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Creating two rooms

How to upgrade a party wall when splitting one house into two

The most common approach when converting a large single dwelling house into two or more separate properties is to divide it into upper and lower floor self-contained flats. But what about houses that lend themselves to a vertical divide, so that a detached house becomes two semi-detached houses, for example?

Once the feasibility of this type of conversion project has been established, including ensuring the legalities relating to the property allow for sub-dividing, and that it is possible to secure the necessary planning permission and – if applicable – mortgage, attention will turn to the design and construction process. Key to this will be to ensure all the necessary Building Regulations are met, and that the quality of the finished properties will meet buyer expectations.

One of the most important elements of the building in this type of conversion is the party wall that will separate the two new side-by-side properties. This will have to be constructed to offer acoustic performance compliant with the minimum requirements set out in the Building Regulations.

In England and Wales, the separating wall will need to meet Approved Document E. For existing properties that are being divided, this requires a minimum airborne sound transmission level of 43 dB DnTw+Ctr, and the higher the figure the better.

The minimum level for Northern Ireland is the same, using the same calculation, but in Scotland Section 5 of the technical handbook specifies two figures according to the type of building and a different calculation method is used. For what is defined as a traditional building, the minimum standard is 53 dB DnTw, but for all other types of building the requirement is for the party wall to achieve 56 dB DnTw or above.

These acoustic performance standards exist to ensure that people living in connected homes can enjoy a good quality of life, part of which is being able to live without excessive noise from adjoining properties. Studies from around the world have found that nuisance noise can have a damaging effect on some people, causing stress that can result in numerous mental and physical health problems.

Hence, it is vital to get the separating wall’s acoustic specification right – and why acoustics’ best practice should always be to go above and beyond mere compliance performance.

How will an existing wall need to be upgraded to meet the acoustic standards?

Depending on how you propose to split the house, you may choose to retain an existing wall that is currently an internal wall between rooms, such as between two bedrooms. If this is the case, the first step in developing an effective sound insulation treatment is to assess the existing materials used – is it a timber stud, brick block/masonry or something else?

This is hugely important because sound transmits differently through different materials, so the acoustic design will need to consider factors such as the density of the materials that make up the wall and – importantly – the thickness.

In almost all cases the wall thickness will need to be increased to accommodate sound insulation so this needs to be factored into the proposed available floor space in each of the new houses being created.

Generally speaking the thicker the party wall, the better the acoustic performance. A target level of soundproofing which goes higher than the compliance standard, therefore, may not be achievable if there are any restrictions on the depth of the wall lining system. So ask the question,

what is the maximum depth we have to work with to design a solution, such as 50mm, 100mm or 150mm?

Does the floor need treatment when insulating a party wall?

In addition to the construction of the existing wall, the design and materials used for the existing
floor will also need to be considered. This is necessary because airborne sound will transmit through walls using any available sound paths and that includes structures and materials that are joined to it.

So unless the floor structure is considered, even the highest performing acoustic party wall design would be undermined.

That’s why we need to assess what the existing floor structure is within the property and how this floor construction ties in with the wall currently. This will tell us whether there are paths for sound to travel through that need to be addressed.

The risk of impact to enhance the floor construction using rubber-based materials to prevent impact noise transferring into the party wall. Although there is no impact testing possible for situations like these, this issue should be considered when assessing how the existing structure is formed.

How to Soundproof a staircase next to a party wall

Further investigation will also be required if there is a staircase attached to what will become the new party wall. A key design question here is whether this existing staircase will be retained in one of the properties or whether a new staircase will be fitted elsewhere.

This is really important from an acoustic perspective because impact noise from stairs travelling into
party walls can cause serious problems for the occupants of adjoining houses as they go about their day-to-day lives. Impact sound needs to be addressed differently to airborne sound which, if you are planning to retain the position of the staircase on the new party wall, will mean adding rubber-based materials capable of absorbing the thudding of footsteps.

How do I deal with services when creating a new party wall?

Another key acoustic design goal with any separating wall, floor or ceiling is dealing with how any services are run inside. Sound will easily find a weak point in a party wall structure and that can often be with features such as socket boxes and light switches. Issues can be easily avoided, however, if an effective sound insulation treatment for these elements is designed in early, rather than leaving it to chance by outsourcing these decisions to electrical contractors or other trades who may not understand the consequences of their decisions and your efforts to prevent the passage of sound.

How to soundproof a party wall that once featured a door?

In some property conversions where one house is being split into two, it may be that the internal wall earmarked to become the new party wall features a doorway or several. These openings will become infill areas once they are part of the new wall and will require special consideration.

Where possible, be consistent with the materials used to fill this area so the construction can offer the same density and performance as the rest of the existing wall. In a similar vein, take care not to overlook doorways that may have been closed up in the past too, as they could end up being a part of the wall that offers significantly lower levels of sound insulation.

Will the proposed layout of the newly created properties cause acoustic issues?

Finally, it is also worth taking a step back to think about whether the proposed arrangement of rooms inside each of the newly created houses will be a cause for concern acoustically. For example, if your design is proposing a bedroom-to-bedroom situation on either side of the party wall, you may want to consider increasing the acoustic performance of the party wall, depending on whether you feel that the minimum standards are sufficient or if you want the wall to perform better.

How can I get advice on soundproofing a party wall?

As the numerous questions raised here illustrate, building soundproofing is a multi-dimensional topic which requires attention to detail to deliver the right results. Cutting corners or accidentally missing a potential sound path can lead to costly consequences should the property fail a sound test, both in terms of the cost of remedial work and delays in the house being marketed for sale or rent.

If you are working on a property to convert a single dwelling or commercial building into two or more self-contained homes, Hush Acoustics can provide specialist acoustic advice to help ensure the specification will meet the requirements of the Building Regulations and maximise the level of sound insulation possible given the project criteria.

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