Reinsulate converted attic

adocholiday

Hi all,

I bought my house with a converted attic that can only be designated as 'storage space' due to ceiling height. It was used by previous owners as an office/workspace. I want to use it for the same purpose however the knee walls need to be redone and as part of that I need to rethink the insulation.

At the moment there is just basic plasterboard making up the knee walls and ceiling. The plasterboard is not insulated. The rafters are insulated with rigid kingspan boards fitted between them. I'm not sure of the thickness I don't have the measurements to hand. There's a gap between the boards and the non-breathable felt. Between the joists then there's fibreglass wool. Doesn't seem to be a cold space between the floor and the fibreblass.

We were having issues with condensation on the non-breathable felt so we put in soffit vents and 4 inch room vents into each gable. There's a 4 inch vent above ceiling level on each gable too which were existing.

Looking into the crawlspace behind the knee-wall it's possible to see the soffit vents, so the air flowing through them moves freely through the crawlspace and into the attic room itself because the basic plasterboard is not insulated. Pair this with the room vents and it's pretty cold.

What I was thinking of doing is replacing the plasterboard knee walls with insulated plasterboard, and removing the rafter boards entirely up to where the knee wall joins the rafters. I'd leave the rafter board insulation in place from there upwards to save me having to redo the ceiling. I also want to redo the insulation in the joists with the correct size rockwool and ensure that it's as airtight as I can make it. That would keep the downstairs heat downstairs and leave the soffit vents venting the crawlspace with more air to move around the non-breathable felt and stop the attic room getting cold.

Just wondering if this is the correct approach or if I'm way off the mark?

Calahonda52

Having lived in a similar house for many years, an issue is summer overheating due to the rigid failing the decrement delay test.
In addition look up ventilation baffles.stopping the air from passing across the ceiling space under the attic floor is tough.
the solution I have used is cut plywood baffles the full height of the ceiling joists and seal them around the ceiling, rafters and the bottom runner of the knee wall.
Google knee wall and look at images

Its a tough job to do, properly, what you have in mind.

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

adocholiday

Thanks Calahonda. As of now we already have issues with overheating in summer, although less so since the soffit vents went in, and heat loss in winter. I was hoping that by removing the rigid board in the rafters and insulating the plasterboard walls that the vented crawlspace in addition to the new 4 inch room vents would prevent a lot of that overheating. We'd still have rafter insulation from the knee wall to the ceiling though.

Based on that document you shared (specifically section 6.3.3.4) I should have both rafter and plasterboard insulation. This makes life easier for me because the rafter insulation I have seems to be perfectly adequate (to my untrained eye), so all I should have to do here is redo the insulation between the ground floor and the attic space and I'll also do those baffles you suggested.

MicktheMan

My opinion is that while you may be improving you insulation envelope somewhat with you plans, you will not be addressing adequately the likely real reason for your heat loss / gain and that is air leakage. Search around this forum and you will find plenty on the subject.

adocholiday

MicktheMan wrote: »

My opinion is that while you may be improving you insulation envelope somewhat with you plans, you will not be addressing adequately the likely real reason for your heat loss / gain and that is air leakage. Search around this forum and you will find plenty on the subject.

Thanks Mick yes you're right for sure but without putting a massive amount of money into a retrofit here I don't see how we might address this really. We have had to add a significant amount of ventilation to the roof because there wasn't enough and we were getting condensation on the timbers from the non-breathable felt.

What I'm essentially trying to do is seal the attic room as much possible from the voids around it so that air can circulate freely in there while the room within stays insulated. I thought if I could increase the envelope around the room and taking care to make the room itself as well insulated and airtight as possible then the airflow on the far side of the walls wouldn't really matter. The current knee walls are literally just sheets of plasterboard screwed onto the timbers. They're not sealed or anything and there are 3-5mm gaps between some of them so pretty much any work I do there will be an improvement!

Calahonda52

The solar gain is through the rafters in the sloped section of the roof, you need to google decrement delay
eg

http://www.greenspec.co.uk/building-design/decrement-delay/