Many older houses with suspended timber floors have big draughts and inadequate floor insulation; this can cause cold feet as well as higher energy bills. Installing underfloor insulation can address both these problems and give a payback on your investment in 5 years or less depending on whether you do it as a DIY job or use a professional. This article discusses the options for insulating suspended timber floors and the author’s experience of insulating the floors of a 1930’s house .
Why insulate a suspended timber floor?
Consider the two main sources of heat loss in a home:
- heat loss from the fabric of the building (walls, ceilings, floors. closed windows and doors)
- heat loss from draughts through things like gaps in floors, windows, doors, open chimneys and loft hatches.
In a very draughty home, heat loss through draughts can actually exceed the heat loss from the fabric of the building.
Insulating a suspended timber floor can eliminate floor draughts and reduce heat loss through the fabric of the floor at the same time. From a thermal comfort perspective, it does appear that feet are particularly sensitive to colder temperatures, so addressing floor draughts can help improve comfort levels.
Why are some suspended floors cold?
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.
Properties built after 1920 generally have air bricks built into the external walls to get a good cross flow of air to allow the sub-floor area to be ventilated. Properties built pre-1920 often didn’t have air bricks for ventilation or a DPC (Damp Proof Course) so damp may be a problem that needs sorting before you think about floor insulation.
Suspended floors in older houses often have some damage in places; a result of raising floor boards when new central heating pipes were run or the house was rewired.
Some floorboards can warp over the years creating gaps. The cold air from beneath the suspended timber floor then comes through the damaged floorboards, making the house and your feet feel cold.
Even good condition suspended timber floors can suffer from cold air coming through gaps in the skirting boards. You can see the tell tale staining of the carpet round the skirting boards as air from under the suspended floor continuously flows through the carpet.
Alternatives to underfloor insulation
Laying a laminate floor in a room can cut out many of the draughts but it will not dramatically reduce heat conduction and it can be a real shame to cover up some splendid wooden floors found in older properties.
Carpets can also make a difference but they are not air tight and they will not fix draughts through the skirting boards. Air can seep through the carpet, even if it has an underlay, bringing with it dirt and dust which can discolour the carpet if it is a light colour and depositing dust on the skirting board. If this is a problem then 3mm thick hardwood sheets can be fixed down under the underlay and carpet.
Gaps between floorboards can be sealed with varnish or sealant and the gap between the skirting boards and the floorboards can be filled with self adhesive foam strips.
We all know that heat rises, so at the same time it is worth thinking about improving loft insulation and also eliminating any draughts through the ceilings. You can get big benefits from making sure that the loft hatch is not leaking hot air into the cold roof space. The combination of cold air through the floors and hot air out of gaps in a loft hatch can lose heat very quickly. If you can feel a strong draught as you go up the stairs then there is probably some opportunity to reduce heat loss upstairs.
Options for insulating a suspended timber floor
Assuming that you want to sort the issue permanently and reduce your heating bills there are two main methods of applying insulation:
- remove the existing floorboards, insulate and relay the floorboards
- insulate from under the floor using the crawl space underneath.
Removing existing floorboards can be very hard work and risks damaging the boards themselves. If the floorboards are tongue and groove then they will have to be removed in order and if they are well nailed down this can be a specialist task. Alternatively you can saw between each floorboard to cut through the tongue and groove before levering them up. Where the boards run the width of the room you may have to either cut the boards in half or remove the skirting to allow the boards to be lifted. If the existing floor covering is attractive then you may not wish to risk damaging it. Where lino or luxury vinyl like Amtico is installed then lifting the floor is not normally an option. See Insulating A Suspended Timber Floor From Above Floor. In my opinion you are best insulating by removing the floorboards when:
- the room is going to be refurbished anyway and
- the existing floorboards can easily be removed without damaging
- or the existing floorboards are in poor condition and they are going to be replaced
You are best insulating from under the floor when:
- you have a basement or cellar which allows access to the underfloor area
- or there is a sufficient gap under the joists to use the crawl space to actually crawl. In practice that means there are about 5 or 6 bricks below the bottom of the joists (38-45cm or 15″-18″ of space).
- an access hole is already in existence or can easily be created
- when the crawl space allows access to the majority of the house
- the floor covering is too attractive or too difficult to remove without damage
I have tried both methods and on balance would go for the underfloor insulation option in most cases as it creates far less mess and disruption although it does require you to learn some potholing skills. See Insulating A Suspended Timber Floor From Underneath The Floor. If a house is being refurbished or is empty and the floors are in poor condition then ripping out the existing floors and insulating from above would be easier. Where the crawl space is limited on non existence then you may be forced to remove the floorboards or compromise and go for a less ideal option.
Update August 2020: It is also worth checking out a robotic option which has been developed recently. Whilst not cheap, likewise you avoid the unpleasant working conditions found underfloor. See Using Robotics for Underfloor Insulation of Suspended Timber Floors.
Materials For Suspended Timber Floor Insulation
There are three main options:
- Celotex / Kingspan type PIR thermal insulation board. This comes in various thickness (50mm and 100mm are the most common) and in a variety of sizes (2.4 x 1.2m being the most common).
- Mineral wool insulation rolls or batts (mineral wool in slab form) similar to loft insulation
- Natural materials such as sheepswool or hemp
Each of the materials has their advantages. Selecting the right material is a balance between:
- the thickness of material required to deliver the desired U-value
- ease of handling
- ability to cope with damp
An U-value is a measure of the thermal insulation of a material with lower U-values being more effective insulators. For a material which is a worse insulator you need a greater thickness of the material to achieve the same U-value as a better insulator.
The PIR thermal insulation board is the best insulator but it can be a bit more expensive. The boards are quite big so to transport them you need a van or lorry or better still get them delivered. They need to be cut to size before installing between the joists. Measuring and cutting them is very easy with a small handsaw outside but it becomes a little more difficult when you are on your back under the suspended floor with a face mask on. The boards are faced both sides with aluminium-foil to form a vapour control barrier to stop damp. Battens fixed under the board hold it in place or a plastic friction fit support can be used. The method of supporting is the same whether done from above the floor or underneath.
Batt or Quilt Insulation
Mineral wool insulation comes in roll or batts (slab form). It is cheap but not as good an insulator as PIR thermal insulation board and is a pain to cut to the right size. If you can get the right width to match the joist spacing and the joists are spaced evenly then it can work well.
It the floorboards have been removed then the insulation material is held in place with netting underneath (or a vapour permeable membrane) and then a vapour control layer (VCL) is applied above the joists.
If the work is being done from underneath then the insulation can be held in place with either tongue and groove boards or with a vapour permeable membrane. I have seen examples where netting is used from underneath but this is not considered so effective at eliminating draughts.
Natural materials such as wool are more pleasant to handle than mineral wool and are considered superior for older buildings by those that know (English Heritage) as they allow the floor to “breathe” and buffer the moisture reducing the risk of damp. Where you are fighting lack of underfloor ventilation and the lack of a DPC then this could be the best option. It is installed in a similar way to mineral wool insulation.
Insulating The Floor Of A 1930’s House
After considering the options, I chose PIR thermal insulation board as it was the best insulation that would fit between the 4″ (100mm) thick joists. The first room I tackled was the lounge and decided to take up the floorboards to install it. See See Insulating A Suspended Timber Floor From Above Floor.
We were so pleased with the impact the insulation had on the room I decided to insulate most of the rest of the house (the area accessible from the crawl space). As we had the original oak floorboards in the hall and Amtico vinyl in the kitchen and utility area there was no real option of removing the floorboards. Also I had the opportunity when doing the lounge to explore the crawl space under the rest of the house and realised it was possible to do from underneath. I had built an access panel in the lounge which allowed me to access the crawl space very easily. See Insulating A Suspended Timber Floor From Underneath The Floor.
Historic England. Energy Efficiency and Historic Buildings – Insulating Suspended Timber Floors. Available at: https://historicengland.org.uk/images-books/publications/eehb-insulation-suspended-timber-floors/heag086-suspended-timber-floors/ Accessed [7 June 2020].