vapour type layer as their permeability is inconsistent and can vary between production cycles and manufacturing sites.


Key British and European standards governing the specification and use of OSB panels in construction are BS EN300 and BS EN13986.


BS EN300 includes a classification system that defines 4 grades of OSB, in terms of their mechanical performance and relative resistance for moisture. These are:

  1. OSB 1 general purpose boards and boards for interior fitments for using dry conditions.
  2. OSB 2 load bearing boards for use in dry conditions.
  3. OSB 3 load bearing boards for use in humid conditions.
  4. OSB 4 heavy duty load bearing boards for use in humid conditions. However, none of these standard grades are certified airtight.


Traditionally plastic air and vapour control layer AVCL membranes have been used to achieve air tightness. Although, surprisingly there is no recognised industry standards or test method. The degree of vapour diffusion depends on material composition and the success of such systems is dependent on detailing, installation and workmanship on site.


Air and vapour tight OSB is a newer technology, a structural OSB panel which also has integrated vapour control and air barrier properties developed for the timber frame market. Products such as these specialists OSB panels classified as OSB 3 are specifically developed to be airtight and offer an alternative to air and vapour control layer membranes. The panels are guaranteed airtight and prevent interstitial condensation within the timber frame structure without the need for a separate membrane.


What is OSB?

OSB consists of strands of wood from softwoods such as spruce and pine bound with a moisture resistant formaldehyde free synthetic resin into three layers, bottom surface, core and top surface. The wood strands are alternately oriented through 90 degrees so that the core layer is at right angles to the outer layers. The panel is then pressed on the high temperature and pressure are rigid and dimensionally stable wood panel.


Orientation of strands in this way distributes the strength stiffness and spanning capacity of the finished OSB panels, which are approximately twice as strong in the length major axis as in the width minor access. It is this structure that gives OSB its strength making it suitable for a wide variety of structural applications from furniture to the construction of buildings. OSB is a solid panel with no core gaps and no delamination under normal use.


Airtight products. Air permeability.

To overcome the variable air permeability in traditional OSB, carries taken during the manufacturing process to eliminate density variations and air gaps. Some airtight OSB panels have been certified by the Passive House Institute to achieve the lowest level of air permeability of .01 cubic metres per square meter per hour. Airtight OSB is installed with airtight tapes and has achieved N 50 values as low as .09 air changes per hour.


Airtight products. Vapour permeability.

To overcome inconsistent vapour permeability in OSB, the inside face of the panels are coated with a vapor resistant polymer coating. This provides a consistently high water vapour resistance across the surface of the panel and smoothness which ensures a strong adhesion of airtight tape at the panel joints. Tests of airtight OSB panels conducted at the Fraunhofer institute of building physics, found that the coated surface was ideal for tape adhesion and the air tightness of the engineered OSB allowed it to withstand pressures of up to 2000 pascals.


Vapour control in timber frame buildings.

For timber frame buildings in cool temperate climates of vapour control layer is required on the warm side of the thermal insulation, to prevent excess water vapour entering the building fabric via diffusion. Warm air holds greater water vapour than cold air and therefore has a higher vapour pressure. Vapour diffusion transfer occurs when warm moist air inside a building migrates under high pressure through the building fabric to the low pressure cold air outside. If vapour transfer is uncontrolled condensation within the building fabric known as interstitial condensation, can occur as the moist air cools and the vapour condenses on the cold side of the assembly. The purpose of the vapour control layer is therefore to limit the amount of water vapour entering the building fabric.


Air and vapour tight OBS panels can also help to prevent summer condensation by acting as a humidity buffer. Materials made from cellulose fibres are hygroscopic and therefore can absorb and release small amounts of water vapour from the surrounding environment. In the limited number of cases of reverse vapour diffusion, airtight OSB panels can help to prevent condensation within a structure. This occurs where moisture from the outer leaf of a building envelope is heated by summer sun and permeates through to the cooler inner layers, posing a potential condensation risk, if not controlled or designed out.


Research into the suitability of OSB panels as the air and vapour barrier in timber frame structures, found that the quality of the installation was just as important as the intrinsic vapour resistance of the panels. Again, this was quantified by the Fraunhofer institute of building physics in 1989 when investigating the effect of a 1 millimetre tear in the plastic vapour control layer. It was found that the gap increased the vapour transmission from 0.5 grams per square meter to 800 grams per square meter, which is almost 2 pints in 24 hours, as vapour was transferred by convection and not by diffusion. Convection was found to carry 1,600 times more moisture into the structure. Therefore, it is critical for the long term performance of well insulated buildings that, both air tightness and vapour tightness are considered in tandem at the design stage and that suitable fit for purpose and certified products are chosen.


Summary of learning outcomes.

Passive House is a low energy performance standard applied to many building types: homes, care homes, schools, hotels and supermarkets. Air tightness is achieved by creating a single continuous robust airtight layer surrounding the heated volume of the building creating an energy efficient building.


Traditionally plastic air and vapour control layers have been used to provide air tightness. Success very much relies on the standard of installation and workmanship on site, which can fail.


Airtight OSB is now available with integrated vapour control properties and the panels are guaranteed. They are simple to use without the need for a separate membrane.


Airtight OSB is certified by the Passive House Institute as a component in Passive House building.


We have also seen how vapour control in timber frame buildings can be achieved using airtight OSB.

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