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garage conversion - walls |
If you intend to split your garage up into a habitable room and a garage area you need to consider the fire regulations.
A step down will be required but also the dividing wall must be fireproof. If brick or block built this isn’t usually a problem but if it’s a timber stud wall you’ll need to cover the garage side with a layer of fireproof plasterboard (sometimes 2 layers with overlapping joints) and a plaster skim finish. You must also remember that any access door must be fire proof too and have a half hour fire rating. This includes the frame and hardware.
The walls of the conversion need insulating to the required target value given by building control. Every type of building construction has a different ‘u’ value in itself which affects the amount of extra (if any) insulation it will require. An external cavity wall containing 75mm of dritherm fiberglass insualation will require little or no further insulating. The same may apply for an external solid wall having external sheet insulation as part of an insulated render system. It all depends on the existing insulation value and the target value to be achieved.
The thing to remember with any sheet insulation, if you double the thickness you effectively double the insulation value (or half it’s u’ value) e.g. should you require a ‘u’ value of 3.0 and if 30mm of cellotex will achieve 6.0, then 60mm of cellotex will suffice. Check the manufacturers data and the building control target value before you begin. The building inspector will want to know exactly what type of insulation and what thickness you intend to use, as well as the method of installation. Most sheet insulation is available in a variety of thickness ranging from 10mm up to 150mm or more in divisions of 25mm or thereabouts.
There are various types of material available, some requiring differing installation methods, whichever you choose, follow the manufacturers advice.
Probably the easiest way is to use a waterproof sheet insulation against the external wall with any joints waterproof taped to form an insulated vapour barrier. The next stage is to fix metal or timber battens to the wall fixing through the sheet followed by a skin of 12.5mm plasterboard. Remember any electrical cables are better of routed on the room side of the cellotex and taped back in plastic conduit. The 25mm air gap between plasterboard and cellotex offers plenty of room.
The battens will need to be spaced according to the plasterboard manufacturers recommendations. Every 600mm is sufficient for a 12.5mm plasterboard but 400mm may be better, there will be more fixing points and better support, especially where there are children. Vertically spaced is normal but there’s nothing to stop you installing your battens horizontally. Due to the extended length of fixing required it is generally easier to use some sort of purpose designed ‘concrete fixing’. These are generally threaded and hardened screws which require only a pilot hole in a masonary substrate. A long series (150 – 250mm) 6mm masonary drill bit is required and the batten is simply held in place while you drill right through the batten, insulation and into the wall. The fixing is then screwed in with a cordless screwdriver achieving remarkable torque. Some concrete fixings also have 2 separate threads (not twinthread intertwined threads like some woodscrews) that hold into the substrate and allow the fixed item (batten) to be backed off again slightly to achieve plumb or flat whilst maintaining a solid fixing. Sort of an ‘adjustable’ screw, very useful on out of true walls, packers may be added once the required distance is set. If using sheet insulation of 50mm and a batten thickness of 25mm a fixing of 125mm will be required. 4” (100mm) woodscrews just aren’t long enough. Concrete fixing screws are available up to 300mm in length and are easily obtainable these days from most builders merchants / diy stores or online companies such as screwfix or toolstation. If a separate vapour barrier is needed it is usually easier to pin one side up underneath the first batten and the opposite side with the last batten, then you can fill in the remaining battens in the middle. Always remember to keep every third or fourth batten (depending on center distances) exactly 1200mm from the 3rd or 4th one back so the edges of the plasterboard meet halfway on a batten all the way along the joint. If using 6’ x 3’ boards (1800mm x 900) simply adjust accordingly to 450mm centers.
When insulating reveals it may pay to use a sheet of insulated plasterboard, this can be cut to size and stuck in position using ‘no nails’ or gripfill etc. Always remember to cut back the insulation from the corners to the depth of the insulation to allow the plasterboard to meet at the corner. If the construction is single skin and the window / door sits flush with the inside edge of the wall then usually a strip of plasterboard stuck onto the side of the wall insulation will suffice. It may also pay to have the window manufactured with either a ‘large outer’ – the outer edge of the frame is a lot wider than standard or include some ‘frame extensions / knock on’ to the two outer edges. This will facilitate the installation of an insulated reveal without encroaching into the glazed area of the window.
A wall that has habitable rooms on both sides e.g. the garage is an extension type and the other side of the wall is the lounge may well have its own cavity wall insulation so no further insulation is necessary. The direct bond (dot and dabbing) method of fixing plasterboard is usually all that is required although it can be solid plastered instead.
Any stud wall built within the conversion should contain fiberglass insulation. This not only helps retain heat but also reduces the passage of sound.
Electrical sockets are best fixed using ‘dry lining’ back boxes which are cut and fixed into the plasterboard and used in hollow wall situations. Since the insulation does not offer a suitable fixing for an electrical back box this is the most appropriate method. ‘Fire Pads’ may need to be introduced around any holes cut in the plasterboard to prevent the hole becoming a weak spot in the event of a fire, the same may apply to any protrusions from within the insulated area such as a consumer unit or gas meter. Check with the building inspector. Fibreglass insulation is generally regarded as a fire insulator, polystyrene is not.
Should you be installing an organic ‘non waterproof’ insulation then remember to include a vapour barrier between it and the room and a damp proof membrane (essentially the same thing) between it and any single skin / solid wall / source of moisture.
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