Methods for Insulating an Extension: Our Guide Offers Insight into Various Options
Insulating Your Extension: A Comprehensive Guide
Insulating an extension is a crucial step to ensure comfort and positively impact existing spaces. In this article, we will discuss the best types of insulation and methods for incorporating them into your extension.
Walls:
Fully filling wall cavities with suitable insulation is key. Materials such as mineral wool, rigid foam boards, or batt insulation (fiberglass or mineral wool) are common. The insulation should be installed with an appropriate vapor barrier placed on the warm side (usually interior) to prevent moisture migration that could damage the structure. If upgrading the existing house walls too, continuity of insulation and vapor barriers is important to avoid thermal bridging or condensation problems at junctions between old and new walls.
Roof:
For roofs, especially flat roofs typical in extensions, rigid foam boards like polyisocyanurate (PIR) or extruded polystyrene (XPS) are recommended for their high thermal resistance and moisture resistance. Building regulations typically require achieving a U-value of around 0.15 W/m²K or better, meaning insulation thicknesses often range between 100mm and 150mm. Warm roof construction (insulation above the roof deck) is preferable to limit condensation risks. For pitched roofs, insulating at ceiling level with materials like blown-in mineral wool or fibreglass is effective, ensuring vapor barriers are correctly installed.
Floor:
Adding underfloor insulation can significantly reduce heat loss. Choice of insulation (mineral wool, rigid foam) depends on floor type (suspended timber, concrete slab) and moisture considerations. It is important to maintain vapor control layers and maintain thermal continuity with walls to prevent cold spots.
Impact on Existing House:
When attaching an extension, insulating it properly should consider the existing home's thermal envelope to avoid thermal bridging or moisture traps at junctions. Seamless vapor control continuity and matching or exceeding the insulation levels of the existing building walls and roof are crucial for energy performance and durability. Ensuring airtightness between the extension and the old structure reduces drafts and heat loss.
Becky Lane, co-founder and CEO of Furbnow, an expert on insulation and a board member at the Sustainable Housing Action Partnership, emphasizes the importance of insulating period properties with breathable insulation materials like wood fibre or sheep's wool to maintain the building's natural moisture balance and prevent condensation issues within traditional solid walls.
When building an modern extension, a comprehensive approach is required, combining cavity wall insulation, floor insulation, and roof insulation. Phil McWilliam, electrical and general building contractor at Peninsula Electrical Services, explains that for pitched roofs, insulation can be placed above the rafters or between them using rigid boards, mineral wool, or PIR boards.
When considering the cost of insulation materials for an extension, PIR boards are more expensive but offer better insulation with less thickness, with estimates ranging from £20 to £50 per square metre for materials, with installation costs on top. There are guides available to help decide between mineral wool and fibreglass insulation for an extension.
In summary, best insulation for:
- Walls: fully fill cavity with mineral wool or rigid foam, vapor barrier on warm side
- Roof: rigid foam boards for flat roofs, ceiling-level blown insulation for pitched; 100-150mm thick to meet regulations
- Floor: underfloor insulation with moisture control
- Existing House Impact: ensure continuity of insulation and vapor barriers, airtight seals at junctions to prevent heat loss and moisture issues
These methods together deliver energy efficiency improvements and minimize risks of condensation or structural damage at the extension connection.
- To ensure proper insulation in your extension's walls, consider using materials like mineral wool, rigid foam boards, or batt insulation, making sure to install an appropriate vapor barrier on the warm side (interior) for moisture prevention.
- For roofs, particularly flat ones common in extensions, consider utilizing rigid foam boards such as polyisocyanurate (PIR) or extruded polystyrene (XPS) as they offer high thermal and moisture resistance.
- When planning your extension's floor insulation, consider the floor type - suspended timber or concrete slab - and moisture considerations before choosing insulation like mineral wool or rigid foam.
- To maintain continuity with your extension's insulation, consider the thermal envelope of your existing home while insulating the extension to avoid thermal bridging or moisture traps at junctions.
- A comprehensive approach should be taken when building a modern extension, combining cavity wall insulation, floor insulation, and roof insulation for optimal energy efficiency.
- Both mineral wool and fibreglass insulation can be used in extensions; guides are available to help make an informed decision between the two.
- Insulating period properties requires the use of breathable materials like wood fibre or sheep's wool to preserve the building's natural moisture balance and prevent condensation issues in solid walls.
- When insulating a pitched roof in your extension, place insulation materials either above the rafters or between them using rigid boards, mineral wool, or PIR boards for adequate insulation.
- PIR boards provide better insulation with less thickness but are more expensive (ranging from £20 to £50 per square metre for materials, with additional installation costs) compared to mineral wool or fibreglass.
- As you embark on your home-improvement project, ensure seamless vapor control continuity, airtight seals at junctions between the extension and the old structure, proper ventilation, and adherence to home-and-garden regulations for optimal energy performance, durability, and lifestyle benefits.
