If costs were the only criteria, every selfbuilder would choose a timber intermediate floor. However, the performance of timber floors in modern housing, particularly as regards sound proofing, is poor and, as a result, many selfbuilders have chosen masonry construction and pre-cast flooring. They undoubtedly perform well in this respect but they come with a cost penalty. What many people don’t realise is that timber floors can be “improved” to perform just as well as precast concrete ones. There are a number of features which can be designed in, each improving the decibel ratings a little bit: combine several of them and you have a vastly better performance.
• add insulation within the joist void (now mandatory) – adds 3-5dB
• thicker or heavier ceiling board – adds 3dB
• mount ceiling board on separating strips – adds 2dB
• add resilient strips between the joists and the floor cover – adds 2dB
• add a sound deadening layer under the floor cover – adds 4dB
• separate floor joists from ceiling joists – adds 3dB
If you were to do all these measures, you would improve the sound reduction levels from around 35dB up to around 50dB, which is close to the party floor standard required in flats and similar to the best pre-cast flooring systems. The cost? Similar to switching to pre-cast!
Whichever flooring system you choose, you should consider the effect of using hard floor surfaces and downlighters. Hard floor surfaces are noisy, especially as regards impact noise, which can transfer through the structure. If you are really concerned about noise indoors, then consider fitted carpets which are fantastic at sound absorption. Downlighters are another fashionable item which do little for soundproofing: however there are ranges which are rated for fireproofing and acoustic purposes, such as Snaplite, and you should consider fitting these under bedrooms.
Incidentally, there is a very rough and rather crude relationship between soundproofing costs and decibel reduction ratings. The simplest floor, the unimproved timber joisted floor with chipboard over and plasterboard underneath, costs around £35 per m2 and reduces airborne sound transmission by around 35dB. The very best soundproof solutions cost around £60 per m2 and gives just over 50dB sound reduction. Can you see where I am heading? It would be nice to say that each decibel of sound reduction will cost you around £1 per m2, and it’s not so far from the truth. But in reality, the decibel scale is not linear and the decibel rating doubles every 10 decibels, so it gets harder and more expensive to deliver soundproofing at the top end of the scale, so the neat relationship breaks down somewhat. But it’s not a bad guide. If you are prepared to throw more than £50 per m2 at your intermediate floors, you can build in pretty good soundproofing, whatever system of floor you choose.
At 35dB sound reduction, you will be able to hear all too much. Probably every word of every conversation. At 45dB, you will be aware that a conversation is going on but won’t hear more than mumbles, however music or TV will be very perceptible. At 55dB sound reduction, you should be vaguely aware that someone is in the room above or below, but the noise really shouldn’t give you any grief. Unless of course, they decide to hold a rave. Then I think you’d best forget about dB ratings and go out for the evening.