Underfloor is the element people skip, and the one with the most specific requirements. Depth is rarely the problem, because a joist bay is deep. The two things that decide the result are holding the insulation up against gravity, and dealing with a sub-floor that is damp by nature. Get either wrong and you have paid for insulation that ends up on the ground.
In short
- A floor batt is the only one working against gravity. It needs saddles or straps, not friction alone.
- Underfloor batts are cut narrower than wall batts, 415 and 565 mm against 430 and 580 mm.
- What people feel as a cold floor is usually cold air moving up, not heat conducting down.
- With reflective foam, enclosing the sub-floor is worth more than changing product.
- A damp sub-floor is an airflow problem, and it will spoil whatever you install.
Ceilings get done because they are easy and the return is obvious. Walls get done because they are open during the build. Floors get left, and then people spend a winter wondering why the house still feels cold at ankle height.
Why bother with the floor
Underfloor insulation does three things, and only one of them is the one people expect.
- It cuts heat loss down through the floor. The obvious one, and on a raised floor over an open sub-floor it is real.
- It limits cold air moving up from the sub-floor. This is what most people actually feel as a cold floor, and it is the reason the room feels better afterwards even when the bill barely moves.
- It gives some acoustic benefit through the floor structure, because the products made for this are denser than a wall batt.
The second point is worth sitting with. A sub-floor is ventilated to outside air by design, so on a cold morning the space under the floorboards is close to outside temperature and moving. Insulation between the joists puts a barrier between that and your feet.
The one that works against gravity
This is the practical difference between underfloor and every other element, and it is the one that decides whether the job lasts.
A wall batt is held by the studs on both sides and the lining in front. A ceiling batt sits on the plaster. A floor batt has nothing under it. Friction fit gets it into position on the day. Over years, with vibration from the floor above and air moving beneath, unsupported batts sag and eventually drop, and nobody notices because it is out of sight.
A sagging batt is not delivering its R-value. So the fixings are not an accessory:
- Saddles push up into the joist bay from below and hold the batt against the floor. Ours are 70 x 45.2 mm, 250 to a pack.
- Straps run across the underside of the joists. Ours are 190 x 20 mm, also 250 to a pack.
Which you use depends on joist spacing and access, and both fix with standard fasteners. Order them with the batts rather than discovering the need halfway under the house.
Why the batts are a different size
Underfloor batts are cut narrower than wall and ceiling batts, and this catches people out on the order.
| Product | Widths | Pack |
|---|---|---|
| R2.5 Bradford Optimo Underfloor Batts, 90 mm | 415 mm and 565 mm | 8 per pack, 3.85 m² and 5.24 m² |
| Wall and ceiling batts, for comparison | 430 mm and 580 mm | Varies by product |
Floor joists are set out at the same nominal centres as wall studs and ceiling joists. The batt is narrower because it has to sit up between them and stay there, so the fit is tuned differently. Ordering wall batts for a floor is a common and annoying mistake, and the fixings do not come in that box either.
Batt or reflective foam
Two families do this job and they are not interchangeable.
| Dense glasswool batt | Reflective closed cell foam | |
|---|---|---|
| How it works | Bulk, trapping still air in the joist bay | Reflects radiant heat, needs an air gap |
| Typical figure | R2.5 at 90 mm in our Optimo range | System figures rather than a material R-value |
| Fixing | Saddles or straps | Fixed to the joists, maximum span 2.4 m without support mesh |
| Acoustic benefit | Yes, from the density | Minimal |
| Handling | Glasswool, so cover up. Polyester versions exist for this reason | Fibre free, non-irritant, but electrically conductive |
Polyester deserves a mention here because underfloor work is where not itching earns its money. You are on your back in a confined space with the material above your face. Our Martini polyester floor blanket is made for this: a 10 m roll, 430 mm wide and 140 mm thick, that runs out along a joist bay instead of being cut batt by batt.
Enclosing the sub-floor
If you are using a reflective product, this is the single most valuable thing on this page.
Kingspan quotes AIR-CELL Permifloor in an unenclosed sub-floor at RT 0.8 for heat flowing in and RT 1.6 flowing out. Enclose the sub-floor and the same product gives RT 1.6 in and RT 3.0 out.
| AIR-CELL Permifloor, same roll | Heat flowing in | Heat flowing out |
|---|---|---|
| Unenclosed sub-floor | RT 0.8 | RT 1.6 |
| Enclosed sub-floor | RT 1.6 | RT 3.0 |
Nearly double, from a decision about the sub-floor rather than a change of product. If the sub-floor can be enclosed, that decision is worth more than any upgrade you can buy.
Two cautions on that product specifically. It is Class 3 vapour permeable but it is not a water barrier, so do not rely on it for that. And it is electrically conductive, so keep it clear of wiring.
Damp, mould and sub-floor ventilation
Insulating a damp sub-floor without dealing with the damp is money spent twice.
A closed sub-floor holds moist air against the underside of the floor. That is where mould, musty smells and rot in the bearers start, and it is also where the insulation you just installed will sit. Wet insulation loses its performance and stays wet, because a sub-floor has almost no drying energy.
Moving that air out is the fix, and it is usually cheaper and more permanent than treating the symptoms upstairs. Our EcoFan V2 is a 250 mm sub-floor fan with a 24 V DC EC motor, variable speed, moving up to 734 m³/hr, with Air iQ sensing heat and humidity so it runs when the sub-floor is actually damp rather than continuously.
Note why they are stocked in pairs: ventilation needs a path. Two fans placed to pull air across the sub-floor beats one fan pulling from a single point, which is the same principle as intake and exhaust on a roof space.
What the job is actually like
Worth being honest about, because it changes who should do it.
- Access decides everything. A generous sub-floor is a straightforward day. A 500 mm crawl space is slow, awkward work.
- You are working overhead, on your back. Everything you cut lands on you, which is the practical argument for polyester.
- Narrow roll formats exist for a reason. Permifloor 500 is supplied as two 500 mm x 30 m rolls precisely because a 1350 mm roll is unmanageable single handed between joists.
- Services are in the way. Pipes, ducts and wiring all run through the joist bays and all need cutting around properly rather than through.
- Check for damp first. If the bearers are wet or the ground is damp, that is the first job, not the insulation.
What goes wrong
- No fixings. Friction fit alone means the batts are on the ground in a few years.
- Wall batts ordered for the floor. Wrong width, and no saddles or straps in the box.
- Insulating over a damp sub-floor. Deal with the moisture and the ventilation first.
- Reflective product with no air gap. It works by reflection, so pressed tight it does very little.
- Ignoring the enclosure decision. With reflective foam it is worth more than the product choice.
- Foil near wiring. Reflective foam products are electrically conductive.
- Cutting through services instead of around them. Slower to do properly, much slower to fix.
We supply it and we install it
AWBM supplies underfloor batts, sub-floor fixings, reflective underfloor foam and sub-floor ventilation across Victoria and New South Wales, and we install batts, wall wrap and sarking, underfloor included. Send the floor area, the joist centres and how much headroom the sub-floor has, and we will work out the products and quantities and quote it supplied or installed.
Or browse ceiling, wall and acoustic insulation.
Frequently asked questions
Is underfloor insulation worth it?
On a suspended floor over a ventilated sub-floor, yes. It cuts heat loss down through the floor and, more noticeably, it limits cold air moving up from the sub-floor, which is what people actually feel as a cold floor.
What R-value do I need under a suspended timber floor?
Take the target from your energy report or specification. Our standard underfloor batt is R2.5 at 90 mm, and depth is rarely the constraint under a floor, so the practical limits are fixing and moisture rather than thickness.
Can I use wall batts under a floor?
Not sensibly. Underfloor batts are cut narrower, 415 mm and 565 mm against 430 mm and 580 mm for wall and ceiling, because they have to sit up between joists and stay there. Wall batts also do not come with the saddles or straps the job needs.
How do you hold underfloor insulation up?
With saddles that push up into the joist bay from below, or straps that run across the underside of the joists. Friction fit alone will not hold over time, because vibration and airflow work the batts loose and they sag.
What happens if underfloor insulation sags?
It stops delivering its R-value, and because it is out of sight nobody notices. That is why the fixings are ordered with the batts rather than added later.
Does enclosing the sub-floor make a difference?
A large one with reflective products. Kingspan quotes AIR-CELL Permifloor at RT 0.8 in and RT 1.6 out in an unenclosed sub-floor, and RT 1.6 in and RT 3.0 out in an enclosed one. Same product, nearly double.
My sub-floor is damp. Should I insulate it?
Deal with the damp first. A closed sub-floor holds moist air against the underside of the floor, which is where mould and rot in the bearers start, and insulation installed into that will get wet and stay wet.
Do sub-floor fans help?
Yes, when they create a path. Two fans placed to pull air across the sub-floor work better than one pulling from a single point, which is why they are commonly supplied in pairs.
Can AWBM do underfloor insulation?
Yes. We supply the batts, the fixings, the reflective options and sub-floor ventilation across Victoria and New South Wales, and underfloor batts are work we install as well as supply. Send the floor area, joist centres and sub-floor access and we will quote it supplied or installed.
Sources
- AS/NZS 4859.1, Thermal insulation materials for buildings, for declared material R-values
- NCC 2022 Housing Provisions, Part 13.2 Building fabric, for floor requirements
- Batt widths, pack coverage, fixing dimensions and fan capacity from the CSR Bradford Optimo, Martini and EcoFan product data
- System R-values, vapour classification and span limits from the Kingspan AIR-CELL Permifloor product data
General information. Floor insulation requirements vary by climate zone and construction, and sub-floor moisture problems can have structural causes, so confirm the requirement for your own building and have persistent damp or rot assessed by a suitably qualified person before insulating over it.