Cavity Wall Insulation

Changes under Part L 2018 (Conservation of Fuel and Energy) will become compulsory from 1ST November 2019 and will replace Part L 2011.

New Build U-Value Requirements.    Building Element - The Back Stops………

  • External Walls.                               0.18 W/m²K

NOTE: The back-stop U values in many cases will not be enough to satisfy the new NZEB targets.The preliminary BER; building energy rating calculation will identify the U value targets and also the performance required from associated building materials.This will include the airtightness level, the heating/ventilation system type and efficiency, the U values for the windows and the external doors and also thermal bridging measures.

External Walls Insulation Guides.

If you are building your own home the U value target for the external walls should not be higher than 0.18 W/m²K.

You only get one shot at insulating the cavity walls. Don,t listen to the question; what can I get away with....

The lower the U value the slower the heat loss; the slower the heat loss the greater the savings.

 

Cavity Wall Insulation  - U value achieving 0.18 W/m²K

  • 19mm Sand and Cement Render
  • 100mm Dense Block
  • 40/50mm Cavity              Low E Reflective cavity due to foil facing on insulation boards.
  • 100mm Polyisocyanurate Foam (PIR) Foil faced cavity wall insulation fitted tigtly to the inner leaf block work. (no gaps)  Insulation Thermal conductivity 0.022 W/mk
  • 100mm Dense Block
  • 10mm Sand and Cement Render

 

Cavity Wall Insulation  - U value achieving 0.18 W/m²K

  • 19mm Sand and Cement Render
  • 100mm Dense Block
  • 40/50mm Cavity              Standard un-ventilated cavity.
  • 110mm Polyisocyanurate Foam (PIR) Foil faced cavity wall insulation fitted tigtly to the inner leaf block work. (no gaps)  Insulation Thermal conductivity 0.022 W/mk
  • 100mm Dense Block
  • 10mm Sand and Cement Render

 

Cavity Wall Insulation  - U value achieving 0.18 W/m²K

  • 19mm Sand and Cement Render
  • 100mm Dense Block
  • 40/50mm Cavity              Standard ventilated cavity.
  • 115mm Polyisocyanurate Foam (PIR) Foil faced cavity wall insulation fitted tigtly to the inner leaf block work. (no gaps)  Insulation Thermal conductivity 0.022 W/mk
  • 115mm Dense Block
  • 10mm Sand and Cement Render

The installation of the cavity wall insulation will determine the U value result; not the paper exercise:

On many construction sites it can be difficult to acheive a perfect fit when installing rigid cavity wall insulation boards.There are many reasons for this and the first one is the quality of the block work. Uneven block work and mortar snots will restrict the rigid insulation from sitting flush with the internal leaf block wall. Mortar droppings must alway be cleared away from the top of the cavity wall insulation boards otherwise the boards will not fit tightly togther.

Mortar dropping must also be cleared away from wall ties as the mortar droppings can create a path for water to flow from the outer leaf blockwork to the inner leaf blockwork. If the rigid cavity wall insulation boards are sitting off the inner leaf block work we get what is called a chimney effect. This can greatly reduce the thermal performance of the external wall as the cavity wall insulation can not provide the thermal resistance to heat loss due to the energy flowing upwards within the chimney space. Warm energy will always take the path of least resistance (the space between the inner leaf block work and the rigid insulation boards is the path of least resistance) 

My advice is that if the paper exercise; U value result recommends 100mm insulation thickness to expect a U value achieving 0.18 W/m²K add an additional 20mm to the insulation thickness. Inspect the quality of the inner leaf block work and inspect the installation of the rigid cavity wall insulation. If it is not possible for you or your architect to be on site every day to carry out inspections your contractor can provide you with up dates and images from a smart phone or I Pad if you agree this as part of the contractor prior to awarding the contract.