Beam & block covering cavity: thermal bridge

[James_H]

I'm not sure exactly which would be the right sub forum but hopefully this is the right one.  Though I'm not posting in the insulation board to avoid a biased answer.

 

I'm having an extension built with construction details and thermal standards to be broadly the same as the newish build existing house.  The beam and block floor has just been put down, but it is unlike any drawing I have seen where the door openings are to go.  My construction drawings do not detail the openings, but match to what I would expect in that the ends of the beams and blocks go as far the edge of the inner block where it meets the cavity.  

 

For the sliding door openings, as built the beams and blocks have completely bridged the cavity.  This bothers me that it might not be right and leave me with a thermal bridging problem.  This is compounded by having no insulation in the cavity at or below the beam/block level.  I have attached the drawings and a sketch of what has been built so far.  The dotted lines will be finished floor level and patio outside.  Note that DPC level for walls started already is higher up than that line as required.  

 

Am I right in being concerned about what has been laid? If not right, is it just not best practice, or is it something building control would be unhappy about?

 

 

 

Edited October 26, 2019 by James_H

[epsilonGreedy]

Looks wrong to me, my main concern would be moisture ingress into the b&b floor structure though thermal bridging is a secondary issue. 

[PeterW]

It’s fine - just make sure the DPM /DPC is properly lapped under the insulation and sealed properly to each other. 
 

The screed needs to be isolated from the outer wall, beams are effectively cold anyway so that’s not unusual into an opening. 

[Oz07]

You will have insulation ontop of beams so not to disastrous. As @PeterW says lap some dpc from where they sit right over the ends. Insulation should carry on below the cavity 150mm below top of floor insulation. 

[James_H]

Thanks for the replies. It's not quite as much of a concern as I had first thought. I anticipate the DPC layer to be between the blocks and insulation.

 

The PIR wall insulation technical doc advises to take it down to the bottom of the beams for this type of construction. Though with no cavity left at the door openings, the perimeter can have no more than the 120mm planned floor insulation depth below its top. Though it can go lower other than at the door openings. 

Edited October 27, 2019 by James_H

[epsilonGreedy]

Is there any dpc between the vertical joint at floor blocks and outer wall?

 

Also was the floor delivered as a designed kit from the beam manufacturer?

[James_H]

1 hour ago, epsilonGreedy said:

Is there any dpc between the vertical joint at floor blocks and outer wall?

 

Also was the floor delivered as a designed kit from the beam manufacturer?

There's no DPC and the floor blocks and beam ends are tight to the outer wall.

 

I cannot be sure but I do not believe it was as a designed kit.

[PeterW]

As long as there is a proper DPM lapped onto the outside brickwork under the DPC then there won’t be an issue. Just make sure there is a decent thermal break when the slab is cast. 

[epsilonGreedy]

12 minutes ago, PeterW said:

As long as there is a proper DPM lapped onto the outside brickwork under the DPC then there won’t be an issue. Just make sure there is a decent thermal break when the slab is cast. 

 

Is there no concern about moisture the entering the B&B floor structure below the DPM at those door thresholds?

[PeterW]

Via what route ..?? Brickwork isn’t porous enough to let that much moisture through - more likely to get damp from below. 

[Cpd]

Just a quick question as people are talking about this, with a beam and block floor what is best Practice with regards to the gap below the slabs and down to the ground below ? Obviously insulation etc is all above the beam and block but is there are recommended gap below or can this be back filled ? Again I am just interested as a friend is building with beam and block and it’s perked my interest. (I expect it’s whatever the SE has told him to do but please humour my ignorance) thanks   

[Temp]

Our BCO wanted the void vented despite there being no wooded joists to rot.

 

We left it empty as we had run out of bodies that needed disposing of ?

[epsilonGreedy]

9 hours ago, PeterW said:

Via what route ..?? Brickwork isn’t porous enough to let that much moisture through - more likely to get damp from below. 

 

 

Why was the concept of a DPC ever invented?

 

From memory an F2 standard brick can absorb 12% water which sounds porous to me. Block & Beam manufacturers emphasize the importance of not letting the dpc under beam ends shift out of place as beams are moved into final position, they also caution about the risk of dpc getting punctured as heavy beams are dropped into position.

 

The creation of a large moisture bridge as shown in the OP's hand sketch is clearly contrary to the design intent of keeping the B&B structure as dry as possible.

[epsilonGreedy]

2 hours ago, Cpd said:

Just a quick question as people are talking about this, with a beam and block floor what is best Practice with regards to the gap below the slabs and down to the ground below ? 

 

 

Not so much "best practice" it is more a case of clear absolute minimum clearance as defined in building standards. It depends on the the soil type and risk of heave. 200mm for clay, less for other soil types, 100mm or 150mm I think.

Edited October 28, 2019 by epsilonGreedy

[epsilonGreedy]

3 hours ago, Temp said:

Our BCO wanted the void vented despite there being no wooded joists to rot.?

 

My BCO popped over for a chat the other day when inspecting the neighbour's self build. We talked mainly about the building control industry landscape post Grenfell, when I asked if I had enough void vents for my 3 sided outbuilding workshop he said oh plenty, they are just there to placate the concrete beam manufacturers who are concerned about condensation settling on the underside of the beams.

 

More evidence the floor structure should be kept as dry as possible, I am sure we have all driven under a 1970's motorway bridge suffering from concrete cancer.

Edited October 28, 2019 by epsilonGreedy

[PeterW]

6 minutes ago, epsilonGreedy said:

 

Why was the concept of a DPC ever invented?

 

From memory an F2 standard brick can absorb 12% water which sounds porous to me. Block & Beam manufacturers emphasize the importance of not letting the dpc under beam ends shift out of place as beams are moved into final position, they also caution about the risk of dpc getting punctured as heavy beams are dropped into position.

 

The creation of a large moisture bridge as shown in the OP's hand sketch is clearly contrary to the design intent of keeping the B&B structure as dry as possible.

OK in over 25 years I’ve only ever seen moisture on the inside face of a brick once, and that was due to it being buried - concrete beams by design can be submerged in water indefinitely, this is such a minuscule risk it’s not worth worrying about. 
 

If the OP is worried then slip a piece of DPC between the front and back faces, but where it is really needed is when the screed is done, and also this needs to be insulated. 

[PeterW]

2 minutes ago, epsilonGreedy said:

More evidence the floor structure should be kept as dry as possible, I am sure we have all driven under a 1970's motorway bridge suffering from concrete cancer.

That is utterly irrelevant unless you are going to be post tensioning and salt gritting your beams on a regular basis, whilst using a semi porous top coat and allowing thousands of gallons of water to flow over them over a 30 year period. 
 

Or alternatively, I’d love to see how your BCO can make condensation occur on a beam that is the same temperature as the outside air..??? 

 

 

[epsilonGreedy]

1 hour ago, PeterW said:

OK in over 25 years I’ve only ever seen moisture on the inside face of a brick once, and that was due to it being buried - concrete beams by design can be submerged in water indefinitely, this is such a minuscule risk it’s not worth worrying about. 
 

If the OP is worried then slip a piece of DPC between the front and back faces, but where it is really needed is when the screed is done, and also this needs to be insulated. 

 

 

This is a very odd thread. Some of your posts give the impression you view rising damp and dpc as a passing industry fad though in other posts you emphasize the importance of the floor dpm lapped to dpc on top of the B&B floor which by implication says you accept that damp gets into the floor structure.

 

There is clear industry consensus that effort should be put into stopping rising damp at the bottom of a B&B floor. The OP's builder has deviated from B&B construction norms by creating a moisture bridge.

 

[epsilonGreedy]

1 hour ago, PeterW said:

Or alternatively, I’d love to see how your BCO can make condensation occur on a beam that is the same temperature as the outside air..??? 

 

 

My BCO is past normal retirement age and went private after a full term career in local authority building control. He was also the UK representative at the annual European congress of building control inspectors for some years, he is a vastly impressive expert who has already saved me significant dosh.

 

The BCO did not necessarily agree with the condensation issue, he was explaining the thinking at the B&B design office when they specify the number of floor void vents as part of a floor kit specification. Do you agree with the concept of floor void ventilation?

[PeterW]

They need a ventilated void but it’s a very small flow rate.