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 insulation 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 masonry substrate. A long series (150 – 250mm) 6mm masonry 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 twin thread 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 centre 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 centres.
