Thatched roofs are one of the oldest and most visually pleasing forms of roofing, providing humanity with shelter since back when we were all wearing mammoth hides. After a serious decline in the 80’s and 90’s owing to widespread concerns over fire, hygiene and extortionate insurance and maintenance costs, the last two decades have seen a resurgence in thatched roof popularity. More stringent building regulations have reduced the associated fire risks and brought down insurance premiums. At the same time, the dramatic shift of environmentalism and sustainable living from the fringes into the mainstream has driven a revival of traditional, renewable building methods. Thatched roofs aren’t just for for the eccentric anymore, now they’re for everyone. Thatched roofs are cool.
How does a thatched roof work?
Okay it seems like a simple question, but all the best knowledge is built in stages. To understand it’s thermal properties, we first need to understand its structure.
Thatching is the practice of harvesting, drying and layering vegetation such as straw and water reed to create a robust, waterproof roof. Traditionally, thatchers use locally available materials, but in more recent times a significant amount of thatching materials have been imported. This is pretty serious business in the thatched roof community as many feel that it undermines the character and spirit of the practice, while also depriving local suppliers of business.
There’s a lot to love about thatched roofs. They are based on renewable materials, traditional historic methods, they’re carbon neutral and soundproof surprisingly well.
How well do thatched roofs insulate?
Actually pretty well. Thatching creates air pockets within structure of the thatch that trap and hold heat, insulating a building in both warm and cold weather. This natural propensity to maintain thermal mass is what we in the biz call the U value, and at the height of it’s game a thatched roofs’ U value can be impressively high. Overall, U values vary greatly according to thickness and density of the thatch, in fact depending on the materials, thickness and condition of the thatch, some thatched roofs can come close on their own to meeting modern standards for thermal insulation. Other factors include ventilation, pitch and moisture content. Evidently there are a lot of variables, but as a rule of thumb you’d be looking at 0.2/m2k for 450mm of well maintained reed or 350mm of straw. This may sound good, but insulation is consistently one of the most high impact, cost effective methods of cutting energy consumption. Making the most of it is imperative to an ecologically efficient house.
Hot or cold roof insulation for thatched roofs?
Cold roofs tend to be the best course of action for those who have no habitable use for the space between their ceiling and roof. It’s also extremely important for anyone whose thatching is relatively thin, subject to gaps at areas of transition to other materials, or penetrable by outside air.
Generally the cheapest solution, cold roof construction involves laying insulation above the top floor ceiling and can be done at home in a matter of hours. Since thatched roofs require good ventilation at all times, some people choose to open their eaves to provide increased air flow. In this situation it’s important to ensure that the ventilation paths are clear.
The second option is to create a warm loft. Ventilation and moisture balance is vital in this kind of improvement, owing to the threat of expensive premature rotting. In areas that use materials other than thatching the difference in temperature and U-value can cause cold bridging, so it’s important to ensure these areas are targeted.
In terms of materials, we’re big fans of sheep wool for this kind of job. It’s moisture balancing properties allow it to absorb water vapour in damper conditions, holding it until it’s able to evaporate come sunnier days.