A site visit last year to a friends recent barn conversion with a barrel roof highlighted the condensation risk in barn conversion roofs with these types of conversions. There was significant damage to the inside face of the roof with water continuing to appear, particularly on windy cold days. Looking at the detail used, it was clear this was due to cold bridging and condensation.
Under Planning requirements, it is often the case that they do not want the external envelope to increase in width or size, so the developer and architect have fixed challenges.
For existing roofs, this can cause a significant issue depending on the method of construction. if you have steel beams or metal trusses (flat, barrel or pitched), then we strongly recommend you try to insulate OVER these elements to avoid significant condensation risk.
Insulated Between steel beams or trusses – The Issues:
- Steel and other metals conduct temperatures rapidly. If the outside of the steel is cold and the inside of the steel is warm, the temperature will quickly conduct to the cold causing a temperature difference with adjacet surfaces attracting moisture from the air.
- The ventilated roof means you have cold air cooling the top of the steel constantly, causing this significant cold bridge.
- You also have the outside air temperature and no thermal protection.
- If a barrel roof or pitched roof, then moisture build up may collect due to gravity or a connection which may look like a roof leak.
Here is a cold drinks can. The cold liquid on the inside rapidly cools the metal skin. As the outside air touches the cold can, the moisture in the air condenses and forms the moisture droplets you can see. This is what happens to steel or other highly conductive materials with cold bridging. if you keep it exposed to cold elements, you are at high risk of significant condensation on the inside that vapour contol layers will only push to another party of your building, or fail.
Your building structure is not a drinks can, keep it warm by wrapping the outside in insulation and keep the condensation out of your building
What can be done?
The example below is a section through a barrel roof based on a recent detail by another Architect:
You can see in the cross section the potential effect on the steel beam or truss. And you also get thermal breaks from the timber joists. You then heavily rely on a vapour control layer which is sensitive and can be damaged during construction.
Insulated over the steel beams or trusses – The right approach:
What can be done:
- Keep the structural line as it is shown, between the steel beams or trusses
- Move the insulation line above the structure to form a WARM roof (unventilated) and ensure the insulation connects with the wall insulation at the perimeter to avoid cold bridging at that junction
- This keeps the structure in the ‘warm’ zone top avoid cold bridging and condensation
- This relies on thermally broken connections to the roof insulation deck above.
- We strongly recommend liaising with roofing manufacturers to get advice for their products and systems to ensure you are building to the correct detail to avoid future defects.
Below is a revised diagram showing the insulation line above the structural zone:
The above details are relevant for the project we are discussing, but each project is different and you should consult with the project team to ensure these issues are being considered and designed out.
Need further advice?
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