Insulating a suspended floor but hit a snag.

[Wockawocka]

I'm insulating a floor today with 100mm pir and the joists are also 100mm. Not a problem, however the joists rest on beams in several places.

I'm worried about cutting off the airflow in the space underneath.

There are holes in places where the brick hits the concrete base but I'm concerned these aren't enough. Would cutting the beam in places be advised of should I leave well alone?

I've also considered tapering the 100mm up to 50 across those points.

Images attached for further explanation

 

[DAZB]

You should be ok with the existing holes in the sleeper walls and check outside for the number and location of any airbricks to gain an idea of the ventilation and any crossflow and you may be able to add more airbricks in locations if you need to.

 

[Ivor Windybottom]

I think you're right to be concerned. I'm also looking at the wall across the bay - is there a route through this?

You'll definitely constrict the airflow. A typical sleeper wall has gaps between every brick. It looks like you have all of three, all way down away from the floor.

I reckon you need a look at the whole room, see where the airbricks are and work out if you're going to create a problem.

I'd be tempted to get rid of the lot, put thicker PIR down and a concrete floor on top. I don't know whether it would be acceptable to build up from the concrete oversite subfloor you already have - if so then it would be pretty simple, a huge improvement, super-warm, rock solid and no worries about damp and rot ever.

Just an unqualified suggestion! But possibly worth thinking about, and may be less work and similar expense.

 

[Wockawocka]

Thanks Daz. I was looking at it earlier and thought it could be an idea to remove a brick in places under the beam the joists rest on (where there's a full one between the joists)?

 

[DAZB]

Thanks Daz. I was looking at it earlier and thought it could be an idea to remove a brick in places under the beam the joists rest on (where there's a full one between the joists)?

It wouldn't hurt to knock out more bricks along the top course, say one every 750mm -1000mm or so between joist spacings, as the beam is spanning everything and it will not affect it structurally. The centre beam looks like a wider timber so you may have a double course of bricks there so good solid support.

 

[Wockawocka]

Yeah I think that's the solution. I've also considered popping a vented brick in there. I'm going to start early on it today so I'll post the results here to later.

Concreting the floor would mean doing the whole ground floor and time, as well as costs prevent me from doing this. I also like having the supply cables underneath.

 

[cdbe]

I'd just ramp up the celotex to 50mm over the wall plates - knocking out bricks is time consuming and could destabilise the wall.

 

[Koolpc]

I'd just ramp up the celotex to 50mm over the wall plates - knocking out bricks is time consuming and could destabilise the wall.

I disagree....

 

[cdbe]

I disagree....

Fair enough. I can only speak from my own experience, this is what's happened to bits of my dwarf walls after someone has knocked a few bricks out. I suppose in fairness I don't have a wall plate and I don't know how carefully they were removed (probably a lump hammer given how much of the surrounding brickwork they loosened). When more than adequate airflow can be provided over the top of the wall plate why bother.

 

[Notch7]

Thanks Daz. I was looking at it earlier and thought it could be an idea to remove a brick in places under the beam the joists rest on (where there's a full one between the joists)?

I would do this

removing a brick keeps the air flow all in one plane and ensures integrity of the insulation u value

tapering the insulation means a more difficult path for the airflow

 

[Koolpc]

Fair enough. I can only speak from my own experience, this is what's happened to bits of my dwarf walls after someone has knocked a few bricks out. I suppose in fairness I don't have a wall plate and I don't know how carefully they were removed (probably a lump hammer given how much of the surrounding brickwork they loosened). When more than adequate airflow can be provided over the top of the wall plate why bother.

View attachment 357662

Yikes!

 

[phatboy]

Tapering the insulation would force the air to pass more quickly directly under the insulation, which could result in cooling it. Which isn't what you want.

Taking a brick out wherever is easy would be preferable IMO.

 

[cdbe]

My god! There's some serious physics going on in this thread. My mate works at BAE, I'll see if he can rig up this arrangement and test it in the wind tunnel in his lunch break!

 

[Wockawocka]

Update:

So after all the advice here (Thank you) I stepped back and looked at things in a logical way.

There's one air brick letting air in from the East leading to another two West. Then there's three at the North side of the house. In all these cases there's gaps in the sleeper walls letting air through.

As I pulled up more of the floor I realised that yes, between the floor joists there was a lot of space for the air to flow through but whenever it hit a supporting wall it tapered down to just a few joists with brick between the joists. On that basis I realised I didn't need all that air flow so took 50mm out of the insulation on every third piece. This would have minimal impact ot the U value and based on how ****ed the joists were anyway (31cm to 34cm apart in places) making it hard to get a super tight 100mm depth fit even with expanding foam seemed more detrimental to the u value than cutting a bit of insuation.

When I looked at removing bricks many were still supporting half bricks above and I just really didn't want to weaken anything. I also considered air bricks to replace the ones I looked to remove from the sleeper wall but they're still mostly brick and the sleepers are imperial so I noped out of that.

Also I thought practically, just how much air flow do you need in the void. It's to prevent damp and moisture build up, so long as there's a continuous flow wth some turbulance it should even out the moisture in the space and flow out.