What are batts?
Batts are flexible slabs of insulation, made to standard widths to suit common stud and rafter spacings. They are cut slightly wider than the gap and pushed into place, where they stay by friction without fixings.
Unlike rigid boards, batts are not structural and are not finished directly. They fill the depth of a frame, with boards, membranes and plaster or cladding on either side.
Wood fibre and hemp compared
We stock three batts: two wood fibre and one hemp.
|
STEICO Flex 036 |
Pavatex Pavaflex |
Inditherm |
|
|---|---|---|---|
|
Fibre |
Wood |
Wood |
UK-grown hemp |
|
Density |
50 kg/m³ |
50 kg/m³ |
45 kg/m³ |
|
Thermal conductivity λD |
0.036 W/mK |
0.038 W/mK |
0.040 W/mK |
|
Vapour resistance μ |
2 |
Not stated |
1.3 |
|
Reaction to fire |
E |
E |
E |
|
Widths |
375 or 575mm |
375 or 575mm |
370, 440 or 570mm |
STEICO Flex 036 has the lowest thermal conductivity of the three, so it needs the least thickness for a given performance.
Pavatex Pavaflex is a similar wood fibre batt, slightly less insulating per millimetre. Both wood fibre batts have a high specific heat capacity (2,100 J/kgK).
Inditherm is made from hemp grown in the UK. It is the most vapour open of the three and comes in an extra 440mm width, useful in older buildings where framing isn't at standard centres.
In practice the three are fitted the same way. The choice usually comes down to the thickness available in the depth of the frame, the widths that suit the framing, and whether UK-grown material matters to the project.
Where they are used
-
Between the studs of timber frame walls
-
In stud walls built for internal wall insulation on solid masonry
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Between rafters, often combined with a rigid wood fibre board over or under them
-
Between floor joists and ceiling joists
In Bio Space's build-ups, batts fully fill the timber frame in the Timber Frame Clad and Timber Frame Rendered walls, and the stud zone in the Stud Wall IWI.
Most of the performance of a batt depends on how well it is fitted.
-
Cut slightly oversize. Cut each batt about 10mm wider than the gap, so it compresses lightly against the studs. Check the manufacturer's guidance for the exact allowance.
-
Fill the full depth. Leave no void behind or in front of the batt, unless the build-up specifies a service void.
-
No gaps at edges and ends. Gaps let air move around the insulation and carry heat with it. Butt the ends of batts tightly together.
-
Work around services. Cut the batt to fit around pipes and cables rather than compressing it heavily or leaving voids.
-
Use the right tools. A sharp insulation knife or an insulation saw gives clean, square cuts. Wear a dust mask when cutting.
Fitting batts well
-
Batt held by friction. Compressed slightly against each stud, with no gaps.
-
Gap beside the batt. Air moves around the insulation and heat bypasses it.
-
Timber stud. A cold bridge in itself, so gaps beside it add to the loss.
Schematic plan, not to scale. Check the manufacturer’s guidance on how much to oversize batts.
Most of the performance of a batt depends on how well it is fitted.
-
Cut slightly oversize. Cut each batt about 10mm wider than the gap, so it compresses lightly against the studs. Check the manufacturer's guidance for the exact allowance.
-
Fill the full depth. Leave no void behind or in front of the batt, unless the build-up specifies a service void.
-
No gaps at edges and ends. Gaps let air move around the insulation and carry heat with it. Butt the ends of batts tightly together.
-
Work around services. Cut the batt to fit around pipes and cables rather than compressing it heavily or leaving voids.
-
Use the right tools. A sharp insulation knife or an insulation saw gives clean, square cuts. Wear a dust mask when cutting.
Protecting the batts
Batts work best within a build-up that is airtight on the warm side and wind-tight on the cold side. In timber frame, that usually means a vapour control and airtightness membrane on the inside and a wood fibre sarking or sheathing board on the outside. In a stud wall on masonry, a variable vapour control layer goes on the room side of the batts, behind the lining board.
Airtightness matters as much as the insulation itself. Moist indoor air leaking into the frame can condense within it, so membrane joints, laps and penetrations should be sealed with suitable tapes.
