Roger Bisby takes a walk across some Isosonic Dekfloor 30 as it is being installed in a refurbishment project.
Sound insulation can be a tricky business – things that work well in the laboratory may fair less well on site. There are so many things that can undo all that design work – a nail, a screw or a badly filled service duct. Even a tiny air gap is enough to let the sound through, so it is essential that the installer understands what they are trying to achieve, and that the devil is always in the detail.
Fortunately, in the case of Thermal Economics, the company has a wealth of experience, and a team of good technical sales people who will guide you through the products and process.
I went out to meet one of them on a conversion project in the Midlands to see the installation of Isonic Dekfloor 30. The product comprises 22mm t&g chipboard, with a resilient rubber sheet that is bonded to the underside of the board.
From the builder’s point of view the boards are glued and laid like any other chipboard floating floor. It is essential that no fixings are used and that a gap of 20mm is left around the edge. This must be filled with acoustic foam before the floor coverings and skirtings are fitted. The skirting boards must be fixed clear of the decking on an Iso edge 6/75 strip to prevent sound transmission into the wall.
The particular job I went to see was perhaps not your typical refurbishment. The building is just over 200 years old, and the evidence of countless changes of use was everywhere. It had stood empty for seven years, but is now about to see a new lease of life as luxury flats.
In some ways it would have been quicker and easier to remove all those old floor joists and start again, but shelling out a 200 year old building can often lead to problems with structural stability, so leaving those huge timbers that were built into wall sockets is no bad thing. They have also done all the moving and shrinking they are ever going to do so why go through all that again?
However, the problem for the builders was the floor was 70mm higher at one end of the building than the other, and it went up and down a bit, so they had to find the high spot and work from there using a laser level to mark and scribe the new build up laths on every single joist.
It was a demanding task, but they achieved it with considerable speed and success. I am not sure how it would have worked out without a good laser but they deserved a bit of help from modern technology.
It is important that the floor has minimal deflection, so getting those joists nice and level was key to the success. The rubber back has far more compressive strength than PUR foam backed boards so you wouldn’t expect to see the floor
bed in or settle over years. Interestingly, that increased resilience doesn’t lead to sound transmission because the make-up of the rubber is granular, so impact noises are absorbed in a very localised area rather than across the sheet.
By comparison, the laying of the Dekfloor was a piece of cake. The builders said it provided real time and cost savings over some of the multi-component systems they have use where you have to build the layers up. Apart from the fact that the rubber backing blunted the circular saw blades a little more rapidly there was really nothing else to distinguish it from ordinary floor deck.
The only thing I noticed is that the rubber backing prevents it sliding, so instead of being able to kick it in as you would normally with chipboard sliding on joists they had to give it a hefty whack. It would have helped to have a couple of knocking blocks to protect the t&g.
The joints are glued in the usual manner to prevent any squeaks and also prevent airborne sound coming through. The fact that they were using existing joists meant that the board ends didn’t land on the joist. In an ordinary floor you might have trimmed them back and butted the board ends but the T&G has to be maintained.
The usual practice is to build in a couple of noggings to take the board ends, but as this deck was being over laid with some Gypsum board, and underfloor heating topped off with some engineered oak, there was no chance of those board ends receiving a point load. The very fact that they are using hardwood floors in an apartment is a testimony to the effectiveness of the system. In many flats it is a condition of the lease that you have carpets, which is a tacit admission that the sound insulation isn’t that effective.
Another difference with this refurbishment was that the ceilings were very high, so there was ample room beneath the first floor for a suspended ceiling, in-filled with Rockwool. In a more typical build you would suspend the ceiling directly below the joist using Isonic cleats. These have resilient rubber pads to reduce sound transmission.
Once the boards were down I had a chance to jump up and down on them and there was absolutely no detectable deflection. The builder then had to walk some hefty steel beams across the newly laid floor and, unbeknown to them, I sneaked into the flat below to see what was happening on the underside of the decking.
It was proof that even with a 600 kg steel beam being walked across the floor there was no noticeable compression or deflection. I can’t imagine a more extreme test but, of course, we have to bear in mind that these huge old joists were set at 400 centres. If they had been at 600 it could have been a different story, but a 22mm board is always better than an 18mm.