insulate garage workshop for heat and sound

5 May 2015
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United Kingdom
Hi all,

I'm going to be workshopifying my garage soon, hurrah :)

From lots of scouring of forums and manufacturer websites, I have a plan of action, but before I jump in I'd really appreciate a sanity check.

The garage is standalone ~5m x ~5m with single thickness brick walls, pillars in the middle of each, pitched roof and uninsulated roller doors. I want to insulate it both *thermally* and *acoustically* (neighbours' houses & gardens are close, and routers, grinders, saws etc. are pretty loud tools). Also, I don't want to seal off the main doors (at least one will be for bicycle, lawnmower, etc). I want to be able to use it in winter without undue amounts of temporary heating or physical exertion!

What I'm thinking is:

ceiling - from the inside out: plasterboard, resilient bars, 100mm high density mineral wool (Rockwool RWA45 or Knauf equivalent), 100mm normal loft insulation.

walls - from the wall inwards: breather membrane, 25mm battens (mini cavity), 50mm cellotex GA4000, VCL, OSB sheet (so I can hang shelves easily). I lose ~90mm of floorspace on each wall.
[Reference: celotex garage conversion "application datasheet" page 8, linked to from (it uses PL4000 which has bonded plasterboard and appears to have VCL builtin)].
This Celotex page ( suggests DPM behind the battens instead of membrane; I suppose it depends on how exposed the walls are?
I know some people advocate tanking but the celotex pages don't show that option.
To avoid sound leaking up the walls straight into the roof space I think I need the tops of the battens/celotex/OSB to snug up to a resilient bar, not direct to the roof trusses.

front doors/walls - the roller doors provide no meaningful insulation; inside that build a ~100mm partition of, from the outside in: plasterboard, 100mm high density mineral wool
(rockwool RWA45 or knauf equivalent), plasterboard. The bottom ~2m of one of these "walls" will be doors hinged outwards through the roller door opening.
I'm considering using metal studs for lightness and possibly less sound transmission than timber.

floor - not sure whether to bother - from the slab upwards: DPM, 20mm cellotex, VCL, 18mm chipboard. Join the dpm & vcl to that in the wall. Except of course at the main doors, where I'll have to edge with timber.
[Reference: celotex garage conversion "application datasheet" page 6 & 7]
I plan to do the walls from slab upwards, and if I decide to insulate the floor then lay this inside the insulated walls.
How thick does the plasic sheet have to be to qualify as DPM?

pedestrian door - meeting U values is straightforward, but acoustic data hard to come by; lots of construction options, with upvc seeming cheapest, perhaps dense (and expensive:() composite or steel skin ones better?

window - double glazed unit; again quantified sound insulation data is hard to come by. Any recommendations very welcome!

Is it going to work? My estimated numbers are:

For comparison, U (W/m^2K) for conversion for residiential use (building regs) is:
walls = 0.3
ceiling = 0.16
floor = 0.25

With the materials above I estimate:
walls U = 0.39, (bricks U=3, GA4050 U=0.44)
wall with door + window U = 0.6 (assuming door U=1.6, window U=1.4)
wall/roller doors U = 0.27 (plasterboard U=2.5, 100mm RWA45 U=0.35)
ceiling U = 0.18 (plasterboard U=2.5, 100mm RWA45 U=0.35, 100mm loft insulation U=0.44)
floor U = 0.57 (using calculator below, assuming 20mm celotex)
which is probably sufficient: it should be comfortable and warm up quickly without losing heat too fast; any more insulation is unlikely to cost in.

slab calculations: Floor Calculator/Ground Floor Calculator.htm
RWA45: rw slab datasheet.pdf

Sound seems much harder to control (& estimate). I'm /assuming/ the SRI values are meaningful for the frequencies of powertools.

As a reference I believe the regs for sound reduction are, Rw (SRI):
45dB between rooms
50-55dB for places like (school) music rooms
I assume that 50-55 amounts to barely hearing the music at all, so anywhere in the 45-50 region is probably ok/good for an outside workshop.

Walls: single skin unplastered brickwall, R = 42 dB; the cellotex, OSB etc will increase that a bit but probably not a lot.
I could add a layer of plasterboard on the inside, on the basis that (i) it adds mass; (ii) possibly more absorbant than OSB, though without resilient bars I'm /guessing/ it may not make much dfference and I don't want to sacrifice too much floorspace.

Main doors/partition:
In a Knauf brochure they claim:
metal studs, 12.5mm wallboard each side, 25mm knauf earthwool, 100mm total R = 42 dB
metal studs, 12.5mm knauf silentboard each side, 50mm knauf earthwool, 100mm total R = 59 dB
This is in a brochure for Knauf Silentboard and they're being a bit sneaky in changing both the boards and doubling the amount of insulation. Their graph suggests silentboard is ~10dB better than "standard wallboard", so with standard plasterboard and 70mm dense mineral wool (it's also the thermal insulation) it might be around 50dB.

The resilient bars should prevent sound travellng along the roof trusses and straight onto the tiles & out the door fascias etc. The 100mm mineral wool should absorb some noise.
But not sure about the single skin of plasterboard - I believe that the sound insulation depends really on two separate-ish heavy layers (plasterboard) separated by a gap, so without one of the layers of plasterboard it might be a bit hopeless. I guess I'll try it & see. I could always add a layer above at a later date if really required, though that would probably require retrofitting a VCL too, which would be a total pain.

So, what do you reckon? Any howlers?

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