Insulation for Party Walls and Apartments

In attached housing and apartments the wall between two dwellings is doing two jobs the code cares about: fire separation and sound separation. Both are properties of a tested system, not of the batt inside it, and both are checked. That makes this the one part of a building where substituting a product with the same R-value is most likely to cost you.

This is specification driven work. If you are supplying or installing on a Class 2 apartment building or attached Class 1 housing, the documentation matters as much as the material.

What a party wall is actually doing

In attached housing the party wall is the fire separation between two dwellings. In an apartment building the walls and floors between sole occupancy units carry both fire and sound separation requirements, and those are code requirements with tested system ratings attached rather than comfort extras.

Note the order. On a party wall the thermal number is the least of the three, and on an internal separating wall it may not be a requirement at all. That is the opposite of how most people approach insulation.

The rating belongs to the system

This is the point that decides everything else, and it is worth stating as plainly as possible.

Fire performance belongs to the tested system rather than to the material on its own. So does the acoustic rating. The wall achieves its rating with a specific lining, a specific framing type, a specific fixing pattern and a specific insulation product, tested together.

What follows:

• You cannot swap the batt and keep the rating. Even at an identical R-value, a different density or a different material is outside the tested system.
• You cannot swap the lining either. The insulation is only one component, and changing the sheet is just as significant.
• The document you need is the system report, not the product datasheet.

When we are asked to supply for a rated wall, the useful question is not what R-value you want, it is which tested system the wall is being built to. That tells us exactly which product goes in it.

The joints are the weak point

A party wall is usually built well. The joints in it are where the separation is won or lost.

In attached housing the vertical joints in the party wall are the vulnerable line. A joint packed with combustible material, or left as a void, does not maintain the separation the wall was designed to provide. That is why non-combustible stone wool products exist specifically for this: they hold their form and their integrity under heat, which is what keeps the joint doing its job.

Two format details from our range show how the products are shaped by the problem:

• Party wall batts are cut 1200 x 168 mm at 100 mm thick, five to a pack, sized for the joint rather than for a wall cavity.
• The party wall sealer comes in 4000 mm lengths, two per pack, because a continuous seal has fewer joints than one pieced together from shorter batts. Fewer joints in a fire seal means fewer places for it to fail.

There is a thermal and acoustic side benefit to sealing those joints properly, but it is a side benefit rather than the reason to specify it.

Getting the acoustic side right

Acoustic insulation is specified by density to suit a tested system, and the R-value is a by-product. In separating walls that distinction stops being academic.

A high density acoustic batt at 90 mm and a standard wall batt at 90 mm are different products doing different jobs, even where the R-values are similar. The dense one is in the system because the system was tested with it.

One useful detail from the products themselves: the Soundbreak range is dyed green so you can see at a glance which cavities have been done. On a job with hundreds of separating walls, being able to walk the floor and see which bays carry the acoustic batt rather than a spare thermal batt is worth something.

Our full treatment of the difference is in acoustic vs thermal insulation.

Flanking in apartments

You can build a perfect separating wall and still hear next door, because sound goes around walls as readily as through them.

The paths that matter in multi-residential work:

• Over the top, through a continuous ceiling void that runs across both units.
• Through services, where a duct, pipe or conduit passes through the wall and the penetration is not sealed to the same standard as the wall.
• Back to back fittings, where power points or recessed fittings on opposite faces of the same wall leave a thin path between them.
• Through the structure, where the floor slab or the frame carries impact sound around the wall entirely.

The first three are ordinary detailing problems and they are solvable. Building the partition full height to the slab closes the ceiling void path, and where that is not practical, baffles dropped into the void above the wall line close it for far less.

Where thermal comes into it

It is worth being clear about when a separating wall is a thermal element and when it is not.

A wall between two heated dwellings is not part of the building envelope in the way an external wall is, because both sides are conditioned space. The energy requirement usually sits on the external walls, the roof and the floor.

Where a separating wall does carry a thermal requirement is at the edges: where it meets an external wall, where it becomes an external wall above a roofline, or where one side is an unconditioned space such as a garage or a common corridor. Those junctions are also where thermal bridging shows up, and where the detail is worth checking rather than assuming.

Our guide to what R-value goes where covers the element by element picture.

What goes wrong on site

• Substituting the batt to match an R-value. The rating was tested with a specific product at a specific density.
• Substituting the lining. Just as significant as changing the insulation, and easier to overlook.
• Unsealed service penetrations. Both fire and sound take that path.
• Gaps at the head track or the perimeter. Sound goes wherever air goes, and so does smoke.
• Joints packed with whatever was on the truck. The joint material in a fire separation is a specified item.
• Back to back power points. A small detail that undoes a good wall.
• Batts not touching the frame on all four sides. An acoustic batt with an edge gap is not performing as tested.

Ordering for this work

Send us four things and we can get this right first time:

1. The tested system the wall is being built to, or the acoustic and fire reports that name it.
2. The wall build up: framing type and depth, lining, and whether the studs are staggered or the mounts are resilient.
3. The quantities by wall type rather than as one total, because the separating walls and the internal walls are different products.
4. The joint detail, which drives the party wall batt or sealer size.

We will match the product to the system rather than to the R-value, and where a product in our range does not carry the documentation the specification names, we will say so.

Frequently asked questions

Sources

General information, not a fire engineering or acoustic opinion. Fire resistance levels and sound insulation ratings belong to tested systems and to designs assessed for the specific building, so work from your project's fire and acoustic documentation and confirm any substitution with the relevant consultant and the certifier before ordering.