Solving Dormer Roof Insulation Problems: Faqs Answered!

Steve London Flat Roofing 07802300099
In this video, I’m addressing a question from a customer about how to insulate the dormer window of her shallow chalet bungalow. She sent me photos of her 1970s bungalow and a drawing of how she wants to insulate it from the inside. We’ll dive into what’s right, what could be improved, and the differences between a warm roof and a cold roof for this situation.

The customer planned to form a cold roof under the dormer and use 300mm of Rockwool for the rest of the loft. One key element missing from her plan was the Air Vapour Control Layer (AVCL), which is essential for preventing moisture problems.

I suggested an alternative: using a warm roof approach rather than the cold roof method over the dormer. Here’s why:

Why a Warm Roof is Better:
Better Thermal Efficiency: Placing insulation above the roof deck keeps the entire structure warmer.
Simplified Ventilation: A warm roof is sealed, removing the need for internal ventilation.
Less Interior Disruption: Insulating from above avoids the need to drop ceilings or add insulated plasterboard inside.
Issues with the Customer’s Proposal:
The customer intended to add soffit vents to connect airflow between the dormer and main loft. This is not advisable as mixing airflow can increase moisture content and cause condensation issues. Ideally, each section should be vented separately.

She also wanted to add 120mm of insulation between joists, 200mm of fibreglass, and 50mm of insulated plasterboard, but didn’t mention the AVCL. An AVCL is vital to prevent moisture from reaching cold areas and condensing, leading to potential damp problems.

My Suggested Approach:
Install Independent Ventilation: Keep dormer and loft ventilation separate.
Consider a Warm Roof: Adding insulation above the roof deck creates a sealed environment, offering better insulation without internal airflow issues.
Use an AVCL: Proper insulation must include an AVCL to control vapour and prevent condensation.
Practical Considerations:
A warm roof may have a higher upfront cost, especially for an existing roof, but it offers lower energy bills, less condensation risk, and improved comfort in the long term.
Installing a warm roof means insulation on top, with no ventilation between rafters—making it a simpler, more effective solution.
Final Thoughts:
For those considering a similar project, a warm roof is generally the better approach despite higher costs. It provides improved thermal performance, saves on energy bills, and prevents future structural issues.

Hopefully, this video helps those facing similar challenges. Unfortunately, I lost the customer's email, but I hope she sees this video and reaches out if she needs more help.

Thanks for watching—drop your questions about insulation, roofing, or anything else in the comments below. I’ll do my best to respond!
00:00 Customer asks question
00:46 Cold roof venting explained
01:40 How to do a warm roof
02:07 How to vent and add a AVCL to the cold roof
03:34 Warm roof details