Green Building Forum - Retrofit project Fri, 14 Aug 2026 16:43:01 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310638#Comment_310638 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310638#Comment_310638 Wed, 13 May 2026 08:29:03 +0100 ChrisGT Cavity walls with 80mm glass wool batts, clod roof, dot and dab plasterboard walls and a terrible airtightness test
Planning to:

Fill cavities below dpc with ecobead.
Create warm roof by increasing rafter depth with intello membrane on inside and blown in gutex or warmcel and solitex plus counterbattens and battens above.

150mm eps from 300mm above ground to foundation fixed to exterior brick walls. Timber frame made from 2 x 50x50mm timber with noggins to create 150mm cavity to be filled with gutex and 40mm rendered woodfibre board outside. Intello against bricks connecting to foundation and roof vapour control layer.

Does this sound like a good plan. Have considered parge coat in wall or passivpurple but intello seems cheaper and more eco.

Triple glazed windows hung outside current wall in picture frame.

Any suggestions or thoughts on improvements?]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310639#Comment_310639 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310639#Comment_310639 Wed, 13 May 2026 10:35:59 +0100 fostertom Posted By: ChrisGTTimber frame made from 2 x 50x50mm timber with noggins to create 150mm cavity to be filled with gutex and 40mm rendered woodfibre board outsideWas suggested to me that short (like 1200mm) offcuts of timber I-beam may be available cheap from suppliers, wd save much 'studwork' fabrication on site.
Does the external render-board have to be an expensive insulation board? I thought with thin-web I-beams, thermal bridging doesn't need to be killed?
With I-beams, are 'picture frame' window surrounds needed?
To prevent standing ground-water in the trench, hitting the insulation value of the below-ground CWI and EWI, turn them into french drain? provided you've got somewhere to drain that out to - a soakaway that also fills with groundwater will just pipe water back into the trench.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310641#Comment_310641 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310641#Comment_310641 Wed, 13 May 2026 10:58:33 +0100 djh Posted By: fostertomWas suggested to me that short (like 1200mm) offcuts of timber I-beam may be available cheap from suppliers, wd save much 'studwork' fabrication on site.
Does the external render-board have to be an expensive insulation board? I thought with thin-web I-beams, thermal bridging doesn't need to be killed?
With I-beams, are 'picture frame' window surrounds needed?I'm not sure what you're suggesting to do with the short lengths of I-beam, Tom? I'm not sure there's enough work involved in building the framework Chris suggests to justify trying to save it.

I agree about the outer board - a stronger render board might be better.

The picture frames are needed to box out the light well from outside to inside; otherwise the blown-in insulation will just fill all the space.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310645#Comment_310645 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310645#Comment_310645 Wed, 13 May 2026 13:44:48 +0100 fostertom Posted By: djhnot sure what you're suggesting to do with the short lengths of I-beamScrew em to the brickwork, butting short lengths end to end - dead easy. Just spacers, not doing any spanning.
Posted By: djhThe picture frames are needed to box out the light well from outside to inside; otherwise the blown-in insulation will just fill all the space.
The I-beam spacers can be located to form the sides of the window 'picture frame' - more ditto nogged across top and bottom]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310646#Comment_310646 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310646#Comment_310646 Wed, 13 May 2026 16:12:42 +0100 djh Posted By: fostertom
not sure what you're suggesting to do with the short lengths of I-beam
Screw em to the brickwork, butting short lengths end to end - dead easy. Just spacers, not doing any spanning.Sounds more work than building the space frame Chris proposed to me, but each to his own.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310647#Comment_310647 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310647#Comment_310647 Wed, 13 May 2026 17:20:08 +0100 Doubting_Thomas
If the beam isn't structural then it can be just made of a patchwork of different lengths and still work.

Having said that, our carpenter did not enjoy cutting these engineered beams due to the different densities of timber. They were also able to lift each beam into place in one lift, unlike this approach.

So once you factor in the extra cutting, fixing and general labour, I think djh is right - it might not stack up financially, despite potentially being slightly 'greener'.

Worth doing the sums on your particular job though. We had to wait weeks for a special order, so the time factor might swing things.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310648#Comment_310648 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310648#Comment_310648 Wed, 13 May 2026 22:01:08 +0100 djh Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310687#Comment_310687 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310687#Comment_310687 Sun, 17 May 2026 22:03:11 +0100 ChrisGT Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310692#Comment_310692 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310692#Comment_310692 Mon, 18 May 2026 10:49:20 +0100 djh
Passive Purple is indeed a layer of paint. I believe it works but I think it's quite expensive.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310782#Comment_310782 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310782#Comment_310782 Tue, 09 Jun 2026 11:04:56 +0100 ChrisGT With the garage conversion roof with skinny 70x 35mm roof trusses he and the architect have suggested ripping them out and replacing with a glulam ridge and I joists. It would create a useful space I suppose instead of a forest of timbers. Any suggestions. Im surprised there arent some standard details for dealing with these issues.]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310783#Comment_310783 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310783#Comment_310783 Tue, 09 Jun 2026 17:30:45 +0100 djh Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310784#Comment_310784 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310784#Comment_310784 Tue, 09 Jun 2026 19:23:48 +0100 ChrisGT Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310785#Comment_310785 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310785#Comment_310785 Wed, 10 Jun 2026 10:30:37 +0100 djh Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310786#Comment_310786 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310786#Comment_310786 Wed, 10 Jun 2026 19:49:08 +0100 ChrisGT
100mm booster rafters on top of main house rafters which are 1965x46mm

Take off the garage trusses and replace with hlulam ridge and ibeams rafters.

Unless anyone has better suggestions??]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310787#Comment_310787 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310787#Comment_310787 Wed, 10 Jun 2026 20:59:06 +0100 djh Posted By: ChrisGT100mm booster rafters on top of main house rafters which are 1965x46mmPresumably 195x46 as mentioned before?

How are the booster rafters attached to the existing rafters?]]> Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310815#Comment_310815 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310815#Comment_310815 Fri, 12 Jun 2026 13:25:04 +0100 ChrisGT Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310816#Comment_310816 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310816#Comment_310816 Fri, 12 Jun 2026 13:25:56 +0100 ChrisGT Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310818#Comment_310818 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310818#Comment_310818 Fri, 12 Jun 2026 15:12:37 +0100 djh Retrofit project https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310821#Comment_310821 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18435&Focus=310821#Comment_310821 Sat, 13 Jun 2026 20:42:48 +0100 Peter_in_Hungary Posted By: ChrisGTI would have thought with screws every 30cm. The SE suggested nail plates every m.
When I had to strengthen some rafters when going from corrugated sheets to tiles my SE said 100mm booster rafters on top of existing screwed with structural screws every 30 cm with PVA glue between new and existing. His reasoning was that without this joining the new and existing wouldn't act as one timber and so would be structurally inferior.]]>