Insulating under floor

[Pford75]

As the dining room progresses, I am going to be insulating under the timber floor (which is in the cellar, so easy to access). I've purchased a mixture of celotex and glass fibre rolls (reason for the mixture is that the cellar is v.large and I got them at Diss auction rooms at a bargain price).

Since we also have a pump in the cellar which runs intermittently, I'd like to add to the insulation to both help with sound proofing and also to hold the insulation in place. I'm looking for something quick and simple...and was going to use either plasterboard sheets or chipboard across the floor joists from down in the cellar (to hold the insulation in place). Any thoughts/advice/suggestions/warnings?

 

[Penners]

If you want sound insulation, you'd certainly be better off using a solid sheet material to hold the insulation in place, as opposed to just netting or something like that.

I was talking to an engineer the other day, who is something of an expert on acoustic (and thermal) insulation. He recommends using two or more layers of thin plasterboard, rather than one thick layer, to maximise acoustic insulation.

So if you were planning to create a cellar ceiling of, say, one layer of 12mm plasterboard, you'd be better off with two layers of 9.5mm - nothing between them, just sandwiched together, with the insulation material above them.

In order to create the best possible barrier against the movement of air through the floor (and of course air movement means sound movement), make sure your insulation is tight against the sides of the joists. Also, when you fix the plasterboard to the joists below the insulation, run a generous bead of acrylic mastic (sometimes called decorator's caulk) along the joists before you screw the plasterboard on.

A suspended floor, by its nature, will always have a tendency to resonate, but with the above structure below it, and a good carpet with underlay on top, you should be able to ameliorate this considerably.

 

[MdB]

Also, when you fix the plasterboard to the joists below the insulation, run a generous bead of acrylic mastic (sometimes called decorator's caulk) along the joists before you screw the plasterboard on.

Is this to stop the plasterboard from directly touching the joist and to add something flexible between them?

 

[AMc]

I used to read www.avforums.com, in their DIY section there were lots of threads covering construction of dedicated home cinema rooms. The subject of sound proofing came up a lot . It would be worth having a browse and searching on "soundproofing" as I believe there are specialist board materials that have a much greater impact than regular plasterboard.
http://www.avforums.com/forums/room-acoustics-audio-video-calibration/

Something to bear in mind is that acoustical isolation needs air tightness to stop higher frequencies and mass to reduce conduction of low frequencies. The former should be reasonably easy to achieve stuffing wool above the celotex to block the gaps, the latter will depend a bit on how much extra weight you're prepared to screw to the joists above. Many of mine are quite bouncy and I wouldn't want more than a layer of plasterboard hung from them.

 

[Penners]

Also, when you fix the plasterboard to the joists below the insulation, run a generous bead of acrylic mastic (sometimes called decorator's caulk) along the joists before you screw the plasterboard on.

Is this to stop the plasterboard from directly touching the joist and to add something flexible between them?

Exactly so, and to create as much of an airtight seal as possible between the floors. Sound can wriggle through the smallest of air-gaps, so the more that you can eliminate such gaps, the better.

 

[christianb]

Weyroc moistureboard is pretty good. Will at least privide something substantial, and not too costly.

C.

 

[MdB]

Also, when you fix the plasterboard to the joists below the insulation, run a generous bead of acrylic mastic (sometimes called decorator's caulk) along the joists before you screw the plasterboard on.

Is this to stop the plasterboard from directly touching the joist and to add something flexible between them?

Exactly so, and to create as much of an airtight seal as possible between the floors. Sound can wriggle through the smallest of air-gaps, so the more that you can eliminate such gaps, the better.

Just to go off subject further. I had a look at various materials to go between joists and floorboards to stop sound transmission and decided that £120 a roll was a bit much. Instead I'm going to use roofing felt (the rubber type) which at £28 a roll is a little bit more reasonable. It looks like it has the right properties and it should help some even if it isn't as good as the proper stuff.

 

[Pford75]

Thanks for the helpful responses.

The noise isn't sufficiently loud or worrying for me to spend lots of time or money on this aspect (heat insulation is the main reason for this). Since I will be using whatever insulation I manage to pick up cheaply at the auction, I'm stuck with that. I will heed the advice that two thin sheets is better than 1 thick sheet