Converting an Integral Garage Into a Room

Working backwards from the number the floor has to reach

A garage is outside the heated envelope and a bedroom is inside it, and that change is what brings the energy standard down on work that is not an extension. In England the hook is regulation 22 of the Building Regulations 2010, Requirements relating to a change to the energy status of a building, with Approved Document L Volume 1 supplying the limiting standards the floor, walls and any new opening then have to meet. In the United States it is the insulation and fenestration table in Chapter 11 of the International Residential Code, or the corresponding table in whichever edition of the International Energy Conservation Code the jurisdiction adopted, read against the climate zone. Both numbers move between editions and between zones, so look yours up rather than carrying one across from the last job.

The arithmetic runs the opposite way to the way people do it. Start at the target, subtract the resistance of everything going in anyway — screed or deck, finish, surface films — and what remains is what the boards have to supply alone. Multiply that by the board's declared thermal conductivity for a thickness, then hold it against the head height you measured. Product choice stops being a preference at that point: polyisocyanurate declared to BS EN 13165 typically sits near 0.022 W/mK against expanded polystyrene to BS EN 13163 at nearer 0.038, so the same resistance lands roughly seventy per cent thicker in the cheaper board.

One caution about what a layer sum is and is not. Adding resistances gives you the resistance of the construction you are laying, and that is the number you need to choose a thickness. It is not the finished floor's U-value, because a floor loses heat to the ground in a way that depends on the ratio of exposed perimeter to area — a long thin garage on the gable end of a house behaves quite differently from a square one tucked between two heated rooms. The calculation method for that is BS EN ISO 13370, Thermal performance of buildings — Heat transfer via the ground, and it is what a building control submission will be assessed against.

The damp line runs across the threshold, where the door used to be

Almost every garage conversion that goes wrong goes wrong in the same metre of floor. The drive was laid to fall toward the garage, the slab sat low to take that water, and the house's damp-proof course was set at a height chosen for the house. Take the door away and you have a habitable room whose finished floor may sit level with, or below, the ground immediately outside it. Approved Document C asks for clearance between finished external ground level and the damp-proof course — a hundred and fifty millimetres is the figure the guidance works to — and on a freshly infilled threshold that clearance has to be created rather than assumed.

The membrane's job is to arrive at the wall and join something. Dress it up the perimeter to above finished floor level, lap it into the existing course in the direction that sheds outward, and where the infill wall is new, build its damp-proof course so the floor sheet laps into it on the same day rather than being tucked behind it later. Sheet material classes for this duty are in ASTM E1745 and the installation practice in ASTM E1643; the masonry course it has to meet is the subject of BS 8215, Code of practice for design and installation of damp-proof courses in masonry construction. Every lap gets taped, including the ones nobody will ever see again, because a lap left dry under a screed is not a lap.

The threshold itself needs three things the rest of the perimeter does not. A vertical damp-proof course at each reveal, where the new infill meets the cut face of the old opening and the cavity is closed. A cavity tray or a lintel with an integral one at the head, with weep holes above it, so water crossing the cavity is put back outside rather than onto the new inner leaf. And somewhere for the drive to drain now that the fall no longer runs into a garage — a channel drain across the old opening, or the drive re-graded away, connected to a legitimate outfall rather than pointed at the flower bed. Skip the third and the first two are being asked to hold back standing water for the life of the house.

One thing changes simply because the floor becomes occupied. In a radon-affected area a slab that was acceptable under a garage is now under living space, and the document is BR 211, Radon: guidance on protective measures for new buildings, read with the local radon map. A damp-proof membrane and a radon barrier are not automatically the same specification, and the difference lies in the sealing at penetrations and the perimeter rather than in the sheet.

Boards on a floor with a fall built into it

The fall you measured has to go somewhere, and there are only three places to put it. Level the slab first with a bonded screed or a levelling compound and lay flat boards on a flat base; let the boards follow the fall and take the level out in a screed of varying thickness above; or, on a small fall, absorb it in the deck build-up. The first is the most predictable and eats the most height at the low end. The second saves height and leaves a screed that is thin somewhere, which is where it will crack. The third only works on a fall a floating deck can bridge without rocking.

Board grade matters more here than anywhere else in the envelope, because this insulation is under load. The property to specify is compressive stress at ten per cent deformation, determined to BS EN 826 and declared in the product standards — BS EN 13163 for expanded polystyrene, BS EN 13164 for extruded, BS EN 13165 for polyisocyanurate, with ASTM C578 and ASTM C1289 covering the same ground in North America. A board suited to a wall cavity under a screed carrying a bed, a wardrobe and somebody standing on a chair will dish, and the dishing shows in the finish long before anybody suspects the insulation. Where the screed is a floating one, BS 8204-1 sets the minimum thickness over insulation by loading class, and the figure is thicker than the one most people carry in their head.

Setting out is quick and the waste is not what a rule of thumb says. Boards go down breaking joint, butted tight, taped at the joints where the manufacturer's system calls for it, and cut close around anything that comes through. A garage floor typically has more of those than a room does: the old gully or drainage channel across the door, a soil pipe that was routed through because it was convenient, sometimes a duct. Three cut-outs on a fifteen square metre floor is not a ten per cent job.

1. Grind or scabble the slab back to a sound, clean surface and fill any hole deeper than the membrane will bridge.
2. Establish finished floor level off the house floor, not off the garage slab, and mark it round the perimeter before anything is laid.
3. Lay and lap the membrane, dress it up the walls past that mark, and tape every seam and collar.
4. Fix the perimeter upstand strip to the full height, so the screed or deck can never touch masonry.
5. Lay the boards breaking joint from the low corner, cutting into the perimeter strip rather than against the wall.
6. Lay the separating layer, then the screed or deck, then leave it alone for as long as the material needs rather than as long as the programme wants.

The strip around the edge is doing more work than the middle

A solid floor does not lose heat evenly. Out in the middle, the ground under the slab is nearly as warm as the room and very little moves; at the edge, the path from the floor surface out through the slab edge into an external wall and the air beyond it is short and cold. That is why a floor's calculated U-value depends on its exposed perimeter at all, and why a perimeter upstand strip that stops at the last easy corner has undone a disproportionate share of what the boards achieved.

It has to run continuously from the top of the membrane to finished floor level, everywhere the floor meets a wall: both reveals of the old opening where the infill returns into the existing jamb, and the short return beside the house door. Where the old slab edge is exposed at the threshold, carry board down that edge too. What is being controlled is the linear thermal transmittance of the floor-to-wall junction, calculated by the conventions in BR 497 and assessed alongside the surface temperature factor covered by BRE Information Paper IP 1/06. Most jobs demonstrate the junction with a published construction detail rather than by modelling it, which is fine — provided the detail on site is the one that was published.

Filling the hole the door left

Start under the threshold, because that is where the money hides. A garage door opening is very often built off a thickened slab edge or a shallow concrete toe rather than a foundation, since nothing was ever going to be built on it. An infill wall is a load, modest but real, and building control will want to know what it bears on. Dig a trial hole before the job is priced: if what comes back is a toe, the infill needs its own foundation to the small-building provisions of Approved Document A, which means excavating inside a doorway — the commonest reason a garage conversion overruns programme and budget together.

The lintel above stays and stays loaded. Whatever spans the opening was designed for a clear span and is still carrying the wall over it, so the infill beneath is non-loadbearing: build up to the underside with a normal bed joint and do not wedge, pin or pack it tight in an attempt to be helpful. Check the bearings at each jamb are undisturbed when the reveals are cut into for bonding, and if it turns out the head was formed by a beam within a frame rather than by a lintel in the masonry, stop — that is a question for an engineer, not for a bricklayer with a bolster.

Tying the new work to the old is a choice between toothing into the cut jamb and fixing stainless steel wall starter profiles to it. Profiles are the usual answer on a conversion because they are quicker, they do not disturb the existing coursing, and they take the vertical damp-proof course cleanly behind them — but they have to be the profile made for the leaf thickness you are building, and they have to be plugged into sound masonry rather than into the render.

The face is the part the street judges, and bond is less forgiving than colour. Match to the wall as it stands today, from a sample panel laid outside on a dry day rather than from a photograph — but if the beds do not line through with the existing courses across the reveal, the join is visible from the pavement forever, and no tinting fixes two millimetres of drift accumulated over fifteen courses. Most infills also carry a window, which brings its own lintel, a cill, a cavity closer at the jambs and a hole in the middle of the brick count.

Behind the brick, the leaf nobody sees does the insulating

The outer leaf keeps rain off and matches the house; everything thermal happens behind it, by one of two routes. A blockwork inner leaf with the cavity insulated matches how the rest of the house was built and keeps the wall thin. A stud liner inside the masonry, insulated between and over the studs, is slower and thicker but has one large advantage here: the garage's own external walls are frequently a single skin of blockwork never meant to be part of a dwelling, and a liner brings the infill panel and those walls to one plane, one insulation depth and one plasterboard line in a single operation.

If it is batts between studs, the failures are the ordinary ones and they matter as much here as in a new wall. Full depth, friction fit, split around cables rather than crushed behind them, no rounded shoulder at the head, nothing left slack in the bay above a noggin. What batts cannot do is cross the studs themselves, so if the target U-value is tight the answer is a continuous layer over the framing rather than a deeper bay. That argument, and the framing factor arithmetic behind it, is set out fully in the wall insulation guide and is not repeated here.

Where the vapour control layer goes is decided by what is outboard of it and by the climate, not by habit. BS 5250, Management of moisture in buildings, is the code of practice for the question, and the condensation risk analysis it points to is what settles a borderline case rather than a rule of thumb about the warm side. Two details are worth writing on the drawing regardless: the layer has to be continuous where the new stud liner meets the old garage wall, and it has to be sealed at the floor, where it meets the membrane you dressed up the perimeter.

Pricing a job with no roof and no foundations in it

A garage conversion is priced badly because the instrument people reach for is an extension rate. That rate carries substructure across a whole footprint, a roof, four external walls and the drainage that goes with them, and a conversion buys none of it — the box exists. What it does buy is a set of items that look small on a drawing and are not: a floor stack built by hand in a room with one way in, an infill panel over a foundation of unknown adequacy, making good where new plaster meets old, and services extended from a house that was not expecting the load. The rate is therefore far too high as a measure of the work, and the finished figure per square metre still often lands near an extension's. Neither comparison tells you anything.

The extension model is still the right shape to build the estimate in, provided the shell line is used honestly. Set the build rate low, or to nothing, and put the real money in the discrete lines: the infill and its foundation where the structural opening line would sit, the floor build-up and the making good together, and the fees. Then watch the share of the total that is not the shell. On a conversion that share is dominant, which is the arithmetic reason a small garage costs nearly what a large one does and why halving the floor area does not halve the price.

Two fee lines are commonly missed and both are cheap to establish early. Building control applies to the whole of this work, and the application belongs before the door comes off. And in England, while an internal conversion is not usually a planning matter, the elevation is changing — and houses on newer estates frequently carry a planning condition requiring the garage to be retained for parking, or sit under an Article 4 direction removing permitted development rights for alterations. That is established by asking the local planning authority, not by looking at what a neighbour got away with.

Three holes in a morning decide the contingency

The variance on this job sits almost entirely in things you can find out cheaply and choose not to. A trial hole at the threshold says whether the infill has a foundation or needs one. A core says the slab's thickness, what is beneath it and whether a membrane exists — and a taped sheet of polythene left overnight on swept slab says for nothing whether the floor is passing moisture, which is the principle the in-situ probe method of ASTM F2170 and the surface hygrometer method of BS 8203 apply properly when a moisture-sensitive finish is going down. A hose run onto the drive for ten minutes says where the surface water goes once no door is catching it.

Price the answers, not the fear. A conversion where all three came back well is a low-variance job and does not deserve a large percentage. One where the slab is bare, the threshold is a toe and the drive falls toward the house is a different project wearing the same name, and those belong in the estimate as identified provisional items with a stated basis rather than buried in a percentage somebody will argue about later. The contingency then covers what is genuinely unknown — plaster behind the garage wall, the routing of a soil pipe, whatever the previous owner did — instead of covering three questions you declined to ask.

One line that is not money. Breaking out a threshold and grinding a slab in an attached garage generates respirable crystalline silica, in a room with a door into the house and usually with the family still living in it. On-tool extraction or water suppression, the door to the dwelling sealed rather than closed, and the standards to work to are OSHA 29 CFR 1926.1153 in the United States and the Control of Substances Hazardous to Health Regulations with the HSE's construction dust guidance in Great Britain.