applying render to SIPS

Structural Insulated Panels (SIPs) are a high performance building material consisting of an insulating foam core, sandwiched between two panels of OSB board. There are many benefits to building with SIPs panels, including insulation and airtightness. With SIPs panels, the insulation is highly effective due to the fact that the panels fit together tightly enough that air leakages are minimised. They’re a popular building material because the required U-values to meet building regulations can easily be achieved with a very quick speed of build.

Rendering SIPs panels can pose a bit of a challenge due to the fact that the render cannot be applied to the OSB board directly. In order to be able to render the SIPs there are various steps that need to be undertaken before the rendering can go ahead.

Installing Timber Battens onto the SIPs Panels

The very first step in applying render to SIPs panels a breather membrane needs to be secured to the external OSB panels with galvanised staples at 600mm centres horizontally and 300mm centres vertically. We recommend using the Dupont Tyvek Housewrap, as this membrane is totally vapour permeable and can be left exposed for up to four months.

Once the breather membrane has been applied, vertical timber battens need to be installed onto the SIPs panels. These are installed as a means of supporting the render carrier board which is secured to the battens, and so the size of your render carrier board will dictate where you position the battens. Once the battens are in place, you can secure the render carrier board to them using the appropriate wood screws.

The render carrier boards should have a slight gap of 3-5mm between each board. This needs to be covered over using Pavatex Render Carrier Board Joint Tape, which is another breathable membrane except this one is a self-adhesive tape and is designed for the purpose of jointing render carrier boards before render is applied directly on top. The joint tape will further secure the boards and prevent the wet materials seeping through the gaps.

Thin Coat Render Systems on SIPs

We recommend using the Premium Basecoat for rendering SIPs as this is our strongest basecoat and will therefore ensure the system is highly secure and crack-resistant (read about how to fix cracked render here). We always recommend that Fibreglass Mesh is embedded within the basecoat as a means of increasing the tensile strength of the system; each strip of mesh is overlapped by 10-15cm all the way around, and therefore once the basecoat is set the mesh should absorb any movements within the render carrier boards without cracking.

After the basecoat has set for 24-48 hours, a render primer needs to be applied. The primer used will depend upon which kind of render you are using, so if you’re going for an Acrylic Render you will use our Mineral and Acrylic Primer, but the silicone-based renders require the SiSi Render Primer. This can be painted onto the basecoat and left to dry for 12 hours, and then finally you are ready to render!

Achieving a nice render finish on SIPs depends upon thorough preparation, so by now you should have a perfectly stable and flat basecoat that will provide the perfect surface for the thin coat render. The render is applied in such a thin layer (the thickness that you apply the render matches the grain size) that any large imperfections can be quite apparent, particularly if you’ve gone for a smaller grain size i.e. 1mm. Our most popular choice for installers is our Silicone Silicate Render in a 1.5mm grain size because it is breathable and offers good coverage at a price that is great value.

The render should be applied using a notched trowel, and then it should be rubbed up using a plastic render float to bring out the texture. Once applied, leave it to set for 24-48 hours.

Wood Fibre Insulation onto SIPS Panels

If render carrier board onto battens seems like too much hassle for you, then why not use Wood Fibre insulation? The Wood Fibre can be attached directly to the SIPS panels using stainless steel wood screws and universal fixing discs. After this, you simply apply the thin coat render system on top as explained above. Not only is this a simple and easy way of doing it, it reduces the risk of condensation and also improves the thermal efficiency of the structure.

And there you have it; how to apply render to SIPs panels. We have also covered how you might go about applying render to other types of substrates, including ICF and high performance blockwork. So check out those blog posts and stay tuned for future content!

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