Trench block or cavity with infill below DPC advice required?

[ruggers]

Looking for some advice for a self build.

I need to raise the height of the house including floor level by 225mm which will include raising the outside finished floor level (FFL) around the house.

As per sketch, would it be best to add another course of 225mm trench blocks on top of the existing course, or would it be better to add 2 rows of 225x100mm wide dense solid blocks and infill the 150mm cavity with a lean towards the outside?

 

Cost wise theres not a lot in it, Trench blocks will be quicker for a builder, but I'm thinking of whats best for the construction & are trench blocks ok closer to the surface with freezing? They'll be out of the ground for a good few months before the outside ground is build up to the new FFL.

[Russell griffiths]

A few pointers,

you are working backwards

start at ffl and work down with your measurements not up.

FFL  is internal.

FGL finish ground level is external

so start at FFL and measure down, you can then work out if you need to add anything, if it was me I would try to add more concrete to the foundation. 

Then use a concrete trench block not an aircrete one, laid flat so 100mm build up per course, this gives you much more flexibility with height difference. 

With your one course that comes up so high, if your foundation is 50mm out of level ( it will be) you have no flexibility to adjust the blocks to take up this inaccuracy.

 

[ruggers]

Thanks Russell.

FFL needs to be at a certain height to achieve a drainage fall on a pre existing main.

I've got a site that at the back slopes 315mm from one back corner to the other side of the build. The structural engineer has set a dig depth of 750mm from FGL to the invert of the foundation on the higher side for a 225mm concrete strip. I have a couple of areas that need to go down much deeper where trees were to avoid heave, these will be mass filled to bring the foundation top back up to match the standard depth for the rest of the build.
 

Theres 450mm of top and sub soil to be stripped to take it down to clay level ready to mark out the foundations, this will then be returned with a little more to build up to the new FGL at a later date once the build is at DPM or beyond. Due to stripping this 450mm off, it means at the lowest corner, the concrete foundation could be above ground level, so options are to either dig a course deeper so its underground again, shutter the lowest corner until ground around it is replaced which seems a messy idea, or add compacted stone (150-250mm) across the area that the house will sit on, then the foundations to be excavated through it so it acts as a retainer for the concrete on the lowest corner & keeps the site clean when digging out I'm advised. It seems strange to cover a site in stone to then dig back through it & have it taken away. The house needs built up one course to level it with the surround area & road access regardless, but the other issue is potentially needing the foundations to be another course deeper because some points are low.

 

The trench blocks I seen were W355 x H215 x L440mm. Not physically seen them yet, are these usually aircrete & a bad idea?

Are the ones you mention 355mm wide x 100mm height x 440mm long?

[Nickfromwales]

Use this opportunity to install more insulation into the floor, and more screed, for an all-round better performing heating system ( assuming UFH )?

For inverts for drainage you should be referencing TOC ( top of concrete ) not FFL . TOC for you would be top of screed. 

[Iceverge]

Here Here, 

 

With SE's thumbs up just install more insulation to mitigate the wall floor thermal bridge. 

[ruggers]

17 hours ago, Nickfromwales said:

Use this opportunity to install more insulation into the floor, and more screed, for an all-round better performing heating system ( assuming UFH )?

For inverts for drainage you should be referencing TOC ( top of concrete ) not FFL . TOC for you would be top of screed. 

Meaning keep the B&B height the same height but add more insulation to build up to a higher FFL? I'll be installing wet UFH yes, was going to be 165mm PIR & 55mm cemfloor = 0.12W/m2k. It could be upped to 250mm & 50mm screed, it's just so expensive even EPS doesn't seem much cheaper by the time you go 50% more.

 

Yeah my drains are all taken from FGL minus 300-320mm for the first chamber invert which is the shallowest I can have. I've just got the terminology wrong on here. There will be a 150mm step dow from inside FFL to outside FGL path with exception of the low threshold front door.

 

Basically the reason I was working from FGL down, is because I have a set height that FGL needs to be at for all drainage to work, then knowing how deep my soil is, and where my clay levels are, I need to either go down a course deeper because of the lowest ground point, or we don't remove as much soil from that lower side so the founds have a natural shutter but are still laid onto solid ground. It might just be a case of factoring in the cost and seeing what it's like during the dig.

[ruggers]

Would the telescopic vents install ok as per sketch where the T beams & slip bricks are set on a course of brick?

[Nickfromwales]

8 hours ago, Iceverge said:

Here Here, 

 

With SE's thumbs up just install more insulation to mitigate the wall floor thermal bridge. 

Yup, noted earlier that there is no insulation ( and expansion ) upstand around the perimeter of the heated slab ( screed ). Needs a bit more detailing before we allow this to proceed   

[JOE187]

  I have recently ued https://www.insulationuk.co.uk/ for 240mm Eps in new build of  floor 84m area. 

 2 layers of 100mm and 140mm staggered.

  I think you would find 260mm eps 70 at 250mm  around 30 to 35% cheaper than  the 160mm of Pir!

[Iceverge]

The PIR manufacturers have tremendous marketing. Every merchant and builder in the land cannot seem to see beyond it. 

 

Saved €1300 from memory in our floor insulation by using EPS.  

 

[ruggers]

6 hours ago, Nickfromwales said:

Yup, noted earlier that there is no insulation ( and expansion ) upstand around the perimeter of the heated slab ( screed ). Needs a bit more detailing before we allow this to proceed   

I was just focusing on the block selection rather than the correct detail for floor insulation. I understand that but haven't worked with foundation blocks before. All the ones online seem to have strange sizes. I was hoping to just find some foundation blocks 440mm long x 215mm high in varying widths to suit my walls, 350mm, 300mm & whatever will go under the walls supporting 100mm beam ends that will only have single load bearing walls built above them.

Most foundation blocks seem to be 3.6Kn not 7.2Kn and made from aerated material. Are these ok to use or problematic like the aerated blocks used above DPC which always have cracking issues?

One of the builder I was looking to use still use the double dense block with infill method. Other than time, are there any cons to using this method nowadays? 

6 hours ago, JOE187 said:

  I have recently ued https://www.insulationuk.co.uk/ for 240mm Eps in new build of  floor 84m area. 

 2 layers of 100mm and 140mm staggered.

  I think you would find 260mm eps 70 at 250mm  around 30 to 35% cheaper than  the 160mm of Pir!

I just had a quick look, jablite 70 at 240mm would probably save me £700, the 160mm PIR was going to cost £1800. I''d need to raise the internal garage floor up by 75mm though to keep the heights correct between the house and garage step down. Jablite customer service staff were the least educated of any company I've ever dealt with in my life. Two women taking calls who said they have no training and no data at all on their own product.

[Nickfromwales]

30 minutes ago, ruggers said:

Most foundation blocks seem to be 3.6Kn not 7.2Kn and made from aerated material. Are these ok to use or problematic like the aerated blocks used above DPC which always have cracking issues?

One of the builder I was looking to use still use the double dense block with infill method. Other than time, are there any cons to using this method nowadays? 

I cannot think of a reason to reinvent the wheel here tbh, and as long as there is a thermal break ( with one course of aerated ) then you're as good as it needs to be afic.

Is this in motion yet / or still a blank canvass? 

 

[ruggers]

I'm all for simplicity at this stage, not yet broke ground but too far to make any big changes. Building regs are passed we are just swapping over from slab to B&B on the plans for a spring start I hope. No one seems overly concerned about adding or taking away a course in height as long as it's all vented correctly & following the standard construction. 

 

When you refer to a course of aerated block, do you mean like this image attached, or one of the below DPC courses?

[ruggers]

Slight edit to the original post sketch. The height of the lean cavity infill can be lowered to suit.

 

Builder doesn't like to run the first course in trench blocks, something to do with them not being as easy to cut and level up if the founds arent level.

 

The first inner leaf block on top of the beam and block floor will be an aircrete type like my last post.

 

My question is, I'd like to know if the cavity wall insulation is fine as it is or has to go down abother 225mm?

 

I cant see any benefit to it being lower down. The screed has insulation under it, the vented void will always be cold air, the inner leaf has the insulated block on it.

 

The way drawn works out well for pallets of xtratherm tongue and groove pallet orders, if i go deeper its into another £1500 pallet so I'd have to do the base coarse in stocked square edge or xps.

[Iceverge]

If I was you I would bin the lean mix (With SE's approval) and the PIR boards in the cavity. Just use EPS bonded beads and fill the 150mm cavity right down to the foundation or as low as the engineer will let you. The masons will thank you for not having to fiddle with the boards too.  

 

EPS beads are about the same price as concrete (£100/m3) and far far cheaper than PIR boards.  You'll be at about £15/m2 for beads installed vs £23.50+fitting for the boards with PIR.  You'll have a vastly better performing wall too.

 

The boards CANNOT be installed properly. I've never seen it happen. 

 

 

 

[ruggers]

I appreciate what you've said but I've covered this before and I'm due to start in 2 months. Can't use EPS bead we are in exposed weather zone 5 face brick exterior so it's no good. It would be a nightmare to get passed by anyone locally because it's not used, everyones either timber frame or partial fill where we live and behind the times on anything new like weather comp or heat pumps within 70 miles. I agree the blown bead would be a winner and easier.

 

I'm paying extra and using tongue and grooved 100mm PIR to save the hassle of taping the joints and it not sticking in bad weather or people just skipping it. We've had to do a last minute change from slab to beam and block and everyones been very slow compared to other peoples services used so I daren't make a single change bar getting this below DPC agreed. 2 years to get some plans sorted is too long. I just want to get on with it now and be done with changes but I have this last bit to finalise. 

[Iceverge]

Ah sorry, I thought I recognised the username, I remember your situation now.

 

It'll perform as well as it can do in that case, there's a small benefit from taking the insulation lower down but it's only marginal in your case. It makes a bigger difference when the wall has a much lower U value.  I wouldn't worry in the slightest about the drawing as is.

 

A couple of neighbours used closed cell  injection foam after the PIR boards near me as a belt and braces if it all goes peat tong. They seem happy with it as far as i know. 

 

Good luck and keep us updated with pics. 

 

 

[ruggers]

I think if the founds are decent the builders happy to use trench blocks for the first course, I've had a few mistakes with my plans so just trying to get the last part right. I'm not sure how much building control hold you to the plans for below DPC. They've already passed them, we are just changing to suspended beam for heave reasons. If taking the insulation down another block course was beneficial for cold bridging I would but if its so minimal I'd rather spend my time and money in other areas are all to a good standard.

 

Thanks & I will be posting some progress up for others in the near future.

[Canski]

Xtratherm tongue and groove boards are not easy to work with. They tend to be slightly oversize causing problems with vertical spacing or wall ties. Always allow a bit extra width on the cavity as well to allow for a gap between your retaining clips and the faced brickwork. We charge double for fitting these boards over traditional cavity bats and it’s still not enough. 

[MikeGrahamT21]

47 minutes ago, Canski said:

Xtratherm tongue and groove boards are not easy to work with. They tend to be slightly oversize causing problems with vertical spacing or wall ties. Always allow a bit extra width on the cavity as well to allow for a gap between your retaining clips and the faced brickwork. We charge double for fitting these boards over traditional cavity bats and it’s still not enough. 

 

My builders used Xtratherm T&G on my extension, and they thought it was a revelation, made it so much easier, and no snagging your trowel in the fluff!

[BadgerBadger]

We've just completed a fairly similar below DPC build-up.  Our cavity fill came up to the level the beams sat.  The telescopic airvents simply sat in gaps in the slips under the edge blocks.  Don't forget you'll need to line them up with the external brick bond, so the embedding them in cavity fill might limit your flexibility.

 

Rigid insulation is hard to get right, but doable with care.  Cutting around airvents and cavity trays is particularly time consuming, and I imagine many contractors would take short cuts.

 

Our boards were flat edged and taped, and to be honest I feel tongue and groove would have made it harder, particularly with masonry build and wall ties etc.

[ruggers]

13 hours ago, Canski said:

Xtratherm tongue and groove boards are not easy to work with. They tend to be slightly oversize causing problems with vertical spacing or wall ties. Always allow a bit extra width on the cavity as well to allow for a gap between your retaining clips and the faced brickwork. 

I was looking at the boards and imagine the tongue at the top and one side adds the small over size but just needs slit into by the wall tie to make the T&G meet tightly together rather than square edge that neatly slips between but then has all the problems of the gaps if incorrectly fitted or a bit of wind gets at them while building. There will be 100 PIR and 50mm cavity spare, 350mm in total so I think 275mm wall ties are the correct ones, need to check the drip loop in the middle is away from the 100mm board.

12 hours ago, BadgerBadger said:

We've just completed a fairly similar below DPC build-up.  Our cavity fill came up to the level the beams sat.  The telescopic airvents simply sat in gaps in the slips under the edge blocks.  Don't forget you'll need to line them up with the external brick bond, so the embedding them in cavity fill might limit your flexibility.

Yes good point, I was thinking of this, I was planning on fitting the vents the same way, it will mean they probably can't be spaced equally from outside, it will just be a case of finding gaps between the beams that align with the brick bond outer wall and within the maximum recommended spacings.

 

I was thinking to build the infill mix up to the top of the first brick on the inner leaf which would be just under the telescopic vent, then we'll have to  infill before fitting the vents to the slip coarse. It depends on how high it needs filled to provide support. I'm sure I seen it needs to have a 150mm gap to the underside of the insulation which would work out well. (So lower than my last sketch)

[Canski]

39 minutes ago, ruggers said:

I was looking at the boards and imagine the tongue at the top and one side adds the small over size but just needs slit into by the wall tie to make the T&G meet tightly together rather than square edge that neatly slips between but then has all the problems of the gaps if incorrectly fitted or a bit of wind gets at them while building. There will be 100 PIR and 50mm cavity spare, 350mm in total so I think 275mm wall ties are the correct ones, need to check the drip loop in the middle is away from the 100mm board.

Yes good point, I was thinking of this, I was planning on fitting the vents the same way, it will mean they probably can't be spaced equally from outside, it will just be a case of finding gaps between the beams that align with the brick bond outer wall and within the maximum recommended spacings.

 

I was thinking to build the infill mix up to the top of the first brick on the inner leaf which would be just under the telescopic vent, then we'll have to  infill before fitting the vents to the slip coarse. It depends on how high it needs filled to provide support. I'm sure I seen it needs to have a 150mm gap to the underside of the insulation which would work out well. (So lower than my last sketch)

Sorry I haven't mastered the quote function yet.

 

yes correct ....you need to slit the board for the tie to sit in plus some more for the thickness of the tie then push it back and clip it on. All good until the boards are 5mm over sized and springing up off the board below because the plastic has delaminated from the PIR top and bottom and is acting like 2 springs. in 4 courses of boards you are 20 mm out of gauge then the fun begins ... chuck a standard joist height in of 2400 mm and you have another problem unless you have 300 mm joists and hit the tie course at 2700mm by which time your board will be at 2730 mm. OK I may have had a few bad packs on a site or two but the size of the first site has slipped my mind but the second  was about 400 m2 of facework.

 

I find that its better to ensure that the last 2 brick courses below  the beams are in coursing bricks and set the vents out correctly to the facing bricks and use a 600 mm PC lintel in the top course over the vent.  All done sealed and tidied before the beams go on and you can put your bet your bottom dollar that very few of the beams leave room for a vent to line up with the facework.

[ruggers]

@Canski *on quotes just delete the text unwanted and then tap enter twice at the ned of the sentence to split paragraphs*
It's always good to hear from experience. It does sound a bit of a pain if the tolerances of the PIR are out. For the price you pay a tenner a sheet and £4500 quid later it should be machines to size. My joists are going on internal masonry hangers. Door and window reveals say they need a wall tie every 225mm so they'll need slit to.
 

I won't be able to use a PC lintel over the external vent because it's all facing bricks otherwise I would.

 

 

[Canski]

1 minute ago, ruggers said:

@Canski *on quotes just delete the text unwanted and then tap enter twice at the ned of the sentence to split paragraphs*
It's always good to hear from experience. It does sound a bit of a pain if the tolerances of the PIR are out. For the price you pay a tenner a sheet and £4500 quid later it should be machines to size. My joists are going on internal masonry hangers. Door and window reveals say they need a wall tie every 225mm so they'll need slit to.
 

I won't be able to use a PC lintel over the external vent because it's all facing bricks otherwise I would.

 

 

Sorry I meant over the internal side of the periscope vented below the floor beams.

 

Thank you for the quote tip.

Edited January 30, 2023 by Canski
Thanks for the the tip