Part L help - Table 4.2 Limiting U-values for existing elements in existing buildings

[KAZAAD]

Hi all,

 

I am involved in a change of use project (c3) to (c2) but started to receive feedback from building control regarding Part L. 

 

Under notes 6 for Table 4.2  Limiting U-values for existing elements in existing buildings - regarding the 5% internal floor area of the room, does this mean the internal insulation needs to be applied to a lesser standard eg. If I am proposing to add 100mm internal insulation and does not fit accordingly with the room size, can I use an internal insulation for less than 100mm?

 

The internal spaces have been renovated, but building control has requested internal insulation. The property has a wall thickness of 400mm solid with no cavity. Does that mean we need to add external insulation to achieve Part L compliance from building control? Could we get away with an insulated plasterboard for the inside rooms?

 

• Approved Document L, Conservation of fuel and power, Volume 2_ Buildings other than dwellings, 2021 edition incorporating 2023 amendments.pdf

[DevilDamo]

If the existing thermal elements do not meet the threshold values, then they should be upgraded to meet the improved values. Should the upgrade works result in the floor space being quite substantially compromised, then a lesser provision/standard can be applied to the best you can.

[sgt_woulds]

That's not quite right as the lower U-values will need to be justified - Building Regs don't care about room size unless it impacts part M.

 

I work for a company that makes woodfibre insulation for IWI.  It is important to note that we do not generally recommend achieving high U-values with IWI due to the condensation risk to the structure.  There is, generally, a sweet spot between 40-100mm of woodfibre that balances the energy savings, cost, and condensation risk.  Part L of the Building Regulations for England offers flexibility when retrofitting existing walls, roofs and floors; for walls, an improved U-value of 0.30 W/(m2·K) is the target but a ‘threshold’ level of up to 0.70 W/(m2·K) is sufficient, as long as the approach can achieve a payback not exceeding 15 years and is ‘technically and functionally feasible’. 

 

It is worth discussing with your local authority / BCO.  For an older building, most BCOs are now sympathetic to the argument that breathability is more important than the insulation value.

 

It helps to approach them with evidence and we always recommend a WUFI study for any IWI to prove that the build-up won't cause moisture issues long-term; this is very useful when arguing the case for lower insulation levels.  If the change of use is for a domestic home you will probably need to mitigate this performance drop elsewhere, (by e.g. upgrading roof thermal performance).

 

P.S. Insulated plasterboard is the work of the Devil - rarely installed properly and causes more long-term problems than it solves.

[DevilDamo]

1 hour ago, sgt_woulds said:

Building Regs don't care about room size unless it impacts part M.

They will acknowledge the potential reduced room size as a result of upgrading external walls.