Piers Sadler | Piers Sadler Consulting
The risks associated with retrofitting solid wall insulation are well known amongst ‘green’ building professionals and practitioners. These risks include risks to built heritage, interstitial condensation, poor indoor air quality, mould and under-performance of the insulation.
Concern about roof retrofit is much less prominent and the guidance usually assumes simple situations, rather than the often complex reality. The risks are also under-rated, for example, the Guide to Planning Responsible Retrofit (1) rates the risks for different retrofit works as follows:
- External wall insulation: high risk
- Internal wall insulation: very high risk
- Roof insulation: medium risk
The guidance available generally recognises that pitched roof retrofit is associated with risks and well established guidance e.g. BRE262 (2) as well as (3), (4) and (5) advise that a 50mm gap is necessary on the cold side of the insulation and a vapour barrier is required on the warm side. In loft situations, the typical advice is to ventilate at the eaves (and depending on circumstances the ridge) to ensure the cold loft space is protected against condensation.
Building Regulations Approved Document L1b requires that retrofitted retained roofs are upgraded to a U value of at least 0.18W/m2K. This standard applies whenever there is a change of use and also whenever more than 50% of a particular roof area is renovated. Allowing for a 50mm gap above the insulation, this almost always means that insulation will be required on the internal or external sides of the rafters as well as between. Most of the guidance suggests above rafter insulation if re-roofing is required and below rafter insulation, combined with appropriate between rafter insulation if not.
The reality is that the room-in-roof geometry in many houses is complex and the simple rules of thumb provided in guidance cannot easily be applied. The photos below show some examples of these roofs from the inside and out:
Some of the key features of these roofs are illustrated schematically below:
Considering the photographs and schematic section above, it can be seen that the problem is complex in three dimensions and achieving continuous insulation, vapour control and airtightness layers is challenging.
In the discussion below it is assumed that the vapour control layer will also provide an air tightness function, and only vapour control is referred to. The possible presence of insulation of unknown thickness and detailing between the rafters and above the horizontal ceiling may complicate matters further.
The solutions need to address this complexity. Some options for retrofitting are:
1. Follow the line of the edge of the living space, insulating the floor of the room below, the stud dwarf wall, the pitched roof, the flat ceiling etc. whilst maintaining the vapour control layer on the warm side of the insulation throughout, including all access points (doors and hatches).
2. Follow the line of the pitched roof, insulating between the rafters and potentially also on the warm side, creating a vapour control layer on the warm side of the rafters.
3. Insulate above the rafters with a continuous layer of insulation and create the vapour control layer below the insulation and above the rafters.
Option 1 is likely to be the cheapest option but presents several difficulties:
- The warm side insulated plasterboard has to be very thick if the rafters are not very thick. This can interfere with doors, windows and features as well as reducing head height.
- The vapour control layer is difficult to maintain as it will comprise multiple boards fixed to the rafters, joists, stud walls and light well as a minimum and cracks and gaps may open up over time (see photos). Alternatively a membrane fixed between a layer of insulation (between studs) and plasterboard could be used, but this has implications for thickness and continuity of insulation.
- Internal structural walls form thermal bridges.
- The rafters will be cold due to the warm side insulation and it may be difficult to ascertain how good the ventilation on the top side of the rafters is without taking the roof off.
The biggest problem with Option 2 is access, which will involve pulling down ceilings and stud walls and require considerable disruption. The continuity of the vapour control layer and insulation will depend to some extent on the amount of money spent and the level of disruption caused, but internal walls will almost certainly cause thermal bridging, as will parapet gutters.
Option 3 can achieve the required U value and addresses most of the thermal bridges, whilst keeping the roof structure warm and incorporating a relatively simple vapour control layer. Free movement of internal air can be allowed within the roof structure. The insulation can tie in neatly with external wall insulation at the eaves. The disadvantage is that this will require scaffolding, potentially on several sides of the building, and significant re-roofing works. To achieve a thorough job, parapet and valley gutters need to be insulated and raised and new fascias installed. The roof is by necessity raised by this method and therefore planning permission is required. This approach is generally not suitable where the roof runs into that of the neighbouring building.
In summary, room-in-roof settings are common in traditional buildings and often have complex geometry. Guidance on retrofit is usually over-simplified and whilst the inherent risks are recognised, the solutions are insufficiently detailed to provide a clear way forward. What appear to be the simplest options for retrofitting are akin to internal wall insulation and carry with them similar risks. The best approach is to insulate above the rafters, keeping the whole roof structure warm and ventilated and eliminating almost all of the thermal bridges.