Retired air tightness technician Trevor Clark gives his advice on heating your home effectively using barriers.
Insulation comes in many forms and is a very large subject in itself, so the following covers the basic types and, more importantly, where or where not to use them and what to be aware of when installing each type.
Cellulous fibre, granules and bead insulation are all classed as ‘loose fill’ as they come in bags and can be poured. It’s best used where no air flow is present which will move the small particles from their intended location.
Loose fill is basically held within a defined ‘container’. Any air flow must be stopped to prevent ‘wind wash’ or the removal of the particles.
Can be made of fibreglass or rockwool, manufactured as thin strands and rolled and reduced in size to take up less space during transport.
It’s unrolled between joists where it should return to its true thickness, and then at 90 degrees across first layer to build thickness.
Beware this is not self supporting and using in too many layers results in compression of the lower layer, which can negate the performance, occasionally resulting in changing properties from insulation to conduction of energy.
Use only on flat or gently sloping surfaces which support the fibres.
Do not use between vertical studs in timber framed walls (as used to be the case) because this creates problems later when the insulation slips down the wall due to building vibrations to create a void where convective loops speed up the energy transfer through the wall structure.
Semi-rigid and rigid batts is the same material as rolled fibre but treated to maintain shape better. It’s a bit like having lacquer sprayed into your hair to hold it in place.
Cut it into slabs of various sizes and 40cm or 60cm widths in various thicknesses.
Push it into walls and ceilings where the friction between timber studs and batt will keep the insulation in place. It’s not easily crushed and as such a greater thickness can be layered.
Take care not to have any gaps between batts and surrounding surfaces. Laid in brick pattern and when layering stagger joints vertically through thickness.
Be sure to get the insulation into the corners where timber walls and trusses are used to form the void for insulation to be installed.
These three all require a separate air barrier material to prevent air flow on the warm side. Although some may come with a paper covering, this is not considered to be an effective air barrier!
There are many different types of plastic used to manufacturer these boards, some with an ‘aluminium foil’ covering on both faces.
It’s available in various sized sheets and thicknesses. Some have a tongue and groove and these are best used to improve air tightness. It’s easier to use T&G boards but they should still have all joints taped.
Foil faced boards are considered to be an air and vapour barrier, but to become an air tight layer must have all joints taped with the manufacturers approved adhesive foil tape.
Any gaps between boards and/or timber studs should be filled with foam, with excess trimmed and then the joint sealed over with caulk to prevent air flow.
Foil-covered bubble roll
These rolls are a complete waste of money! They have very little insulation value. The reflective surface should be metal if it is to slow radiation and then have a minimum air gap of 30mm to be effective. There are other cheaper alternatives available if you just want a reflective surface, cooking foil has a good emissivity factor which is superior to these bubble insulation rolls. A 12mm thick polystyrene board has more insulation value as these rolls and is probably cheaper.
Claims made for this material in the past have long been disproved, and even when used as an air/radon barrier are not as effective as current materials that are available today.