How to Insulate a Timber Floor
As the days get shorter and the temperatures drop, we receive an increasing number of calls about trying to warm up floors, particularly in older properties. Today, we’re going to discuss what’s called a ‘suspended timber floor’ in pre-war properties. This isn’t meant as a how-to guide or a specification, but rather as a basic understanding of why your floor might feel cold, how it could be improved, and the things to be aware of when considering carrying out underfloor insulation. If you’d like to discuss the most appropriate method for improving the comfort of your suspended timber floors, please visit the Contact Us page and get in touch.
Most pre-war houses will have a ventilated subfloor area (what our American cousins call a ‘crawl space’, where the bare dirt is exposed, there’s an air gap, and then the timber floor joists and floorboards above. This volume is ventilated by air bricks on multiple sides of the building (typically opposite sides). Moisture from the ground is removed by the ventilation from the air bricks, keeping the humidity relatively stable below the floor, but also introduces cold air into the void, taking away heat from the room above both by convection (air movement between the room and the subfloor void) and conduction (heat escaping through the timber floor). Interrupting the flow of air through the subfloor can lead to an increase in humidity and potentially damp problems, so sealing off the air bricks is a bad idea; if your air bricks are sealed, open them up! A ventilated house is a dry house. Suppose you’re going to install insulation between and below your floor joists. In that case, it will likely be necessary to install periscopic extensions to your air bricks to route the airflow down and under the insulation layer safely.
If you’re lucky enough to have a deep sub-floor void, it may be possible to get under the floor and install insulation from below. There are H&S factors to bear in mind that are outside the scope of this article, as it is a confined space. If space is limited, work may have to be carried out from above by lifting the floorboards; as a disruptive process, this is best carried out either when the house is empty, or on a room-by-room basis. Some processes use robots to apply insulation foam to the underside of the floor, and this is gaining some popularity. However, at the time of writing, there are an increasing number of mortgage lenders who are becoming wary of spray foam insulation in lofts and are refusing to lend. This is not to say that all spray foam installations are poorly done and can lead to problems; quite the opposite. However, the mortgage industry is very cautious and tends to tar a product category with the same brush in order to avoid risk. Therefore, we would recommend discussing the matter with your mortgage lender before considering this robot-applied foam option.
There are many different insulation types on the market, each with varying physical properties to consider. In older homes we generally want to use vapour-permeable materials, sometimes referred to as ‘breathable’. These include natural wool, wood fibre, hemp and mineral wool. There are many videos on the internet showing foil-faced PIR boards being used, but we do not feel these are suitable for older homes as they are vapour-impermeable and shrink slightly over time as they off-gas, allowing heat to escape around the edges of the boards, rending the insulation almost useless. There are foam tapes that can be applied to the perimeter of PIR boards to take up any movement (seasonal or shrinkage), but these add up quickly, particularly with narrow joist bays, and do not overcome the issue of vapour-impermeability. Insulation rolls and batts however tend to have more ‘squish’, allowing them to be cut very slightly oversized to get a snug fit that can take up any seasonal movement of the timber joists, while still allowing moisture vapour to permeate. Some of these insulations also have the ability to absorb a small amount of moisture and then dry out during warmer weather without any negative effects.
The Building Regulations Approved Document L (2021 edition incorporating 2023 amendments) sets out requirements for the maximum allowable heat loss through parts of a building, be it a new building, an extension to an existing building, or an improvement to an existing building. For example, if the existing floor has a U-value greater than 0.70 W/(m²K), and most uninsulated suspended timber floors will, the improved U-value must be less than 0.25 W/(m²K). If you were to aim for an EnerPHit retrofit, the U-value would be less than 0.15 W/(m²K). Depending on your particular home, your floor joists may not be deep enough to house the required amount of insulation, in which case it may be necessary to extend that depth with gussets of OSB/3 or structural plywood, attached to the side of the existing joist.
So far, we’ve discussed reducing heat losses via conduction by insulating, but we’ve not addressed what is arguably the greater source of heat loss and discomfort; convection and draughts. The aim is to separate the outside air in the subfloor void from the warm, conditioned air indoors, which you’ve paid to heat or cool. This is generally done with membranes and tapes, but there are sheet products that can also be used to the same effect The membranes are stretched from one side of the room/house to the other, and sealed to the walls to create an airtight layer, preventing the warm and cold air from mixing. This removes the draught and the source of much discomfort. The floorboards are generally then laid back over the membrane. In many cases, another membrane might be installed first, to act as a sort of hammock to support the insulation between the joists and also help reduce air circulation within the insulation layer (called thermal bypass), as this can reduce the effectiveness of the insulation, much like the shrinking PIR boards mentioned earlier.
One of the big sayings in Building Science is “Built tight, ventilate right”. Those draughts between the floorboards did serve the purpose of removing excess humidity and stale air from inside the house, along with other sources of ventilation such as open windows, chimney flues, trickle vents and extractor fans. The extent to which ventilation needs to be improved after reducing unintentional draughts will vary between houses depending on the existing ventilation methods. However, we generally recommend improving the airtightness of a building as much as possible, and installing a ventilation system, that exhausts stale air, but also brings in fresh, filtered air, recovering as much of the heat as possible from the exhausted air to temper the incoming fresh air. These systems are called Mechanical Ventilation Heat Recovery (MVHR) units, and they can be as small as a bathroom wall fan, or as large as a washing machine with ducts that serve every room in the house. With heat recovery of >85%, these can make a huge dent in your heating bill, as well as reducing dust and allergens brought into the home.
If you’d like to discuss the most appropriate method for improving the comfort of your suspended timber floors, please visit the Contact Us page and get in touch.
