Retrofit telescope vents - how???

[Hilldes]

The time is approaching where I need to tackle a job I've been dreading - retro fitting telescope/periscope vents in 140mm concrete blockwork. The bricklayers left them out and in the rush to be ready for timber frame delivery I was reluctant to get the brickies to dismantle the foundation walls.

 

Background is the floor is insulated beam and block - the pre-stressed beams run front to back of the house with many sleeper walls. I specified the telescope vents to go in the side walls. There are slots left in sleeper walls to allow ventilation from left to right.

 

I have standard telescope vents with optional extension. 

 

I guess the 'right' way to do it would be:

 

1. Identify a block in bottom course that is between timber fame studs.
2. Use a mortar rake on and SDS drill to remove the mortar and remove the complete block.
3. Replace block with concrete lintel at the top and two small section of concrete bloc to support the lintel. Telescope vent inserts under lintel between two smaller block pieces. 

 

Any observations please on how to do this safely and rapidly? Alternative vents such as small circular holes to be cut? Lintels are absolutely necessary?

 

 

 

[Moonshine]

That looks an absolute pig of a job, and i thought i was in a bind with mine as below.

 

 

Where it FFL going to be?

 

I can't really make out what the sub structure wall construction is, it it 140mm inner leaf, ~50mm cavity, and 100mm block externally which has been rendered?

 

like this

 

 

or this

 

Edited October 25, 2021 by Moonshine

[Hilldes]

6 minutes ago, Moonshine said:

I can't really make out what the sub structure wall construction is, it it 140mm inner leaf, ~50mm cavity, and 100mm block externally which has been rendered?

 More like the second image - except the infill blocks to the floor beams are EPS and extend down to the bottom of the beams. And I have no Marmox below the TF sole plate.

 

Will have a read of your thread, thanks.

[Hilldes]

This is a section of the front elevation wall where the beam ends can be seen...

 

 

...but the plan was to install the telescope vents in the side walls.

[Iceverge]

Am I correct in assuming the red line I've drawn is the top of the ventilated void?  

 

 

 

If so I can't see how you'll be able to put the telescopic vent in the "side" as the telescopic vent will be obstructed by the beam inside it. 

 

Can you post a picture like the first one but showing a corner of the building so we can see how far back the first beam is set from the 140mm wall?

If it is hard against it you will have to have your vents outside the wall or else in the "end" walls. 

 

 

 

[Hilldes]

6 minutes ago, Iceverge said:

 

If so I can't see how you'll be able to put the telescopic vent in the "side" as the telescopic vent will be obstructed by the beam inside it. 

 Thanks @Iceverge your red line is correct - the bottom course of blocks is the void under the beams. So basically the telescope vent will need ideally to be inserted at the top of the bottom course of block - that way it will vent the underside of the insulated beam and block floor. Putting in a lintel as I had planned will move the telescope vent lower - perhaps even resting on the reinforced concrete ground beam.

 

11 minutes ago, Iceverge said:

Can you post a picture like the first one but showing a corner of the building so we can see how far back the first beam is set from the 140mm wall?

If it is hard against it you will have to have your vents outside the wall or else in the "end" walls. 

 

The separate pic I posted that shows the beam ends is actually a corner - the corner is in the left of the pic.

[Iceverge]

Putting the vertical riser inside the blockwork isn't going to gain you anything. The easiest solution if you have a ground level that is below the red line is put vent straight through the blocks.

 

Assuming your ground level is higher just mark an outline of the vent at the top of the  bottom line of blocks as shown below.  I'd use a diamond blade to cut the mortar and the blocks and remove a place for the vent. The blocks above will be fine as a "lintil". You will need to run your telescopic vents outside the foundation walls. 

 

 

 

 

 

 

 

[Hilldes]

Thanks @Iceverge the drawing is exactly how I saw it working - with the vent on the outside of the block wall in my pics. I like the way you have drawn the hole to be cut in a position below the centre of the course  block above.  Just cutting a hole big enough for the vent is a lot simpler than cutting in a lintel. It does make me wonder though why lintels are mandatory in other penetrations through the main load bearing skin of foundation blockwork e.g. the way a lintel is required for a soil pipe passing through the wall. Same could be said there that a block above bridging a gap of just over 100mm should be adequate - but it wouldn’t be I think according to the building regs.

[Iceverge]

Of course I'm not a structural engineer so beware internet advice! 

 

The way I look at is if you were to make an arch, entirely in compression over the hole, would it be self supporting within the remaining brickwork?

 

All that the lintel does in this situation is support the semi circle of material directly above the hole as well as reducing sideways thrust from the "arch". 

[ETC]

7 hours ago, Iceverge said:

Putting the vertical riser inside the blockwork isn't going to gain you anything. The easiest solution if you have a ground level that is below the red line is put vent straight through the blocks.

 

Assuming your ground level is higher just mark an outline of the vent at the top of the  bottom line of blocks as shown below.  I'd use a diamond blade to cut the mortar and the blocks and remove a place for the vent. The blocks above will be fine as a "lintil". You will need to run your telescopic vents outside the foundation walls. 

 

 

 

 

 

 

 

Sorry for being stupid but is the sub-floor block work solid and if so how will the telescope vents work if there is no cavity to hide the vertical riser? Or are you just going for through wall vents without the riser? Thanks.

[Iceverge]

I was suggesting installing the periscope vent through the wall below the level of the  block and beam floor. Letting the riser come up the outside to be above ground level. It would need to be backfilled carefully to avoid damaging the riser. 

 

 

[ETC]

Hmm. Sounds very strange. I thought from an earlier post that there was a suggestion to use a straight through vent and keep the FGL down as far as possible. Would also negate the need to drain the sub-floor void. Pity the void wasn’t filled. No void - no ventilation.

[Iceverge]

9 minutes ago, ETC said:

Hmm. Sounds very strange. I thought from an earlier post that there was a suggestion to use a straight through vent and keep the FGL down as far as possible. Would also negate the need to drain the sub-floor void. Pity the void wasn’t filled. No void - no ventilation.

It was a suggestion but I imagine OP will want an outside ground level higher than the void. 

[Brickie]

What’s the projection of your finished render to the substructure blockwork? (I.e. the measurement back from one to the other?)

And are you having paving down the relevant sides?

Edited October 26, 2021 by Brickie

[Hilldes]

12 hours ago, ETC said:

Sorry for being stupid but is the sub-floor block work solid and if so how will the telescope vents work if there is no cavity to hide the vertical riser? Or are you just going for through wall vents without the riser? Thanks.

 

There will be 50mm insulation with some form of weather protection (e.g. render or slate tiles) fixed to the outside of the foundation blockwork.. The telescope vent will essentially be buried in the insulation. The finished ground level is approx 150mm below DPC. There is a separate thread on the external skin below ground here...

 

 

[Hilldes]

57 minutes ago, Brickie said:

What’s the projection of your finished render to the substructure blockwork? (I.e. the measurement back from one to the other?)

And are you having paving down the relevant sides?

 

From the outer face of the blockwork to the outer face of the render there is: 9mm OSB + 50mm batten + 18mm for Aqaupanel and thin coat render - so a 77mm projection. There is more information on the detailing for the outer skin in the link I shared just above this post. Yes there will be paving to approx 150mm below DPC.

Edited October 26, 2021 by Hilldes

[ETC]

[Hilldes]

Thanks @ETC that is intriguing and something I haven't seen before. I guess the advantage is that the vertical pipe can be terminated well above ground level so good in areas with a flood risk?  Technically we are in a zone with a flood risk according to DEFRA, but with no historical records of actual flooding to properties.

[Hilldes]

A random thought here on an alternative approach. I'm guessing there will be no point trying to challenge the need to ventilate a [insulated] beam and block floor, given the building regs appear clear that ventilation will be required (see excerpt below para 4.19). IIRC JS Harris for one was not convinced on the need for subfloor ventilation.

 

Approved Document C - Section 4: Floors....

SUSPENDED CONCRETE GROUND FLOORS (MOISTURE FROM THE GROUND)

 

4.19 A suspended concrete floor will meet the requirements if it incorporates:
a. a damp-proof membrane (if the ground below the floor has been excavated below the lowest level of the surrounding ground and will not be effectively drained); and
b. a ventilated air space. This should measure at least 150mm clear from the ground to the underside of the floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts of the floor void. The openings should be not less than either 1500mm2/m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly.

Edited November 3, 2021 by Hilldes

[Gordo]

1 hour ago, Hilldes said:

A random thought here on an alternative approach. I'm guessing there will be no point trying to challenge the need to ventilate a [insulated] beam and block floor, given the building regs appear clear that ventilation will be required (see excerpt below para 4.19). IIRC JS Harris for one was not convinced on the need for subfloor ventilation.

 

Approved Document C - Section 4: Floors....

SUSPENDED CONCRETE GROUND FLOORS (MOISTURE FROM THE GROUND)

 

4.19 A suspended concrete floor will meet the requirements if it incorporates:
a. a damp-proof membrane (if the ground below the floor has been excavated below the lowest level of the surrounding ground and will not be effectively drained); and
b. a ventilated air space. This should measure at least 150mm clear from the ground to the underside of the floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts of the floor void. The openings should be not less than either 1500mm2/m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly.

The concern is minimal cover to reinforcement in PPC T beams and likely corrosion of same. It has been argued by some PPC floor slab manufacturers that they do not have corrosion concerns therefore ventilation is not necessary. Best get a declaration from T beam manufacturer for their particular product. Perhaps they may have a BBA certificate or equivalent to clarify this point and hopefully avoid vents. I would be f the opinion that in the loading up of PPC flooring (where the arching levels out) there will be micro cracks in concrete which may leave reinforcing vulnerable to corrosion. 20yrs down the line who will be left to sort it out if there is a problem.

Edited November 3, 2021 by Gordo

[ETC]

Pity you didn’t fill the void. No void = no  ventilation requirement.

[Gordo]

15 minutes ago, ETC said:

Pity you didn’t fill the void. No void = no  ventilation requirement.

Yeah this is a common approach. One which I personally don’t agree with but that’s for another day maybe lol

[ETC]

I’d be interested to learn why you don’t agree with this approach. Thanks.

[Hilldes]

1 hour ago, ETC said:

Pity you didn’t fill the void. No void = no  ventilation requirement.

The BBA certificate for the insulated beam and block floor system specified a ventilated void, but I generally thought there was no choice - but that is not really talking from much industry experience. Would be interested learn more about builds with beam and block floors with no void. I've got this vision of a pump truck turning up like a liquid screed truck, but pumps in expanding foam to fill the void ?.

[Mr Punter]

As you have beam and block you will need the ventilation for Building Regs and probably warranty.  Too late for a ground bearing slab.

 

We did a place that had external periscope vents to the basement.  They didn't look great but served a purpose.