Green Building Forum - Underfloor hatch advice

Underfloor hatch advice

randomRobin — Wed, 30 Aug 2023 09:21:17 +0100

Hello all
New forum member here.
We will be getting underfloor insulation added to our suspended wooden floor in a few weeks. I have a couple of questions about the access hatches. We currently have decent floorboards with a few hatches already cut from the previous owner of varying quality. They are generally two floor boards wide and 500mm long or so, see pictures.

1) I'm wondering about what to do about insulating these? What is recommended?
The best thing I can think of just now is to ask the installers to put blocks of insulation under the boards that can be removed if required.
I'd want to keep access to the underfloor area for future servicing of cables, pipes etc, and also to be able to show the EPC assessor the insulation.

2) The hatch in the kitchen is not in a great state as the boards have gaps between and wobble. See the photos attached. Is there an easy way to improve this?

Plan is sheep's wool held by netting.
House is ~1900 with decent depth void (800mm to 1m). Solid stone walls.
Thanks

Underfloor hatch advice

Dominic Cooney — Wed, 30 Aug 2023 09:28:41 +0100

Is the 800mm to 1m void below the bottom of the floor joists? and what depth are the joists?

Underfloor hatch advice

randomRobin — Wed, 30 Aug 2023 09:29:44 +0100

photo attached

Underfloor hatch advice

randomRobin — Wed, 30 Aug 2023 09:30:41 +0100

Posted By: Dominic CooneyIs the 800mm to 1m void below the bottom of the floor joists? and what depth are the joists?

Yes, 800mm to 1m void below the joists. Joists are six inches deep I think from memory.

Underfloor hatch advice

Dominic Cooney — Wed, 30 Aug 2023 09:34:25 +0100

So 150mm deep joists with 80 - 100mm void below.
Presumably all the floor boards will have to come up to fit the insulation, then re-lay them?

Could this be a candidate for fully filling with EPS beads instead?

Underfloor hatch advice

WillInAberdeen — Wed, 30 Aug 2023 10:05:36 +0100

Centimetres, not millimetres

Could be wrong, but in the photo it looks like short lengths of the boards have been cut, lifted out, replaced, but not screwed down, so they'll definitely be wobbly. Looks like the cuts do not all line up with the centre line of the joist (best practice) but instead some follow the edge of the joist (easiest but leaves the cut length poorly supported)

To get rolls of insulation under the floor IME I had to make a bigger access than that, perhaps 5 boards cut and lifted with care not to damage the tongues and grooves. Then after completion I screwed a 'lip' of battens all round the hole for the boards to sit on and filled between those with a removable piece of PIR. Then screwed all the cut boards back down with at least 4 screws each.

The area under the house is divided up by dwarf walls which may not relate to the partition walls between rooms. In our case I needed 3 access hatches, all in the hallway, to access the entire ground floor

It's really worth stopping drafts between boards and under skirtings while you are at it . Are you leaving the boards exposed, or covering them with some draft proof layer?

Underfloor hatch advice

Dominic Cooney — Wed, 30 Aug 2023 10:46:29 +0100

Posted By: WillInAberdeenCentimetres, not millimetres

sorry, I clearly haven't woken up yet!

Underfloor hatch advice

djh — Wed, 30 Aug 2023 11:38:13 +0100

We can't see what's underneath the hatch, so we don't know what battens etc have been used to tie the hatch together nor what battens etc have been fastened to the side of the joists to provide support. So it's difficult to make any statements about support or construction of the hatches at this stage.

Richard, what are you planning to do about airtightness? There's not much point in insulation without it. People often avoid the use of organic insulation under floors because of the risk of rot from water falling downwards (or floods coming upwards depending on the site).

As regards insulating the hatches, I'd be looking to make plugs of insulation, shaped like an inverted truncated pyramid, and fastening those to the bottom of the hatches. I'd make corresponding shapes in the insulation surrounding the hatch. This might be easier with something rigid but compressible such as EPS. Do the hatches just pull out? (I've assumed so).

Underfloor hatch advice

Peter_in_Hungary — Wed, 30 Aug 2023 13:52:32 +0100

For insulating the hatches as a last job I would fix netting across the void and fill with the same insulation as the rest of the floor, easier to get a good fit and it isn't as if you will need access more than once in how many years?

I agree with djh - I wouldn't use organic insulation under floors, apart from any rot issues the poison put on sheeps wool to deter moth and alike has been ineffective rare occasions and I wouldn't take the risk

Underfloor hatch advice

Mike1 — Wed, 30 Aug 2023 19:53:25 +0100

Posted By: Peter_in_HungaryI agree with djh - I wouldn't use organic insulation under floors, apart from any rot issues the poison put on sheeps wool to deter moth and alike has been ineffective rare occasions and I wouldn't take the risk

I agree too - I wouldn't use organic insulation under the floor, nor would I use sheep wool anywhere.

Underfloor hatch advice

randomRobin — Thu, 31 Aug 2023 09:15:13 +0100

Hi. Thanks for the comments. Here is a picture of the hatch with the boards removed.

We have bare floorboards on the ground floor - no carpet with a few rugs in places.

I was planning on sheep's wool as the house is ~1900 with solid stone walls. I understand that for traditionally built houses like this maintaining breathability is important, and I saw that English Heritage recommended natural materials for underfloor insulation. This was what let me down the path of sheeps wool as opposed to mineral wool.

For the same reasons as above I don't think I want 100% air tightness. I would like to stop any drafts but maintain breathability (if this isn't a contradiction).

Given the old house construction are there any alternatives?

We have 3 hatches currently, as you say, Will, the sleeper walls don't 100% line up with the interior walls. I expect that the installers will need to cut a further hatch in the lounge as the gaps in the sleeper walls to get under there are really tight for a person, let alone material. The other two existing hatches are less wobbly and better fitting than the one in the photos.

I like the idea of attaching battens round the side of the hatch to aid in sealing. Also, something as simple as screwing down the wobbly hatch in the photos when not in use will probably make a big difference.

Thanks again,
Richard

Underfloor hatch advice

djh — Thu, 31 Aug 2023 11:35:22 +0100

Posted By: randomRobinI saw that English Heritage recommended natural materials for underfloor insulation

I found HEAG086 Suspended Timber Floors; is that what you mean? On a quick skim read it seems they confuse the notions of vapour permeability, breathability and hygroscopicity and make some starnge recommendations as a consequence. They also show insulation fitted just between the joists and not extended below, which is a poor idea since it leaves the joist ends exposed to cold potentially moist air.

For the same reasons as above I don't think I want 100% air tightness. I would like to stop any drafts but maintain breathability (if this isn't a contradiction).

A building can be completely airtight and still breathable. (ours is, for a definition of airtight that includes 0.47 ACH). Breathability is to do with water vapour movement, not air, and the two can be varied independently, although there are many correlations. Don't go any further with your planning until you feel you understand this issue.

As a direct alternative to sheeps wool you could use rockwool or glass fibre wool (though I wouldn't recommend the latter since it is so difficult to work with). You could also use EPS though not other foam insulations such as XPS, PIR, PUR, phenolic.

Underfloor hatch advice

Peter_in_Hungary — Thu, 31 Aug 2023 12:23:19 +0100

Posted By: djhAs a direct alternative to sheeps wool you could use rockwool or glass fibre wool (though I wouldn't recommend the latter since it is so difficult to work with).

Posted By: djhYou could also use EPS though not other foam insulations such as XPS, PIR, PUR, phenolic.

Yes - except that it is difficult to get a good fit between the joists and the insulation and you would probably finish up foaming in between same. (or even plan to foam in at the outset).

With several cm of mineral wool under the floor boards any drafts are going to be v. minimal but could be further controlled by the inclusion of a breathable felt layer (e.g. roofing felt) incorporated.

Having the insulation breathable is IMO important not so much for the building but in the event of the inevitable spillage on the floor as this could seep down through to the insulation and will be difficult to dry out without some level of breathability. In this respect sheeps wool would be worse than mineral wool because the wool could start to rot whilst drying out whereas the mineral wool will not.

Underfloor hatch advice

Osprey — Mon, 04 Sep 2023 14:13:40 +0100

You could also use EPS though not other foam insulations such as XPS, PIR, PUR, phenolic.

Why not PIR in this case?

It seems like every UF insulation type has its proponents and detractors.

Underfloor hatch advice

djh — Mon, 04 Sep 2023 17:06:17 +0100

Posted By: OspreyWhy not PIR in this case?

Because it's a closed cell foam and not permeable.