Last time I wrote about how to build a dormer window on an eco-roof. In this post I focus on the inside of the eco-roof, in particular on the airtight membrane that makes the house practically airtight.
As mentioned in Unexpected complications when rebuilding an old house, the entire inside of the house will be lined with sheeting which forms an airtight layer. This sheeting (or membrane) runs continuously around the inside of the building and has to have no gaps, so builders use a special tape to seal up any joins.
To add this membrane to the inside of the roof, we first boarded over the underside of the Steico beams with OSB ( which mean oriented strand board) and then laid the airtight membrane over that. We fitted this over most of the inside of the roof in the attic room, leaving some areas for adding blown-in insulation, which is a job for later. The company doing the blown-in insulation need to make holes to insert the insulation so it makes sense to leave gaps in the membrane layer for this! However, we did need to put the membrane in areas where the builders needed add the battens that would form the skeleton attic bedroom wall, which you can see in this photo. The rest will be finished off after the insulating the roof.
On the day the builders were to start fitting the airtight membrane, I went over to the site. I expected to find them tucking the membrane down to the ground floor. Instead, I was surprised to find them pulling up floorboards on the middle floor. I had known that they needed to remove the floorboards around the edges of the building to do the tucking down. What I didn’t realise, and nor did they at first, was that the whole floor was rotten with woodworm. On the surface the floorboards looked fine, and it was only when the builders started to take them up that they saw the boards were riddled with woodworm. The boards broke easily, and would eventually have crumbled. They couldn’t stay.
This was an expense I hadn’t budgeted for.
However, had we finished the whole house and then discovered the floorboards on the first floor were rotten, it would have been far more trouble and expense than now. So, overall, I feel relieved this came to light early on.
Because the building will be almost airtight, we will have a mechanical ventilation and heat recovery unit (MVHR.) This will provide fresh air from outside, but will save any heat left in the air, putting it back into the building. You really need this unit if your building is airtight, otherwise the building will seem stuffy and airless.
Additionally, people give off a lot of moisture, from our bodies and in our activities such as washing ourselves, drying clothes, and cooking. An MVHR unit removes that moisture. You may have noticed that in some buildings, there is a build up of condensation on the windows. This used to happen in my cottage before I moved out to do the work. Every morning in colder weather, I would mop icy water out of the windowsills, a nightmare job! The MVHR unit will put an end to this. I am really looking forward to having condensation-free mornings when I move back in.
In the next post, I shall tell you lime render (where does it go?) This is fiddly, long-winded job that caused a lot of drama for us.