Is my plan for the loft sensible?

[Lemna gibba]

We are designing an energy efficient refurbishment to our house to coincide with some internal remodelling. Briefly, half of the house is single brick 1930s, the other half is an 1980s extension. We want to insulate properly and run an ASHP. We completely understand that we need to make big changes to the fabric of our house. For the rest of the house, I think we have a clear idea how to do this. We will replace our conservatory with a SIP built extension. We replace the existing ground floors (suspended timber and uninsulated concrete) with a new insulated slab with 150 mm PIR (happy for comments if more is better) with UFH in screed. We fill the 1980s cavity wall. Currently thinking bonded beads is best. Then we run 100 mm EPS around all of the house (solid and cavity walls). We can change this to Kingspan if it is available - but currently it looks prohibitively expensive. We will replace all the windows with triple glazing. We will make sure we attain a good level of airtightness around the windows, and have would like to use wet plaster throughout. 

 

At the moment we have a PIV system. Without this we have condensation issues. We are under no illusions that we will have an airtight house, but are keen on MVHR due to air quality. The last problem is our loft. Currently it's a bit of a nightmare!

 

The house is made up of two gable roofs joined at 90 degrees. The 1930's part is boarded out. This is probably a mistake. Below the boards is probably around 20 cm of fairly compressed rock wool. The loft feels bone dry, but I haven't lifted the boards yet. At the apex it's 130 cm above the boards. We do use the loft for storage. In the 1930's part: There is plasterboard over the rafters (and above the purloins). These are coming off in places. Below the purloins is what looks like paper. This has mostly degraded and falls apart if you touch it. Then there is an airspace, and a fairly modern looking white membrane. I can feel the batons through this and presumably the slates attach to it on the outside. If I go to the edge near the eaves, there is massive gaps in the current insulation. I can see the edge of the brick wall and the ceiling of the room below. It would appear that the ceiling is made of lath and plaster! 

 

The 1980s part has exposed rafters that look in good condition and what appears to be look like bitumin roofing felt. The old roof has not been taken down where the two roofs join and I can only get in the 1980s part by some contortion under the purloin. Between the two walls looks like no mans land. It is totally devoid of insulation. Parts of the original wall exist and parts of the original roof join them. I am sure some are structural, and most are trash. Many of the slates are still on the roof. They really didn't tidy up.

 

I have been going round in circles about the best option for to properly insulate and get some degree of airtigthness. Currently, I think the best option will be: remove all internal ceilings. Place airtight vapour control layer under the rafters.  We would then leave a service void with batons for electric cables. This can then be re-boarded and skimmed.  We would clear as much of the old roof as we can out. Then re-lay and top up mineral wool. We do some very minimal boarding with a significant air gap above the wool so that we can crawl to things we need like the MHVR and the solar inverter. Removing the internal ceilings sounds doable as we would need to fix them anyway after the internal work. The disadvantage of this system would be that the MHVR will be in a cold loft (although we pipes should run under the insulation), and that we are going to have to cut many holes for ventilation pipes through the airtightness barrier. We also lose any real capacity to store things in our loft. 

 

I've been toying with insulating between and under rafters and moving the airtightness barrier there. However, the complex shape of the two walls and the difficulty working in the tight space make me think that this could be really hard, expensive and open to errors.

 

I'm sorry to have written such a long post. I'm also aware that a very similar question was asked recently. I have been reading a lot, but not always sure that I am reading the correct things and making the correct connections. I'm very motivated to get as energy efficient house as I can, but have no real experience of the practical aspects. We are at the planning/costing stage with the aim of starting work in the summer. We've started talking to contractors, and this last part is bothering me, as it seems like the loft is going to be the main source of air leakage into our house. If you read this far, thanks for taking the time!

 

 

 

 

[Radian]

You seem to have the right sort of ideas and are realistic about your options. One question springs to mind: have you considered stripping the roof(s) and re-laying them as warm roofs? This would simplify the ceiling as fitting a vapour barrier (and making it good) below the rafters is difficult. It would keep the MVHR happy (you already know the pitfalls in cold lofts) and it would be easier to make a continuous insulated envelope that connects with your EWI. Getting the detailing of this right with a cold loft would be a nightmare.

[markc]

@Radian bet me too it, just finished reading and the first thought was strip and redo the entire roof in one go. Removes loads of work and potential grief.

[Radian]

2 minutes ago, markc said:

@Radian bet me too it, just finished reading and the first thought was strip and redo the entire roof in one go. Removes loads of work and potential grief.

 

It's easy spending other people's money (AKA being a government). But this one could be reasonably cost-neutral depending on the work for either approach being contracted out (time being money). 

[Lemna gibba]

Thank you for your quick replies. I had thought about a warm roof, but assumed that it would be too expensive, partly due to the complex shape below (the porch and rear extension are single story so not included here). However, we do need some work done on the roof. Our ridge needs replacing. We are removing the chimneys and we want to add more solar. We'll have the scaffolding in place. I will start to enquire about the price.