Green Building Forum - detailing for a warm pitched roof with a little gable complication Tue, 19 Dec 2023 08:40:37 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 detailing for a warm pitched roof with a little gable complication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299815#Comment_299815 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299815#Comment_299815 Sun, 22 Jan 2023 20:07:22 +0000 number_thirty_three
I'm looking at the insulation options for our upcoming loft conversion and have hit a wall (metaphorically speaking).

Its a typical mid-terrace 1905 period house. The pitched roof has a little gable triangle on the front - I've attached a diagram to illustrate it. The diagram also shows the parapet atop the fire walls breaking the roof line between our roof and the neighbours either side.

Because of those parapet walls, I believe we can raise our roof height on the front elevation a little as it won't be massively noticeable from street level, enabling us to convert it to a warm roof (by which I mean adding around 100mm of PIR above rafters, and filling the 100mm rafters with a further 100mm).

However, I'm unclear what to do about the the little triangular gable. If we added 100mm there, it would make the triangle wider/bigger overall, and that wouldn't be acceptable to do. The only way around it would be to remove the rafters there completely, shrink them down and rebuild, which seems like an awful lot of rework (all the existing roof rafters are in good condition so can remain in place).

I'm keen to go down the warm roof route but this has me blocked. Has anyone seen something like this before / any suggestions?

So far I've considered the following:

a) Adding only a small amount of insulation above the rafters in that section - 20mm say - which would hopefully then not distort the appearance of the triangle. I imagine a downside is a potential thermal bridge where those sections meet the 100mm areas, with inconsistent u-values around the fabric of the front elevation, which I imagine could leave to condensation issues / risk of rot etc.

b) Doing a warm roof on the main pitch, as well as the little gable triangle vertical face, but a 'cold' roof on the pitched roofs of the little gable. There will be intersecting points between the two which would add complication to the detailing even more I imagine.

c) abandoning warm roof aspirations and insulating it as-is (i.e. as a cold roof). This affects head height though, so I'd like to avoid if I can.


Thanks in advance for any suggestions]]> detailing for a warm pitched roof with a little gable complication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299818#Comment_299818 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299818#Comment_299818 Mon, 23 Jan 2023 11:12:41 +0000 WillInAberdeen
So I would be looking to raise the roof on the little gable to make it a warm roof, but if possible retaining the same width of the fascia boards, just shunt them up/out 100mm. This might need some adjustments to the outermost rafter position of the little gable - you might want to do that anyway to avoid it bridging through your insulation layer and to make room for the EWI on the vertical face.

Often good to get a photo of the street and draw the proposed mods onto it to scale, to see how it might look (digitally or with a biro).


The parapet party walls sticking through the roof insulation layer will thermally bridge and bring cold down into your loft, are you planning to insulate the inside face of them?]]> detailing for a warm pitched roof with a little gable complication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299821#Comment_299821 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299821#Comment_299821 Mon, 23 Jan 2023 13:32:36 +0000 djh detailing for a warm pitched roof with a little gable complication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299825#Comment_299825 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299825#Comment_299825 Mon, 23 Jan 2023 17:08:57 +0000 tony
Or run the sheet insulation down the rafter line leaving the gable uninsulated if it sits on the rafters. If a bay window then carry the insulation across its ceiling .]]> detailing for a warm pitched roof with a little gable complication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299828#Comment_299828 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17709&Focus=299828#Comment_299828 Mon, 23 Jan 2023 18:15:17 +0000 number_thirty_three
Posted By: WillInAberdeenOften good to get a photo of the street and draw the proposed mods onto it to scale, to see how it might look (digitally or with a biro).


Good idea, I had just written off the option but I'll do this and see!

The parapet party walls sticking through the roof insulation layer will thermally bridge and bring cold down into your loft, are you planning to insulate the inside face of them?


Yes, I had been planning to insulate the inside face of some parts of these walls - on one side there's a dormer (but it's lower than ours will be), and on the other there isn't, so most of the party walls are effectively external. I have already done IWI with woodfibre elsewhere in the house, with EWI to follow on the rear when the weather becomes mild enough, but not sure I've got enough in the budget to do all of the sqm of these walls with that, so it may end up being PIR on ventilated battens.

Posted By: djhOne other possibility would be to use aerogel insulation, or VIP, over the little gable roof. It would be expensive though. Even phenolic would reduce the thickness a little, and arguably be safer.


Thanks a lot djh! I had briefly thought about Aerogel but had assumed it would be too pricey - But i've emailed Proctor group for a quote now to see. Hadn't come across VIPs before - that looks like a great option here, if they can be used in Pitched Roof scenarios and are perhaps less £ than Aerogel. Will check that out.

Posted By: tonyOr run the sheet insulation down the rafter line leaving the gable uninsulated if it sits on the rafters. If a bay window then carry the insulation across its ceiling .


Great out-of-the-box (or gable) thinking. I had been planning on using the space for storage, so wanted to bring it inside the thermal envelope. But now I'll figure out how much storage we'll actually get though, as it's not loads, so this may be the simplest option overall. Certainly imagine it would be the cheapest - I could possibly even get away with leaving the external section completely untouched during the works as it is weathertight there (or at least it is right now!)]]>