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Don't Get Caught In A Web Of Joists

Self Build Project

By Llyr Davies, Residential Business Development Manger, March 2015

As a self-builder, one of the key decisions you’ll face during your project is whether you go with a timber-frame structure or not. If you do, there are still many options to choose from.

The standard, solid timber joist construction method is one that we’re all familiar with. But, when it comes to designing the services for your building, this can also throw in a few curve-balls. Electrical wiring is fairly simple, given the small dimensions. Plumbing works - not quite as easy but the solid joists can accommodate cut-outs big enough to allow these to run to where they need to.

Ventilation: slightly trickier. Ducting sizes are conventionally either circular 125mm diameter or 204x60mm rectangular PVC ducting. These can be difficult for the solid timber joists to accommodate whilst keeping their structural integrity. You can always put in a service void, but who wants to lose out on that all important ceiling height? So the answer is webbed joists! Now, before you start feeling too pleased with yourselves, please remember a few things when considering your ducting routes and sizes.

Firstly, even using webbed timber joists, check that there is enough depth and width within the web to take either of the standard ducting sizes. Like ducting, webbed joists come in all shapes and sizes so be sure that yours is big enough! If not, you could always utilize a radial ducting system. This consists of multiple runs of 92mm semi-rigid duct from centralised manifolds, so can be the answer when rigid PVC cannot be used.
Secondly, remember that if you go with a branched PVC ducting system, they will come to site in 2m lengths. So be sure to plan in the installation whilst you still have access to slide a 2m length of duct through the webbed joists, as having continuous runs of ducting ensures that the systems stays as airtight as possible.
However, if the scenario arises where you need to cut the PVC ducting into lengths that can be inserted in between your webbed joists, there is a solution to ensure that you retain that all-important airtightness.

This is essential because if you’re cutting a 2m length of duct into four pieces then you need to reconnect these in-situ within the joints. So each length of duct will have an additional three connection points along its length. These connections can be made air tight again via the standard method of tape and sealant, but as you’re doing this multiple times it would be very prudent, not to mention time-saving, to use self-sealing connectors as this will guarantee you an air-tight connection without the need for any tape or sealant.
Nuaire’s Self-Seal ducting range is a fast and efficient method of reconnecting your duct-runs and is available in both circular and rectangular fittings to suit your joist depths. The simple, push-fit connections ensure that air leakage into and out of each duct joint is minimised. This means you get the maximum performance from your MVHR or MEV system by guaranteeing the correct airflow. And dust and debris are kept out of the ducting system. It’s fully compliant with Building Regulations and is a ‘fit-and-forget’ system, so certainly one to consider when planning the services in your timber-frame home. View our Self-Seal range.