Subfloor ventilation required???

[lefthandturn]

Hello all...
I can't seem to find a definitive answer to my quandary so I thought I'd hit up the experts.

I'm currently building a garden room/she shed/he said/we said, never again.

So, I had a pretty solid foundation after knocking down the old garage and decided to build straight on top of it for the new build.
Standard build of 100mm timber frame, celotex/recticel insulation, 9mm osb skin, housewrap, etc.
I built a perimeter foundation of solid concrete blocks to sit the structure on.
Originally I intended to infill this with concrete for a totally solid internal floor, but financially and labour intensively, I've ended up building a timber subfloor instead.
The joists rest upon the perimeter blocks (woth intermittent blocks throughout the middle for support) and underneath there is a full covering of DPM, to suppress any damp from the original slab/concrete foundation.
So at this point, I'm ready to lay my subfloor boards, but I have no ventilation for the joists.
Although there are no signs of damp, and all subfloor is protected from penetrative damp, does anyone see this being an issue moving forward.

Walls and ceiling will be fully covered with vapour barrier attached to ground DPM, so effectively, entire internal structure will be sealed.

 

[JohnD]

Yes.

You have a suspended timber floor.

 

[lefthandturn]

So, yes, you see it being a problem? Can you explain why?

 

[JohnD]

Suspended wooden ground floors are prone to damp, stagnant air, causing rot. That's why they are always ventilated.

One airbrick per linear metre of wall, on at least two sides to give through flow, is not too many.

 

[lefthandturn]

Yeah, that's a definitive answer I keep coming to...but I guess what I'm trying to find out, is why, as in the picture, if you have a vapour barrier that also encapsulates the subfloor also, why that would be a problem? After all, it's all timber.
Basically it will prove very difficult for me to create airflow in the subfloor.

 

[cdbe]

If it's entirely contained within the "dry" side of the building I think it would be ok. Presumably your DPM is continuous up the walls?

 

[lefthandturn]

Yes, a thicker DPM covers complete slab and up the wall to meet the vapour barrier which will be sealed together.
There's about 130mm height from floor to joists which is infilled with celotex also.

 

[mrrusty]

Problem you will have is that the dpm in contact with the ground will be cold, and any warmer air in contact with that dpm will condense on top of the DPM. You need ventilation to keep it dry.

It is extremely difficult to keep water vapour out of a space because of vapour pressure. Air at the same temperature but with different humidity will have a higher pressure the more water vapour it contains, so damp air has a tendency to move into dryer spaces.

 

[cdbe]

Problem you will have is that the dpm in contact with the ground will be cold, and any warmer air in contact with that dpm will condense on top of the DPM. You need ventilation to keep it dry.

It is extremely difficult to keep water vapour out of a space because of vapour pressure. Air at the same temperature but with different humidity will have a higher pressure the more water vapour it contains, so damp air has a tendency to move into dryer spaces.

I'm not entirely disagreeing, because I could be completely wrong, but other than the joists (which appear to be above a layer of PIR) isn't this just a bog standard garage conversion floating floor?

 

[mrrusty]

I'm not entirely disagreeing, because I could be completely wrong, but other than the joists (which appear to be above a layer of PIR) isn't this just a bog standard garage conversion floating floor?

@cdbe Reading it again I think you're right. but surely if it's a floating floor over PIR it doesn't need joists at all? If it's a suspended floor with timbers IMHO it needs ventilation, if it isn't, it doesn't. It sounds like it has 100mm joists with PIR between when in fact the joists are unnecessary and it could just be a floating floor over PIR...

 

[lefthandturn]

Added a pic to better describe.
So, yeah, I used up lots of off cuts of pir to fill in under joists, intended to spray foam all gaps and fill inbetween joists with pir also. My thoughts being more insulation is better than none, especially on cold floor.
The reason i used joists was to make a nore solid base for board and final flooring, and also it brings it up nicely to the door threshold (upvc) so it's not such a big step over it on entry.

 

[mrrusty]

Hmmm. That photo throws up a few comments....

1) You don't need the joists at all - another 100mm layer of PIR and then just float the floor on top. However, with regard to ventilation, you don't need any.
2) Those brackets are about as much use as a chocolate teapot - you're hanging all the flooring load on the contact point of 4 tiny screws. They'll bear down in to the timber in no time. To fix joists like that you use joist hangers https://www.cwberry.com/expamet-bat-type-sl-short-leg-speedy-joist-hanger
3) Unless the span is very short, those 4x2's are going to be bendy, (max span for 4x2's as joists is 1.3-1.5m) https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?class=C16&slope=0&load=1.5
4) I'm guessing it wasn't a sparky ran that cable - I'll leave it to the sparkies to comment on electrical installation....

Best bet - take out those "joists" and put in another layer of PIR. Float a tongue and groove chipboard floor directly over the top. PIR has a compressive strength of about 12 tonnes per sqm, so you don't need joists for additional support!

EDIT - Maybe I am miss-seeing the photo - if the ends of the joists are actually on the blockwork then ignore the comment on the brackets. The bendy comment stands though! If the intermittent blocks are not standing on the PIR, but on the original concrete you may well be OK with that too.

Perhaps I am being too negative... If it is all solid and supported, crack on - The cable needs sorting though - the loop you/someone has made for the backbox is way too short!

 

[lefthandturn]

Hmmm. That photo throws up a few comments....

1) You don't need the joists at all - another 100mm layer of PIR and then just float the floor on top. However, with regard to ventilation, you don't need any.
2) Those brackets are about as much use as a chocolate teapot - you're hanging all the flooring load on the contact point of 4 tiny screws. They'll bear down in to the timber in no time. To fix joists like that you use joist hangers https://www.cwberry.com/expamet-bat-type-sl-short-leg-speedy-joist-hanger
3) Unless the span is very short, those 4x2's are going to be bendy, (max span for 4x2's as joists is 1.3-1.5m) https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?class=C16&slope=0&load=1.5
4) I'm guessing it wasn't a sparky ran that cable - I'll leave it to the sparkies to comment on electrical installation....

Best bet - take out those "joists" and put in another layer of PIR. Float a tongue and groove chipboard floor directly over the top. PIR has a compressive strength of about 12 tonnes per sqm, so you don't need joists for additional support!

EDIT - Maybe I am miss-seeing the photo - if the ends of the joists are actually on the blockwork then ignore the comment on the brackets. The bendy comment stands though! If the intermittent blocks are not standing on the PIR, but on the original concrete you may well be OK with that too.

Perhaps I am being too negative... If it is all solid and supported, crack on - The cable needs sorting though - the loop you/someone has made for the backbox is way too short!

You are correct with your edit sir. The piddly little brackets are for guide only, but in hindsight I should have used hangers, as I've over engineered in many other ways.
Yes, the intermittent blocks are concreted straight on to the foundation (above DPM) and are all within 1.5 meter squared, and is full of noggins throughout pinned with double and triple 100mm screws. It's pretty darn solid.
I was more comfortable working it this way, and also the original slab was way uneven throughout with lumps and bumps everywhere.

My sparky mate said he was fine with 4-6 inch loops for outlets but you're right, I think I'll need to rectify this