Cavity fill lean mix and telescopic air ducts.

[epsilonGreedy]

I am having trouble interpreting the regulations for the height of weak mix cavity fill with relation to DPC and ground level. One interpretation would mean the lower end of my telescopic air vents would be encased by the infill.

 

Would it be ok to partially encase the ducts?

 

 

 

[PeterW]

Why are ducts specified..? Is this for Radon..?

 

Ducts are normally only used in suspended timber floors to ensure there is airflow to stop damp causing issues with the timber

[epsilonGreedy]

8 minutes ago, PeterW said:

Why are ducts specified..? Is this for Radon..?

 

 

I thought they are mandatory for ventilation of a beam & block floor void?

 

Not yet looked into the radon risk, I thought it was a Cornish thing and so this is not yet in my top-10 ToDo list. My plot is located where the Lincolnshire Fens rise up in elevation and become the hilly Lincolnshire Wolds. 

Edited July 18, 2018 by epsilonGreedy

[PeterW]

6 minutes ago, epsilonGreedy said:

 

I thought they are mandatory for ventilation of a beam & block floor void?

 

Not yet looked into the radon risk, I thought it was a Cornish thing and so this is not yet in my top-10 ToDo list. My plot is located where the Lincolnshire Fens rise up in elevation and become the hilly Lincolnshire Wolds. 

 

Depends on who's rules you read...

[epsilonGreedy]

1 minute ago, PeterW said:

Depends on who's rules you read...

 

 

Oh, is it just an NHBC thing?

 

Though it is tempting not to design in a floor void air chilling mechanism I know my plot is drainage challenged hence encouraging airflow feels like the right thing to do.

[PeterW]

Concrete Slab vs Beam/block is an interesting concept - consider them if you don't have clay soil. 

 

Where a sub floor void exists you are supposed to ventilate, but removing the void removes the requirement.... Any reason for B&B btw..?

[epsilonGreedy]

6 minutes ago, PeterW said:

Any reason for B&B btw..?

 

1. It is the done thing in these parts.
2. My veteran building control inspector's normal inclination to towards simplification and ignoring modern building frivolities but even he gave a reassuring wink re. Beam & Block.
3. A minor surface flash flood risk means my DPC is 150mm higher than standard.

[PeterW]

So if you have a propensity to ground water and potential flooding, the last thing you need is an underfloor void of ANY form, vented or not, as water will find its way in !!!!

 

To get anything like a 150mm cavity, 150mm beam, 120mm insulation and min 65mm screed, you will end up with your void at 180mm below ground level which is not ideal in a flood zone

[joe90]

I am on solid clay with a winter water table just under the surface of the garden. We did a slab on ground, 800mm trench founds, cavity of 200mm below DPC filled with XPS down to trench concrete to stop thermal bridge. We also back filled the trench outside with 50mm stone as a French drain with a pipe extended out to a ditch in the road. I would not want a void under my floor!

[epsilonGreedy]

8 minutes ago, PeterW said:

So if you have a propensity to ground water and potential flooding, the last thing you need is an underfloor void of ANY form, vented or not, as water will find its way in !!!!

 

 

We are talking about a 1 in 30 or 1 in 100 year flash flood risk, not Noah's biblical flood ascending to my foundations.

 

I mentioned the extra 150mm in dpc height in order to illustrate the amount of material that would be needed to create a solid floor base. At present all I have is 600mm x 600mm of poured concrete and a remaining trench depth of between 550mm and 650mm, so I can make a tactical change if convinced, happy to keep discussing.

 

Not sure why a passive slab floating on a swamp would be inherently better to a trad foundation standing proud of the swamp with a drying air current wafting through its floor void?

 

Anyhow the site is not a swamp, a fully laden 8 cube concrete wagon drove right up to within inches of my open trenches last week. Not a dribble of water at the bottom of my trenches.

[joe90]

2 minutes ago, epsilonGreedy said:

Anyhow the site is not a swamp, a fully laden 8 cube concrete wagon drove right up to within inches of my open trenches last week. Not a dribble of water at the bottom of my trenches.

 

Ha, at the moment my ground is like concrete and even the jacking legs on my JCB does not dent the ground, go back six months and I could not even drive my JCB anywhere other than the drive or it sank into the mire. Despite owning my own JCB I had to pay a contractor with a swing shovel (tracked digger) to instal my sewerage treatment plant and even that turned to ground into something resembling the Somme.

[epsilonGreedy]

18 hours ago, joe90 said:

Ha, at the moment my ground is like concrete and even the jacking legs on my JCB does not dent the ground, go back six months and I could not even drive my JCB anywhere other than the drive or it sank into the mire.

 

 

I think this would be true of half the building sites in the country but it is a massive extrapolation to conclude beam & block & void is a suspect building practice unsuitable for these sites.

 

The notion of a drainable void sump is something that interests me and another BuildHub member described such a feature recently as part of his foundation design.

[joe90]

6 minutes ago, epsilonGreedy said:

I think this would be true of half the building sites in the country but it is a massive extrapolation to conclude beam & block & void is a suspect building practice unsuitable for these sites.

 

 

That is not what I meant, I was simply saying how different six months made to our site, old maps of the area show it as a boggy moor. Although we did not go beam and block I have nothing against it as a method, just one we did not chose.

[epsilonGreedy]

2 minutes ago, joe90 said:

That is not what I meant...

 

 

I know, I was responding indirectly to @PeterW's intervention.

 

In 6 months our weather has swung from so dire it put the building industry into temporary economic contraction which then dented national GDP and now we have the driest spell since the drought of 1976.

[PeterW]

B&B isn’t the only answer though - it’s quick and simple for volume builders and allows for pretty poor prep and you can just drop them on and you have a working floor. 

[CC45]

B&B is a godsend when you've had no end of rain and you just want a floor in!  I was thankful when ours went in.

[epsilonGreedy]

22 minutes ago, PeterW said:

B&B isn’t the only answer though - it’s quick and simple for volume builders and allows for pretty poor prep and you can just drop them on and you have a working floor.

 

 

Henry Ford had a similar idea and the automobile industry never looked back.

 

There are two approaches to selfbuild, the methodical and cautious approach involves ground surveys, core samples, structural engineers and specialist slab contractors.And while you are at it best bring in a CDM 2015 consultant, its only 1.5% of build costs. Before you know it £30k to £40k has gone just to get up to dpc with a floor.

 

By going mainstream block & beam I have avoided all that. Total costs to date are £650 for the house and garage foundation dig and about £2700 for concrete plus around £300 of hired in labour/advice. I have not costed out the next month to a working floor in detail but it looks like 850 blocks for the floor, about 1000 blocks for the footings, some coursing bricks and engineering bricks and about £800 of labour.

 

On a separate issue I am genuinely interested to know why you think a passive slab wallowing in a puddle is preferable to a trad block & beam house straddling over a puddle with a nice air void below.

[PeterW]

What’s wrong with traditional ground bearing slab ..? No-one is mentioning passive slab here - if you want speed, low cost and simple then ground bearing slab (with UFH if desired) is the lowest cost and most well known solution that’s completed in the shortest time. 

 

I would never want water under a floor - that’s a sure fire way to get damp and settlement issues. 

[joe90]

21 minutes ago, epsilonGreedy said:

There are two approaches to selfbuild, the methodical and cautious approach involves ground surveys, core samples, structural engineers and specialist slab contractors.

 

I disagree , my method has been methodical , the builder who knows the area and the building inspector agreed on trench founds and ground bearing slab based on local knowledge and years of experience, I have not had to pay for any other specialist. The only additional thing we did was incorporate a French drain around the outside to make sure any ground water was taken away, simples. To save the cost of an S.E. for a passive slab I copied the Denby dale passive house foundation detail to alleviate a cold bridge.

Edited July 19, 2018 by joe90

[epsilonGreedy]

40 minutes ago, PeterW said:

What’s wrong with traditional ground bearing slab ..? No-one is mentioning passive slab here

 

 

Because I had assumed that the worst option for a wet site would be such a ground bearing slab. I can see the attraction of a passiv sab house sitting on 300mm of rigid foam and well clear of whatever swampiness lays below.  Surely the next best thing is beam & block & void where techniques to combat rising damp are focused in the footing walls.

 

45 minutes ago, PeterW said:

I would never want water under a floor - that’s a sure fire way to get damp and settlement issues.

 

 

It is there regardless of what you want, nature, rainfall and the watertable control that. The only thing we control is what layers of building strata exist between our toastie UF heated bear feet and winter reality below. I am not sure why you instinctively questioned the viability of 200mm to 300mm of air to combat rising damp and instead recommend stones, concrete and a slither of plastic ?

 

p.s. I am hoping to learn through this debate.

[joe90]

46 minutes ago, epsilonGreedy said:

It is there regardless of what you want, nature, rainfall and the watertable control that

 

Not if you have a French drain and a ditch or a slope to get it away like I have. We also left gaps in the first layer of blocks in the trench linking the inside with the outside so water could not build up under the building.

Edited July 19, 2018 by joe90