Unless you have dormer windows on both sides of your roof that extend to the full length of the room, some part of your loft room will have a sloping ceiling that needs insulating. And because headroom space is at a ,
premium on sloping ceilings, the insulation usually has to be fitted between the rafters as opposed to underneath them. This is where the problems start. You can’t simply stuff the gap full of glass-fibre insulation without causing it to sweat and condense water vapour. Or, alternatively, you can ask a spray foam insulation, Dublin provider.
To make the structure breathable and allow water vapour to be cleared, you need to allow an air gap of at least 50 mm to exist and cross-ventilate it over the top of the insulation. Cross-ventilating it means letting the air in at one end and out at the other. There are proprietary vents that can be built in to your roof (if they aren’t there already) to do this and further advice on those can be found under Roof ventilation.
Because of the required air gap and the given depth of the rafters that exist, you are already limited in your choice of insulation material. You will have to meet Building Regulations standards (and they get more stringent all the time), and you will want to make your home as energy-efficient as possible.
I say your home and not just your loft room, because a third of the heat from your whole house goes out through the roof and good attic insulation up here will make a difference to your home.
Invariably rafters are only 100 mm deep (standard 50 mm x 100 mm), and thus you have only 50 mm left for insulation thickness – not enough for some insulants to meet the regulations. You may need to deepen the space by fixing timber battens to the underside of them, but these really shouldn’t be deeper than 50 mm themselves.
If you can’t afford to lose the headroom from counter-battening, rigid slab insulants can be fitted in two layers, one between the rafters and one thinner layer beneath them, pinned in position.The plasterboard then has to be fixed through it into the rafters with longer than normal plasterboard screws. There is a limit to this second layer (and it is wise to keep it as thin as possible but still meeting the standards) that has more to do with the length of screws available than anything else.
Because of these logistical problems, insulation to sloping roofs has always received more relaxation to the Building Regulations standards than insulation laid flat within roof voids. Even so, the standards are high and the Government has remained committed to reducing carbon dioxide emissions from our buildings.
Because of these increasing standards, some of the old-type insulation products are not so useable these days. Glass fibre, for example, relied on depth with plenty of air pockets trapped between the fibres to
insulate, the higher the standards g0 the deeper the insulation needed to meet them. (I didn’t like the stuff anyway, those airborne fibres can be breathed in without a mask, and they irritated my skin like hell.)
Polyurethane foam boards aren’t much better when you’re cutting them – the exposed edges are fibrous and quite sharp, and need treating with a lot of respect. If these boards are being fitted between rafters, a lot of cutting may be needed to ensure a snug fit. With a thermal conductivity value that is much lower than mineral or glass-fibre products, they can be used in much thinner thicknesses to the same result. The measure of any insulation product is its K value (thermal conductivity) which spells out how good it is at stopping heat from passing through it.The lower the K value, the better it is. The numbers are expressions of watts per metre Kelvin, since they are measures of heat over a square metre in a given temperature state and dryness, which means that they are very small numbers basically and not easy to read at a glance.
Rigid polyurethane and phenolic foam (closed-cell) insulation tends to have a K value of between 0.019 and 0.025, much better than that of glass or mineral fibre (open-cell) insulation at between 0.037 and 0.040. Expanded polystyrene (the same product used in packaging) comes in at about 0.037.
These figures are small and the differences between them appear insignificant, but they couldn’t have more of an effect on things than they do. To realise this you have to
understand that you will get the same amount of insulation from 200 mm of glass fibre as 75 mm of polyurethane foam or 25 mm of multi-layer reflective sheeting. These thicknesses are critical in loft conversions – they make the difference between hitting your head and not hitting your head on the ceiling, and can even decide whether or not you go ahead with the project.