MODULE 9: Suspended floor insulation


The course exam

If you’ve completed all the modules then take the following exam. We’ll email you the results and if you achieve 100%, you will be eligible for a certificate of competence.

Need floor insulation?

If you’re working on a building project and need help specifying your materials, visit Back to Earth.

9.1 Overview

Suspended floor insulation – Basic principles and build-up and risks. Understanding of moisture flow and requirement for airtightness and vapour control.

This is a common problem in retrofit projects as it can be tricky to get anything underneath the floor structure to hold up your insulation. Installing insulation without any support is risky as over time and with the vibration that walking over the floor causes, the insulation can slowly fall out. Equally, taking all your floor boards up to insulate the structure may seem extreme but does give the best result.

From our experience the simplest way to insulate suspended floor structures is to remove your floor boards and suspend a flexible type of insulation between the joists, such as our UdiFLEX or PavaFlex flexible wood fibre insulation. This is achieved by draping a simple breathable underlay material, such as UdiSTEAM Tex 155, to the required depth between the joists and then simply pressing the insulation in to the ‘cradle’ that this creates.

Rigid insulation boards, such as PIR or EPS, are very difficult to cut and install accurately in this scenario and are therefore unlikely to be effective. Additionally, due to their low vapour permeability they tend to focus any moisture flow on to or against the timber joists, increasing the risk of moisture damage in the event of a hole in the membrane or high moisture load at the joist ends. Mineral or glass wools are better but are not able to move or manage moisture and so still more of a risk to timber joists than natural fibre insulations.

9.2 Installing the breathable membrane from above

To calculate how much breather membrane you’ll need, you’ll need to know the dimensions of your floor, the depth of your joists and the number of them across the floor. Take the measurement of the room that runs perpendicular to the direction of the joists and then add the number of joists multiplied by twice their depth. For example, if you have a 5m wide room with 13 joists of 150mm in depth you’ll need to add 13 x 300mm or 3.9m to the width, making 8.9m. You can then multiply this total by the other dimension of the room to calculate the area of membrane required.

Start by stapling the breathable membrane to the top of the first joist. Let the membrane hang down the side of the joist and staple to the very bottom edge of the joist with 3-4 staples across the width of the roll. Bring the membrane across to the bottom of the adjacent joist and staple the membrane again to the very bottom edge of the joist and then up over the top. The membrane needs to have a little tension in it but not so much that it starts tearing. repeat this process across the floor structure. Joints in the membrane should be taped if the airflow underneath the structure is significant, such as a garage, walkway or well ventilated basement.

Once complete you can carefully instal your insulation batts, making sure that the void is fully filled, the insulation is at it’s full depth and that it is not compacted in any way. In addition to installing the breather membrane and insulation, it is very important to use a vapour control layer. This ensures that you do not get condensation in your floor structure and also keeps the floor airtight.

The joints in the VCL need to be well taped and it should also be well taped around the perimeter to the walls. This will ensure that the moisture content of the floor structure is correct and also ensures your floor is air tight.

Once the VCL is installed you can fix your floor boards down again. This should be done carefully to avoid any unnecessary punctures to the VCL. Alternatively you can apply a strip of Butyl tape, such as UdiSTEAM Butyl tape or PavaFix to the top of each joist which will seal up around nail/screw holes should you need to take up the floor boards again.

9.3 Installing the breathable membrane from below

This is the least pleasant way to retrofit insulation as it all has to be done with you standing in the cellar below or lying on you back in a small void underneath the floor. If you’re in a small void please remember that you will need to leave an exit open to get back out!!

When installing from underneath you need to install the VCL first and this has to be stapled to the underside of your floor boards and then come down, around and back up each joist. You will therefore need to quantify the amount of VCL you’ll use in the same way as was done with the breather membrane when working from above.

Once the VCL is installed all the joints should be well taped and the perimeter should be taped to the external walls too if possible. UdiSTEAM Butyl tape or Pavatape will be suitable for this purpose.

With the joints taped the UdiFLEX or PavaFlex can be firmly pressed in between the joists, ensuring a snug, friction fit so that the batts do not drop back out. This does produce dust so protective equipment should be worn such as goggles and a dust mask. The dust is not hazardous (as it is wood) but equally it should not inhaled in large quantities and will hurt your eyes if it gets in them.

With all of the insulation in place the breather membrane can be stapled to the underside of the floor. In this scenario it is best to fix 25mm battens under the insulation (running perpendicular to the joists) at 5-600mm centres as the staples holding the breather membrane will not be enough to hold in the insulation should it want to drop back out in the future.