Pros and cons of different types of underfloor insulation

Home Retrofit
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My planned underfloor insulation and airtightness work has stalled due to suspended wooden floor timbers showing high moisture readings, mostly in the front wall of the original parts of the house (which happens to be south facing).

I’m slowly addressing some of the problems identified, but improving airflow before insulating isn’t proving as simple as it sounds.

The house is detached with filled cavities on E/S/N walls. The W facing wall has a finger cavity. Joists run N-S along the longer walls. There are ample airbricks to 3 sides (E/S/W) of the house, and also several on the N facing rear extensions. There could be more added to the rear extensions but pretty sure they would have little bearing on the through airflow. from front o back of house.

All the 17? airbricks are almost but not quite 2 bricks above ground level and sit directly on the damp proof course, which is bitumen in original part of house and membrane in extensions. The top of the airbricks are lower than the tops of the joists but not below the bottom of the joists. Given that most of these are on E/W facing walls and that joists run parallel and generally quite close to these walls, the joists get in the way of any airflow, and it looks as they would make installing periscope type airbricks tricky.

There are signs of two or three partial openings between the old external wall at the back and the rear extension. These could in theory be opened up to provide effective airflow between the original building and extensions. Except that the rear extensions only have a void of 30cm, and less in places, rather than the 1m in the original building, so15cm clearance or less below joists! Which makes insulating to a sufficient depth tricky as well.

Various people have suggested various ways to insulate the extensions (two phases, one c 1990, and a second c2000) while maintaining airflow through from the original part of the house.

a) remove joists, put in pipes for through ventilation from main building to outside, pour in more concrete and insulate above that
Disruptive, higher embodied carbon, but well understood solution

b) put in pipes for through ventilation from main building to outside, put in thick layer of insulation below and between joists sitting on concrete base
Never heard of this method until today, so wary of implications

c) put in short pipes for through ventilation from main building into extension, insulate between joists as originally planned
Compromised airflow but straightforward and well understood

d) put in short pipes for through ventilation from main building into extension, install floating insulation above joists
Another option that’s new to me. I want to maintain level access throughout the ground floor and The costs and disruption would far outweigh any pros.

Does anyone have any experience of b) and d)?

And of directing airflow from airbricks under joists in tight spaces?

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Do you know whether there is any support for the joists other than at the ends? My own airbricks have largely been lost due to extensions and otherwise resemble yours except that there is a network of subfloor support piers providing extra strength and rigidity to the floor. This means that if would be possible to cut notches or bore large holes though joists blocking airbricks.

It seems to me that you need different solutions for the extension and the original house. I would convert the extension to insulated solid floor. The original I need to know more about before deciding.

You need a plan for the original before changing the extension floor, in case ventilation needs to be routed though it.

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How deep is the underfloor space?
What is the subfloor made of?
Any chance that you know the thickness of the subfloor?
Are there any signs of damp in the brickwork or subfloor?
Is the subfloor fairly horizontal throughout the original house? (Steps or inclines anywhere?)
Is there any flood risk or flash flood risk, taking account of climate change?

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As a potential alternative you could have a look at what Flo Collier has done to her house in Wilmslow https://www.humblebee.eco/post/earth-floor-1-of-2?utm_source=linkedin&utm_medium=blog.post-promoter&utm_campaign=bf8a901b-5417-451c-a8be-152a220a744d

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There are lots of dwarf walls in the original part of the house, but the joists aren’t as hefty as current building regs require, so the advice so far has been to make holes in the dwarf walls to improve airflow. When a new consumer unit was put in for the heat pump, I didn’t realise that the young ‘assistant’ electrician was going to drill holes in the middle of joists to feed a cable through from the router on its journey to the airing cupboard (now the ‘plant’ cupboard’). Obviously the heat pump installer hadn’t told him that insulation would be installed between joists, and I came home in the afternoon to find it done and electricians gone.
I’ve come to a similar conclusion that the extensions probably need a different treatment. And I believe that there is a plan for insulating between joists in original, just that the installer hasn’t shared the details with me beyond a stock sketch, which doesn’t tell me much, and certainly hasn’t shared any detail about how airflow throughout the original part will be affected, beyond the few holes in the dwarf walls that they have already made.

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As I mentioned, my joists are on piers, so free ventilation. I have a brother that once lived in a Victorian house with lattice dwarf walls, a bit like this:

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So the concept of underfloor ventilation was known in the 1860s, unless it was just a measure to save bricks!

How easily could you lower parts of the dwarf walls between joists? Or make lots of holes between them? I wouldn’t want to make holes under the joists in case the strength is compromised.

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Any chance of sharing?

To get the ball rolling, here are mine:

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In fact most of the underfloor has space for 300mm insulation. :+1:t2:

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Apologies for only answering some of your questions earlier.
How deep is the underfloor space?
In the original part of the house the underfloor space is abt 1m.
In the rear extensions there is maybe only 2-5cm below joists in the kitchen and around 15cm in the dining area. But without lifting more of the floor it’s hard to tell.
What is the subfloor made of?
I thought the subfloor in the original house was just rammed earth - but it may be very old concrete. Whatever it is has become covered with loose matter over the decades. As rubble, rotting wood, broken bricks, tiles etc has been removed from under the utility room some fine roots found coming up through/at the outside edge of the subfloor. On a subsequent visit into the void I realised that the subfloor seemed hollow when tapped with a broom handle.
The subfloor of both parts of the extension is concrete.
Any chance that you know the thickness of the subfloor?
No
Are there any signs of damp in the brickwork or subfloor?
In the original house there are what look like dark damp patches of ground, with some lighter, drier patches. Under the bay at the front of the house, where some timber moisture readings are > 20%, the subfloor remains dark and dank, despite having all rubble removed and swept clean several months ago.three airbricks across the bay.
Is the subfloor fairly horizontal throughout the original house? (Steps or inclines anywhere?)
No steps, or obvious inclines seen.
Is there any flood risk or flash flood risk, taking account of climate change?
We are close enough to the River Mersey (to the south) to have had occasional flood alerts from the EA - although the area affected has still been several hundred metres away. There is an underground stream to the north of the house that has caused visible flooding of a school playing field.

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Even if you don’t get flooding in future the flood risk area is near enough to expect increased subsoil water, so damp proofing and resilience need beefing up.

Your dining area looks to be the easiest to deal with if 10-15 cm insulation is adequate for you. Ideally of course you would want more, but the work and cost start to sky rocket. 10 cm would be airtightness/waterproof membrane, 10 cm hydrophobic insulation, another membrane and screed on top. You may be handier than me but I would want professional help for at least the screed. Use a waterproof addmix in the screed.

You could achieve 15 cm by DIY. Check the exact depth and if necessary use self leveling compound to bring the depth to the same thickness as the insulation and chipboard combined. Start off with the same membrane, 15 cm of the same insulation and top off with a floating chipboard floor. This is my preferred choice as it would be slightly warmer and can more easily be taken up again if you ever decide to bite the bullet and upgrade to something thicker.

The kitchen will need a similar solution to the chipboard one above but with much less insulation. There isn’t room to use screed. Alternatively you will need to lower the sub floor, which is a much bigger job.

The rest of the house is trickier.

There could be a skim of concrete over packed earth, the earth could have shrunk where it is driest leaving the hollow you appear to have discovered. You certainly can’t be installing anything over a hollow without knowing that it will remain self supporting for the life of the building. Regrettably, the sub floor needs to come up, hardcore and concrete laid and then the underfloor area waterproofed. (See the cross section above.) You can then fill the void with water resistant insulation and top off with screed or chipboard.

PIR is damp resistant to a point and EPS is more so. For the thinner insulation I suggest PIR for its marginally better U value and for the main house I suggest EPS for its improved waterproofing and price. You can’t have too much insulation and it’s cheaper than concrete. The main house subfloor is deep enough for a radically different product, but at a price. You could use foam glass or Leca. Both are impervious to water.

The resultant floors should be draught free, flood resilient, warm, maintenance free and impenetrable to vermin.

What you cannot achieve is warm(er) extension floors and a suspended timber floor in the main house.

For a price you could include UFH in the new floors. Useful if you have plans for a heat pump or if your boiler can modulate right down. Possibly not a good idea in the kitchen as there would be inadequate insulation below the pipes.

As an afterthought: the house is detached, so you have access to the outsides of the walls all round. Could you manage enough underfloor ventilation below timber depth on three sides to allow a timber floor in the main house and solid in the extensions?

Judging by your description you would need to dig down and use periscope vents. You would also need to address the dampness under the floor, which if bad enough might actually require rebuilding the subfloor as above.

Have you contacted any independent dampness surveyors? Where is your damp proof course? Current building regulations require the DPC to be at least 150mm above ground level but of course once built ground level can change. (My DPC is 15mm above ground level at the back, 300mm at the front.)