I get asked how to install insulation a lot, so here I'll run through the correct way to install loft insulation to help you reduce the risk of interstitial condensation, mould and allow your insulation material to perform to its best ability.
As a residential surveyor I look at a lot of houses and I could count on one hand (and still have a few fingers to spare) the number of existing and older houses that, in my opinion, have had their loft insulation installed the best possible way.
And, if you're a builder or installer, before you start screaming at your computer, this is my opinion and it will probably differ from yours. There are no definitive "rules" that state how to install your loft insulation, but the "traditional" way will likely lead to issues.
If you're renovating, Building control will talk about meeting the minimum energy efficiency values set out in the Approved Documents. But they won’t tell you how to install insulation in your loft and they won’t go into the benefits of vapour barriers or air tightness issues in existing and older properties.
Hydrophobic Insulation - doesn’t absorb and release water vapour readily e.g man made insulation such as fibreglass wool or mineral fibre etc.
Hygroscopic Insulation - DOES absorb and release water vapour readily. So it will take in water vapour during periods of high humidity and release it during periods of low humidity. Examples include; sheep’s wool (best to choose sheep wool that has been treated to prevent moths and other insects eating it), wood fibre or Jute insulation (made from recycled cocoa and coffee bean bags)
Wind wash - where the air moving over the top of the insulation from your roof ventilation (and if your loft insulation has been increased Building Control says it should also be vented) reduces the thermal efficiency of the insulation by drawing heat out of it.
Traditionally the majority of older houses will have very little if any insulation in the loft area. But many people have either had someone come in to insulate their loft or they’ve had a go at it themselves.
This typically involves clearing the loft of any debris or stored items and rolling out 100mm thick layers of fibreglass or mineral fibre insulation between the joists (there are other types of insulation that can be used but the majority I see are either mineral fibre or fibreglass). Then a second layer of 200mm thick insulation is rolled out perpendicular to the first layer to minimise the risk of gaps between rolls.
So far so good? I’m guessing this all sounds familiar? If not have a look at the images below.
Is this the right way to install insulation?
The short answer is no! While this method does include some of the correct steps it also misses out a few very important steps.
By installing or upgrading the loft insulation in your home you’re trying to make your home retain it’s heat for as long as possible. So it makes sense to try and protect that insulation from wind wash and from water vapour from within your home. If you can do that, your insulation will perform better and you’ll reduce the risk of condensation issues. Think of it like putting a breathable waterproof coat over your wool sweater before going out for a walk in the rain. Sure without the waterproof coat the wool sweater would absorb a lot of moisture before you start to feel wet but if you can stop the moisture getting into the wool in the first place you can stay outside indefinitely.
Clear your loft area - if you have any insulation in your loft I’d suggest removing it, then clean the loft area, a vacuum will lift any small pieces of insulation and dust that could cause sealing tapes (mentioned in a later step) from sticking.
If you have eaves vents you will need to ensure that you do NOT block these and you may need to either reduce the thickness of the insulation as it gets to the eaves or preferably install eaves ventilation spacers which will maintain the ventilation space (see the last image on this page). You can buy these designed for refurbishment projects that fit between the rafters.
Any recessed lights going through the ceiling must be covered and sealed within airtight and fireproof enclosures such as fire hoods (if not the light fitting could overheat and cause a fire and water vapour can also get through the light fitting). Traditionally electricians just pull the insulation back from around the recessed fittings leaving sections of the loft uninsulated.
Services such as wires and pipes that go through the ceiling will also need to be sealed airtight.
Lay a vapour barrier such as 500 gauge polyethylene as this will stop water vapour getting into the insulation. If you can lay this in one piece so much the better but if you have to joint it you’ll have to use a good quality jointing tape (not gaffer tape or parcel tape). Leave enough barrier at the edges so that it can be sealed to any vapour barrier that may be in the walls, or if no vapour barrier has been installed in the walls then it will need to be sealed to the walls within the roof void and the timber wall plate at the eaves. This will provide a proper airtight seal. When laying the vapour barrier it should go right down into the gaps between the rafters to form pockets into which the first layer of insulation is laid (see the final image on this p[age) and the insulation should be tight to the joists with no gaps.
As above but instead of laying a vapour barrier you would lay a vapour control layer (VCL) which lets a controlled amount of water vapour through into the hygroscopic insulation. This means that in periods of high humidity the insulation can absorb some of the water vapour and then when humidity levels are low the insulation releases it. This should reduce the risk of condensation and mould within your home. As above this layer should be sealed and taped to any vapour control layer in the walls or alternatively to the walls and wall plate within the loft area.
Lay your insulation (high density batts are preferable as they resist sagging over time). Lay 100mm between the joists (the horizontal timbers on the floor of your loft area) and then lay another layer of 200mm thickness (you could add 300mm if you prefer) perpendicular to the first layer as this will reduce the risk of gaps in the insulation. If you have a water tank that sits on or close to the joists then you should NOT insulate below the water tank as this will help reduce the risk of freezing. You should also insulate the sides and top of the water tank and install a good fitting lid. If your water tank is elevated above the joists then you should insulate below the water tank. Pipework in the loft area will also need to be well insulated.
If you are going to install a storage area or a walkway to access a water tank etc. you should install additional joists or stilts to raise the finished floor level so when you floor these sections they only slightly compress the insulation by 5-10mm and form a tight seal with no gaps.
Lay the air tightness layer on the top of the insulation and seal all edges and joints.
Stand back and admire a job well done.
So now that you know how to install insulation in your loft there is nothing stopping you from making your home warmer, cheaper to heat and a more pleasant place to be. The same principle can now be applied to your suspended timber floor and here is an article on adding cavity wall insulation and the issues connected with such insulation.