Underfloor Thermal Insulation Of Suspended Timber Floors
Why is my home cold?
Many older houses with suspended timber floors have inadequate floor insulation and suffer from draughts which cause higher energy bills as well as cold feet. Installing underfloor insulation can address both these problems and give a payback on your investment.
A cold draughty room can feel very similar to one in an ice hotel
In a home there are two main sources of heat loss:
- heat loss through the building structure like walls, ceilings, floors, windows and doors surfaces
- heat loss from draughts through gaps in floors, windows, doors, open chimneys and loft hatches.
Why is my floor so cold?
If your floor is cold you may well have a suspended floor. The space under a suspended floor is designed to be well ventilated to avoid the build up of damp. So it is no surprise that the air under the floor is close to the outside air temperature; on the coldest days this air could be below freezing.
If you're thinking of replacing the old floor covering with a new one, especially on the ground level now is the best time to also consider thermal insulation of the subfloor. And this is more important if you want to install a valuable hardwood floor.
Schematic - using air brick to vents the sub floor
Removing the original pine boards
Is underfloor thermal insulation the solution?
Underfloor thermal insulation of suspended timber floors involves removing the existing pine floorboards or any other type of boarding. This to eliminate the empty spaces between the joists that allow the cold draughts to reach the subfloor.
A galvanised mesh bed is fixed between the joists to properly hold in place the insulation. This is an important stage of the work since it has to be done accurately in order to last for decades and this is the only time when there will be free access into the subfloor. If the mesh bed fails it will cause not only ruin the thermal insulation but also will block the air circulation under the floor causing a build up of damp which in turn will cause the joists to rot. For this reason it is highly recommended to entrust this type of work to skilled and responsible tradesmen.
Applying thermal insulation
Complete thermal insulation
Is the floor really insulated with mineral wool?
Once the galvanised mesh bed is properly fixed the insulating material can be fitted. We highly recommend mineral wool or even better sheep wool. Sheep wool is a natural insulator and particularly helpful for those who suffer with allergies. Nowadays the PIR boards (i.e. Celotex boards) are the preferred solution but they have a big drawback when used in old homes… they do not allow vapour diffusion which may cause damp buildup through the years.
After the thermal insulation materials have been well inserted in all possible spaces 18mm hardwood plywood is thoroughly screwed over the joists. The alignment of the new wooden floor will determine the orientation of the plywood boards.
New plywood subfloor
New wide engineered flooring oak boards
What if there’s a surprise under my floor?
When working with old suspended timber subfloors there is always a risk of finding many faults and errors from previous building interventions. We have discovered leaking pipes, rotten joists, missing noggins, loose electrical cables and much more. For this reason it is good to anticipate that other trades may need to be called to rectify these issues, especially plumbing and electrics as that will be the best time to resolve them.
Knowledge, care and patience are needed while working with an opened subfloor to avoid causing new problems like punctured pipes or cutting electrical cables. Such disasters will affect the integrity of your property as well pose a safety risk to you and your tradespeople. But when done well by experienced tradesmen underfloor thermal insulation will go a long way to warm your home, keep your feet cosy and reduce your heating bills.