How much ventilation under floor is enough?

[SJRSJR]

Hello

I've currently got some floorboards up in a ground floor utility room room which wasn't previously heated but soon will be. In the external cavity wall there's an air brick to the outside. I can't see from the inside but I can stretch towards it and feel a draught so I assume it's there. There's a 12" void below the joists which continues to the other side of the house where there's also an air brick, which is clear. It's not directly in line, does that matter?

By the outside wall I can see that some joists have been replaced so there's obviously been some damp in the past. Towards the inside of the room the boards that I've pulled up have rusted nails but the joists seem okay. I'm wondering if this is from previous damp or a sign of continuing damp. (The boards don't look like they've ever been lifted.)

An an internal brick wall backs on to a downstairs toilet and below floor level there are inch gaps between the bricks that I can put a ruler through. It looks intentional as there's no crumbing mortar. Would these be for ventilation between the rooms? Is it likely to be enough or should I try to remove a full one?

It's a 1920/30s house and I want to make sure there's enough ventilation before the flooring goes back, so any advice would be appreciated. Thanks

 

[Deluks]

One airbrick at each end doesn't sound enough, you can get dead spots if not enough flow, although 12" is a good air space.

If you can get another couple in it might be worthwhile.

The gappy bricks might be a sleeper wall, these are usually built with gaps to allow air flow.

Assume you are putting in insulation whilst the floor is up?

 

[JohnD]

Older houses often have insufficient airbricks, and those that exist are often clogged with cobwebs and dust. It sounds like you only have one at the front and one at the back. Clean them out and add some more. One every two metres is not too many.

Brush and shovel any dirt and rubble out of the void. A builders canister vac with a long pipe can suck a lot of loose stuff out.

If you hold a camera or phone under the floor it can probably take photos or video of the parts you can't see.

 

[SJRSJR]

Thans Deluks, I'll give it a go. I do have sleeper walls elsewhere, but the gaps are brick size, in a chequered pattern. I hadn't thought of insulation but now you mention it.... I presume it'd have to be PIR board? What's the minimum thickness I could get away with for it to make a difference? Even though it's only a utility room it's 20 sq m so if it's going to be big money I'd prefer to allocate it to the living room.

John D, thanks. I'll do that.

 

[Dereekoo]

Look at clause 4.14 of Approved Doc C of building regs for guidance

 

[endecotp]

I presume it'd have to be PIR board?

You can put mineral wool between the joists, held up with something like bailer twine stapled to the joists.

When this has come up before, I have suggested finding a small child, or a member of your local caving club, whom you can send down into the crawl space to do it for pocket money.

 

[SJRSJR]

As regards clearing the rubble, there's all sorts down there. Can bricks and the like stay? All in all it's cold down there but not musty.

 

[SJRSJR]

When this has come up before, I have suggested finding a small child, or a member of your local caving club, whom you can send down into the crawl space to do it for pocket money.

I think I'll tackle it myself first, I can't be doing with Building Regs and Social Services on my back

 

[SJRSJR]

Look at clause 4.14 of Approved Doc C of building regs for guidance

Thanks. I suppose I should be calculating the entire void, and not just the room. A couple more should do it.

 

[aveatry]

You can put mineral wool between the joists, held up with something like bailer twine stapled to the joists.

When this has come up before, I have suggested finding a small child, or a member of your local caving club, whom you can send down into the crawl space to do it for pocket money.

I did mine the same but my space is about shoulder width in height. I didn't want to cut 3 access holes as the floor was being sanded floorboards so I took out a section of the hit and miss dwarf wall so I could crawl to the next section. What a job that was but whilst I was under there I put in a lot of surround sound cabling and cat5 to all the TV stuff
At the time WiFi was crap and Bluetooth surround speakers were not a thing

 

[SJRSJR]

What about something like this, cut to a couple of inches wider than the joist centres. Staple it to the side of the joists so it forms a cradle?

That said, I only took a couple of boards up to install a radiator, insulating it will mean getting the lot up!

 

[aveatry]

When I crawled under mine I used rockwool slabs that are stiff enough to hold themselves - a tight fit gripping the old rough swan joists. But I did also use 2 or 3 pieces of wire stapled across the joists to give extra support, you can get coils of wire for gardening very cheap

 

[Deluks]

What about something like this, cut to a couple of inches wider than the joist centres. Staple it to the side of the joists so it forms a cradle?

That said, I only took a couple of boards up to install a radiator, insulating it will mean getting the lot up!

View attachment 394349

That's what I used, with rockwool loft roll in between. Rockwool slab is denser, but more expensive. PIR is better thermally, but more expensive again.

What I should've done is use a breathable membrane on the cold side, apparently the draught underneath can 'suck' the heat from the fibres of the insulation.
It's still an improvement though, get a decent staple gun, or better an electric one.

 

[SJRSJR]

You can put mineral wool between the joists, held up with something like bailer twine stapled to the joists.

Could anyone suggest how thick the twine should be and the spacing needed to stop the insulation sagging? It’s Knauf 44 100mm. Thanks

 

[endecotp]

Could anyone suggest how thick the twine should be and the spacing needed to stop the insulation sagging? It’s Knauf 44 100mm. Thanks

It will depend on how snug the fit is, i.e. exactly what the joist spacing is. I have also used the orange fencing in stud walls. If crawling under a floor, I think a spool of something would be easier to work with. Put in as much as seems necessary at the time. Don’t worry about the insulation becoming more saggy later, it’s pretty stable stuff unless it gets wet.