Timber frame closed panels are constructed from factory-fabricated wall, floor and roof panels that are pre-filled with insulation. Such panels are made from studs, rails and rigid boards on both sides. Different types of insulation can be used and linings are fitted to control the passage of water vapour. A vapour barrier is fitted to the warm side of the insulation and a breather membrane is fitted to the outer face of the panel.
Panels are designed to be airtight with vapour permeability increasing as you move through the thickness of an external wall, from the internal surface to the external surface. The internal surface is usually a polythene vapour control layer (VCL) and then there is the insulation thickness, the external sheathing board and, finally, the breather paper wrap.
If internal moisture does penetrate the VCL, it will find it easier to move in the direction of the external surface where it will evaporate away naturally. This avoids vapour being trapped in the panels and then condensing, otherwise known as interstitial condensation.
The rigid boards serve to provide resistance to racking. This is the tendency of a rectangular shape to deform into a trapezoidal shape as a result of an external pressure. In a house, it is the wind force on the side of a house that attempts to deform the rectangular shapes (internal and external walls) in the house. The racking boards used are commonly OSB (oriented strand board) but they can also be plywood, MDF or Fermacell (made from gypsum and paper).
Such panels can be fully finished in the factory, by providing service voids, internal plasterboard, external cladding and even windows and doors already fitted. Different suppliers offer different levels of prefabrication. The advantage of factory fabrication is that site construction time is minimised and a superior standard of insulation is achieved. U-values from 0.25 down to 0.10 W/m²K can be achieved.
Types of panel
The above description is that of a simple closed panel. The reality is that there are many varieties of this basic panel and you will also see different specifications according to whether the panel is designated an external wall, an internal wall, an internal non-loadbearing wall, a floor or a roof component.
Benefits of Closed Panel
- Rapid assembly on site
Generally, it takes just a few days to assemble the closed panels on site and a week to achieve a weathertight stage on a standard project.
- Package shell is available
Instead of dealing with a number of different trades, you are dealing with one shell manufacturer who will take sole responsibility for delivering and assembling the weathertight shell. The manufacturer may provide additional services, such as house design, organising the slab foundation, internal fitting and even plot search and planning in some cases. The choice is yours, from ordering just the basic shell to commissioning a turnkey solution.
- Ease of scheduling
In adverse weather conditions, standard brick and block construction struggles to continue. With Closed Panel, the assembly of panels into a weathertight state only takes a few days and this is achievable in all but the most severest weather. This permits you to schedule the follow-on trades with a high degree of certainty. You can maximise the extent to which the Closed Panels are completed off-site by specifying the inclusion of windows, doors, cabling, switches, sockets and even plumbing.
- Consistent quality
The factory-fabrication of a closed panel ensures that all design tolerances are actually achieved. Insulation is accurately installed and panels are precision cut and fitted to make it easy to deliver the expected airtightness and ever-important U-values.
- Predictable costs
Closed panel shell systems can be purchased at a fixed price, subject to the usual conditions regarding timeframes and currency fluctuations for imported timber.
Closed Panel characteristics
If you want to make changes on site, it is not so easy with Closed Panel construction. The entire shell has been designed for structural integrity and any major changes will be costly, requiring the involvement of the designer, the structural engineer and the manufacturer. So it’s essential to be very clear about all the detail before you lock down the final design.
Closed panel systems require foundations to be constructed to high tolerances, completely level and square to within 2 or 3 millimetre. This is because the factory-built panels are manufactured square and true and any slight in accuracy at foundation level will be magnified as the build goes higher, to the extent that the panels will eventually just not fit.
If it is impossible for a crane to assist with site assembly, the panels may have to be fabricated in smaller sizes in order to allow manual lifting, even where small-scale lifting tools are available. Similarly, smaller panels may be required if site access is difficult and the packages have to be transferred from lorry to site via smaller vehicles.
A package provider will usually require payment for the entire shell structure to be received prior to its arrival on site. This can be a very large sum and it may not be forthcoming from your mortgage providers until the structure has been assembled on site and inspected by the funders valuers. This could have an impact on your cashflow if you do not plan ahead for this payment step.