Adding Underfloor Insulation to Existing and Older Properties.

Adding underfloor insulation to an older home with suspended timber floors is relatively easy to do. But as with so many things there is a right way and several wrong ways to do it.

Why do we have suspended timber floors?

In the past, floor timbers were often laid directly on top of the ground but this inevitably led to damp and rot in the timbers. To correct this suspended timber floors were introduced that raised the timbers above the soil and created a ventilated void below the floor construction. That way the timbers didn’t get wet from touching the ground and the ventilation (via the sub-floor vents) reduced the risk of condensation. But the downside is that they do tend to be draughty, and an uninsulated suspended timber floor can loose more heat than the walls or the loft area. Yet this is an area that is very often overlooked when it comes to upgrading the insulation in older properties.

Uninsulated Suspended Timber FloorUninsulated Suspended Timber Floor

Why Add Underfloor Insulation?

  • Because suspended timber floors are draughty! Most suspended timber floors comprise simply of floor boards laid on top of timber floor joists. These draughts are often reduced slightly by liners laid below timber or laminate floor finishes, but these will not stop the draughts completely as the edges are not typically taped to make them airtight.

  • Well installed underfloor insulation will mean your house will heat up quicker and loose heat slower - provided the underfloor insulation is correctly installed.

  • Correctly installed hygroscopic insulation in an older property’s suspended timber floor will absorb some of the water vapour within the property during periods of high humidity (reducing the risk of condensation and mould on interior surfaces such as walls and ceilings) and release this water vapour slowly when the humidity levels are low.

A Few Useful Terms to Understand

  • Hygroscopic Insulation - easily absorbs water vapour from the air during periods of high humidity and then releases it again slowly when humidity levels drop.

  • Hydrophobic Insulation - resistant to the absorption and release of water vapour.

  • Vapour Barrier - prevents water vapour passing through it (although no material is 100% impermeable to water vapour, vapour barriers are as close as you’ll get). 

  • Vapour Control Layer - designed to allow controlled amounts of the water vapour in your home to pass through. 

  • Jointing Tape - used to provide an airtight seal at the joints of the tape. You can get different types of tape for sealing joints to different materials.

  • Wind wash - is the movement of unconditioned air around or through buildings in such a way as to diminish or nullify the intended thermal performance of the home. In this instance it is where the ventilation in the sub-floor area draws heat out of loose unshielded insulation.

What Can Go Wrong With Underfloor Insulation?

Worst case scenario you trap moisture from spills or interstitial condensation up against the timbers within hydrophobic insulation and in turn risk rotting the floor joists.

You spend a lot of time and money installing underfloor insulation incorrectly only to have it do very little because it has gaps around it negating the insulation properties.

You suffer from wind wash and loose a lot of the insulation properties.

You install the wrong type of insulation that is incapable of absorbing and then releasing water vapour or you don’t install the correct barrier products and sealing tapes.

So if you’re going to go to the hassle and expense of lifting all your floor boards and adding underfloor insulation, then surely it makes sense to do it correctly first time! 

How do we Traditionally Add Insulation to Suspended Timber Floors?

The easiest way is to simply lift the floor boards and install the insulation between the floor joists (after you have fully checked them for damp and rot and carried out any necessary repairs). Then relay the floor boards and finish with your chosen floor finish. 

Sounds simple, right? Wrong!

As with any form of insulation in your home, it’ll only work effectively if you use the correct type of insulation, protect it from moisture, wind washing (where the air movement via the sub-floor vents draws warmth out of the exposed insulation) and by making sure the installation is as airtight as possible. 

Other Examples of Incorrect Ways to Insulate Suspended Timber Floors:

Some of the most common ways I see insulation installed below suspended timber floors in older properties may initially sound correct, but in reality aren’t that effective and can lead to other issues:

  • Rigid Insulation boards (closed cell foil backed insulation sheets) cut and laid between the floor joists - the theory behind this sounds plausible, however floor joists in older properties don’t always run straight and so cutting the boards to fit tightly between the floor joists is a major pain, and if they’re not a tight fit the insulation won’t be efficient. Plus they have a closed cell structure so they won’t absorb and release the water vapour that will be present within your home as this type of insulation is hydrophobic rather than hygroscopic. 

    You could potentially seal the edges with spray foam, but that will set rigidly and if there is any movement to the floor you’ll get cracks and therefore air gaps. The foil back to the insulation boards is a vapour barrier (prevents water vapour passing through), provided the joints are correctly taped. However to reduce the risk of condensation internally you’re better to use a hygroscopic insulation material which can absorb and release water vapour, so a total (or as close as you can get to total) vapour barrier isn’t ideal.

Mineral wool or Fibreglass wool - this could be simply packed between the floor joists and supported by chicken wire stapled to the bottom of the floor joists. Sounds good right? These types can be packed in between the edges of the joists avoiding the issue with gaps but their structure is quite loose and can be affected by wind wash. They’re also going to be open to the elements so any spilled liquids from above and water vapour (leading to interstitial condensation) can penetrate the insulation reducing it’s insulation properties significantly. Plus wind washing reduces the effectiveness of the insulation. 

Note: While mineral fibre and fibreglass wool are hydrophobic they will still allow moisture to pass through them. Water spills from above will pass through and will compress it (I speak from personal experience, despite what the suppliers may tell you).

The Correct Way to Add Underfloor Insulation.

To insulate the suspended timber floors in your existing home you need to consider three things:

  • Thermal performance.
  • Moisture control.
  • Air tightness.

So you need a system that will hold the heat well, allow water vapour (but not liquids) to be taken in when the humidity levels are high and released again once the humidity levels reduce and provide a barrier to make the installation air tight. If you can achieve this you’ll avoid draughts and wind washing and get an insulation installation that performs well.

  • The first step is to lift the floor boards to expose the floor joists and inspect and repair any defective joists etc. You could at this stage lay a vapour barrier on the clay surface below the floor (if you have clay/soil exposed) as this will reduce the risk of water vapour from the ground. Next lay a breathable airtightness membrane over the joists forming wells for the insulation to be installed between the joists. This will stop wind washing and allow any residual water vapour to escape.
airtightness membraneAirtightness Membrane for Suspended Timber Floor Insulation
  • Then install a high density (to avoid settlement over time) natural hygroscopic insulation product such as batts of sheep’s wool (but it will need to be treated to prevent moths and other insects eating it), wood fibre or Jute insulation (made from recycled cocoa and coffee bean bags) as these will regulate the water vapour by absorbing it when necessary and releasing is again slowly once humidity levels reduce.
Hygroscopic Insulation for Suspended Timber FloorHygroscopic Insulation for Suspended Timber Floor
  • Finally you will need to lay a vapour control layer on top of the underfloor insulation (this slows the passage of water vapour through to the insulation but does not stop it like a vapour barrier would). All the time ensuring the correct overlaps and the correct taping of all edges Including the joint between the floor and the walls. Then you can relay your flooring.
Correctly Insulated Suspended Timber FloorCorrectly Insulated Suspended Timber Floor
How to Install Suspended Timber Floor InsulationCorrectly Installed Suspended Timber Floor Insulation

As you can see from above it isn't really much more difficult to install underfloor insulation correctly. Provided you take your time and plan the installation of the breathable airtightness layer before starting (and ensure you leave a flap at the edges so that it can be taped to the walls). Then lay the insulation so that there re no gaps and again when laying the vapour control layer you'll need to turn it up the wall at the edges and then taped. 

If you can do this you'll have a well insulated, breathable and airtight floor.

Next step is to do the same to your loft insulation and walls.

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