Green Building Forum - Target air tightness for retrofit Mon, 11 May 2026 03:34:18 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260719#Comment_260719 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260719#Comment_260719 Thu, 24 Aug 2017 09:02:07 +0100 gravelld .

For planning my retrofit I need to think about a target air tightness level. But I really have no idea what to aim for, or even what is achievable. The current AT is not measured, but I'm pretty sure permeability is high.

The existing property is:

- Masonry cavity, 1950s build, extended lots of times (plenty of steel!). Wool in some cavities, EPS bead in others.
- Solid floor, uninsulated
- Fully rendered (with plenty of cracks and popped sections)
- Cold loft, lots of ceiling insulation

The retrofit will involve:

- AT layer to be the external render and (I think) the layer on top of the existing rafters. WT to be outbound of all insulation
- EWI, including down to founds (currently thinking truss + cladding rather than sticky EPS blocks)
- New joinery (high spec I hope)
- Removing/narrowing some windows, enlarging others
- Filling the cavities completely (those that aren't already)
- Insulation over the roof to connect to the EWI
- Nothing happening to the floor

Anybody got any kind of stats for what retrofits can achieve?]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260726#Comment_260726 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260726#Comment_260726 Thu, 24 Aug 2017 11:19:51 +0100 Peter_in_Hungary Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260731#Comment_260731 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260731#Comment_260731 Thu, 24 Aug 2017 12:47:27 +0100 gravelld Posted By: Peter_in_HungaryWhats the building construction, cavity wall, solid brick, stone/rubble, TF or what have you?Thanks, edited...]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260746#Comment_260746 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260746#Comment_260746 Thu, 24 Aug 2017 18:50:11 +0100 Nick Parsons
Highly air-tight? Or high permeability?

MVHR?

I'd look at aiming for 3 or less, and hopefully significantly less. I know 2 people who have got around 1 ACH on retrofit.]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260749#Comment_260749 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260749#Comment_260749 Thu, 24 Aug 2017 19:50:37 +0100 tony Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260750#Comment_260750 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260750#Comment_260750 Thu, 24 Aug 2017 20:12:50 +0100 Nick Parsons Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260752#Comment_260752 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260752#Comment_260752 Thu, 24 Aug 2017 20:42:49 +0100 jfb At the start of the project I wasn't even sure if I would need MVHR as I had no idea what AT result I would achieve so I am glad I did install mechanical ventilation. I was also the one doing most of the AT detailing which is maybe harder if you are trying to get a reluctant builder to do it.
A lot comes down to detailing of wall/roof junctions and between different materials (what are you doing from the external render to new windows?)
My point is I think if you are committed to achieving a good result (which it would appear you are) you might be pleasantly surprised by what you can achieve.

What is the planned roof build up?]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260756#Comment_260756 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260756#Comment_260756 Thu, 24 Aug 2017 22:01:12 +0100 gravelld Posted By: tonyThat won't be easy due to the cavity wall constructionBut... cavities filled, and inbound of the AT layer anyway, because AT layer going over the roof. Still a problem?]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260757#Comment_260757 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260757#Comment_260757 Thu, 24 Aug 2017 22:01:40 +0100 gravelld Posted By: Nick Parsons''The current AT is not measured, but I'm pretty sure it's high.''
Sorry, meant AT is low, permeability high.]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260758#Comment_260758 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260758#Comment_260758 Thu, 24 Aug 2017 22:05:02 +0100 gravelld Posted By: jfbA lot comes down to detailing of wall/roof junctions and between different materials (what are you doing from the external render to new windows?)Tape I guess. Would be installed in the insulation layer, so not sure on the detail exactly.

Posted By: jfbWhat is the planned roof build up?
Again, not finalised detail, but probably:

plasterboard
Rafters+insulation
AT layer
Insulation
WT layer
Covering

I realise I need to be careful about moisture regards the AT layer in the middle of insulation. But the reason it's there is so it can more easily meet the layer at the current external render.

Thanks for the numbers people, it's good to know that 3 is at least something to aim at, and 1 is even achievable.

What I haven't said yet is this is an incremental retrofit... just to add a bit of complexity.:tongue:]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260775#Comment_260775 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260775#Comment_260775 Fri, 25 Aug 2017 14:52:09 +0100 jfb OSB handy for walking around on the roof while installing the insulation on top.]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260777#Comment_260777 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260777#Comment_260777 Fri, 25 Aug 2017 15:51:15 +0100 gravelld Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260792#Comment_260792 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260792#Comment_260792 Sat, 26 Aug 2017 12:30:39 +0100 jfb There are quite a lot of previous posts on GBF on ways of making OSB airtight.]]> Target air tightness for retrofit https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260803#Comment_260803 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15252&Focus=260803#Comment_260803 Sat, 26 Aug 2017 20:50:35 +0100 gravelld