Airtightness Membrane....same as above ground?

[Thorfun]

I'm researching the next detail for our self-build and in about 3 or 4 weeks the basement walls will be finished and the block and beam floor (was originally going to be hollow-core concrete slabs) will be laid to create the ceiling/ground floor. I want to continue the airtight membrane/VCL from the above ground levels down in to the basement.

 

Here's the detailing the architects and structural engineers have for the basement ceiling and I've added a blue line showing a VCL running around the block and beam/hollow-core which can then be taped to the VCL above ground and also continued to the basement floor level. 

 

I think that above ground this is called a 'Tony tray'? my questions on this subject are:

 

1. is this overkill when considering that the basement is one big concrete box?

2. with the ground floor being block and beam will any VCL just get ripped to shreds with the concrete beams placed on it?

3. if the answers to 1 and 2 are 'no' then can anyone recommend a VCL specific for this scenario or are they all the same?

 

@Bitpipe what did you do in your basement? I don't remember reading any specific detail from you around this.

[tonyshouse]

Is there no surface water drainage below the basement floor?  Looks like thermal bridge at the soleplate 

 

very good idea to wrap the ends and sides of the concrete floor with air tightness barrier, dpc could be used, I would use rubberised and proprietary corners 

[Thorfun]

2 hours ago, tonyshouse said:

Is there no surface water drainage below the basement floor?

there is a land drain around the basement , I just excluded it from the drawing when I took a screenshot!

 

2 hours ago, tonyshouse said:

Looks like thermal bridge at the soleplate 

the external EPS goes all the way to the sole plate. are you suggesting that the sole plate should sit partially on the EPS?

 

2 hours ago, tonyshouse said:

very good idea to wrap the ends and sides of the concrete floor with air tightness barrier, dpc could be used, I would use rubberised and proprietary corners

thank you. I will research rubberised VCLs

[tonyshouse]

Well done with the drainage, and sorry

 

Soleplate and bottom rail of the tf needs to be insulated outboard, worst case is condensation on top of the dpc! 
 

ruberised dpc, is available not sure about vcl 

[Thorfun]

1 minute ago, tonyshouse said:

Soleplate and bottom rail of the tf needs to be insulated outboard

can you supply a bit more detail about this please as I don't fully understand what you mean? 

[tonyshouse]

The soleplate is sitting on insulated block, but it is not insulated from the cold outside. Effectively the outside face of the sole plate is at near outdoor temperatures only heated by heat losses from inside. The tf wall is I presume insulated in the voids. If there is insulation over the inner face of the frame this will make the sole plate and bottom rail even colder, the coldest place will be under the outside bottom corner of the sole plate and condensation goes for coldest places, I see that as on top of the dpc which will get wet and the moisture will soak into the wood eventually causing decay 

[Thorfun]

14 minutes ago, tonyshouse said:

The soleplate is sitting on insulated block, but it is not insulated from the cold outside. Effectively the outside face of the sole plate is at near outdoor temperatures only heated by heat losses from inside. The tf wall is I presume insulated in the voids. If there is insulation over the inner face of the frame this will make the sole plate and bottom rail even colder, the coldest place will be under the outside bottom corner of the sole plate and condensation goes for coldest places, I see that as on top of the dpc which will get wet and the moisture will soak into the wood eventually causing decay 

thanks and it's starting to make more sense and I certainly wouldn't want decay on the sole plate. but I'm confused as I'm sure I've seen lots of sole plate details on here and, iirc, they're all the same as the one in our drawings. even the MBC passive slab and twin wall detail has the outside sole plate in the same way as mine.

 

so, what is the solution here and are loads of timber frames soon to be the victim of decaying sole plates?

 

[SimonD]

1 hour ago, Thorfun said:

thanks and it's starting to make more sense and I certainly wouldn't want decay on the sole plate. but I'm confused as I'm sure I've seen lots of sole plate details on here and, iirc, they're all the same as the one in our drawings. even the MBC passive slab and twin wall detail has the outside sole plate in the same way as mine.

 

so, what is the solution here and are loads of timber frames soon to be the victim of decaying sole plates?

 

 

It's a pretty standard detail, not a lot you can do about it other than design the timber frame to have an external layer of insulation, which is what wood fibre is often used for. You could also bring up the eps to above the sole plate. What is your timber frame buildup as from the drawing it looks like there is something outside the timber frame before the battens and cladding?

[Thorfun]

2 minutes ago, SimonD said:

 

It's a pretty standard detail, not a lot you can do about it other than design the timber frame to have an external layer of insulation, which is what wood fibre is often used for. You could also bring up the eps to above the sole plate. 

thanks and glad it's not just me! I could potentially bring the insulation up past the sole plate but it'll have to somehow have a cutout for the osb. I'll have a think about it and what it might impact and, maybe, speak to the architects. but maybe it's just a detail too far. ?

 

3 minutes ago, SimonD said:

What is your timber frame buildup as from the drawing it looks like there is something outside the timber frame before the battens and cladding?

standard external detail tbh. frame, osb, breathable membrane, battens, cladding.

[tonyshouse]

I had this problem with SIPs a few years ago, to start with the technical department refused to allow any insulation outside the face of the SIPs but the eventually relented. 
 

I suspect that there are problems that are lurking and yet to be encountered, in the old days with poor insulation escaping heat may have been saving the day. As levels of insulation increase the risks of this type of condensation damage increases too. 
 

May be get a wufi model run - model of the thermal bridging with psi values 

[Thorfun]

38 minutes ago, tonyshouse said:

I had this problem with SIPs a few years ago, to start with the technical department refused to allow any insulation outside the face of the SIPs but the eventually relented. 
 

I suspect that there are problems that are lurking and yet to be encountered, in the old days with poor insulation escaping heat may have been saving the day. As levels of insulation increase the risks of this type of condensation damage increases too. 

thanks for this. I'll definitely investigate it further

 

38 minutes ago, tonyshouse said:

May be get a wufi model run - model of the thermal bridging with psi values 

never heard of Wufi before but a quick Google has told me what it is. Every day is a learning day! ? 

[saveasteading]

Excuse me that I have not considered a better solution, but this seems strange. Timber structural wall sits partly on polystyrene (fixed how?) and partly on concrete that sits on more polystyrene.

All these bits of polystyrene, including fancy shapes. Also I don't know what the isolated external square of concrete is for, or the special L required to enclose it.

 

It all seems very complex, not terribly stable, presumably with the intention of good insulation, and and yet there is the direct cold bridge at the sole plates.

 

Happy to have it explained that this is a proven and robust detail, but it seems so complex yet imperfect.

 

Re the basement, what is there to stop it filling with water if the water table rises? It needs to be like a swimming pool in reverse. Perhaps simply not shown or I have missed it.

Presumably there is another detail showing the structure in detail and lots of reinforcement, and waterproofing.

 

 

 

 

[Thorfun]

20 minutes ago, saveasteading said:

Excuse me that I have not considered a better solution, but this seems strange. Timber structural wall sits partly on polystyrene (fixed how?) and partly on concrete that sits on more polystyrene.

All these bits of polystyrene, including fancy shapes. Also I don't know what the isolated external square of concrete is for, or the special L required to enclose it.

 

It all seems very complex, not terribly stable, presumably with the intention of good insulation, and and yet there is the direct cold bridge at the sole plates.

 

Happy to have it explained that this is a proven and robust detail, but it seems so complex yet imperfect.

assuming you're talking about the MBC twin wall system? if so the inner sole plate takes all the load from the house and the outer sole plate is there for the outside of the wall to hold all the cellulose insulation. quite a few on here have this design (and not all from MBC) and it does work. There isn't a cold bridge at the sole plate either as the entire space between the outer and inner sole plates is filled with blow cellulose. personally, if I had the extra money I'd have gone for this over the open-panel system with me fitting the insulation in a heart beat but I just couldn't afford it.

 

23 minutes ago, saveasteading said:

Re the basement, what is there to stop it filling with water if the water table rises? It needs to be like a swimming pool in reverse. Perhaps simply not shown or I have missed it.

Presumably there is another detail showing the structure in detail and lots of reinforcement, and waterproofing.

yep, lots of waterproofing. 2 types in fact. Type A external membrane and Type B waterproof concrete. There's also a land drain around the perimeter which is working well and filling the sump we've dug constantly. here's a photo of it working.

 

 

also a tonne, or rather 9.5 tonnes, of steel in the RC slab and walls. it is on the drawing but I removed the text detailing it as it wasn't pertinent to the question I was asking and didn't want to confuse matters! 

[saveasteading]

Thanks Thorfun. So the only cold bridges are the small spacers shown nail-plated between the 2 faces?

 

Still interested to know what the polystyrene and little concrete infill outside the wall are for.

 

Excellent re the basement....excuse me having been being concerned but not all basements work, and lots of small reinforcement bars is the answer.

[Thorfun]

33 minutes ago, saveasteading said:

Thanks Thorfun. So the only cold bridges are the small spacers shown nail-plated between the 2 faces?

 

Still interested to know what the polystyrene and little concrete infill outside the wall are for.

 

Excellent re the basement....excuse me having been being concerned but not all basements work, and lots of small reinforcement bars is the answer.

yep. just the spacers at, I presume, 600mm centres. so very much reduced cold bridges. I'm also interested as to what the stuff outside the envelope is for! 

 

no worries about the basement, I'm very happy to have people challenge/question what we're doing as it's good to have extra eyes checking for stuff. ? 

Edited May 17, 2021 by Thorfun

[Bitpipe]

I have exactly the same arrangement as in the original post. We have a MBC frame upon our basement) basement wall supports inner wall of frame, outer rests on basement EPS.

 

Airtightness fabric, which is about 4m wide was used and taped to the airtightness board on the inside of the frame wall and to the concrete of the basement wall.

 

Similar detail repeated between timber floors and into the roof. 

[Bitpipe]

14 hours ago, saveasteading said:

Excuse me that I have not considered a better solution, but this seems strange. Timber structural wall sits partly on polystyrene (fixed how?) and partly on concrete that sits on more polystyrene.

All these bits of polystyrene, including fancy shapes. Also I don't know what the isolated external square of concrete is for, or the special L required to enclose it.

 

It all seems very complex, not terribly stable, presumably with the intention of good insulation, and and yet there is the direct cold bridge at the sole plates.

 

Happy to have it explained that this is a proven and robust detail, but it seems so complex yet imperfect.

 

Re the basement, what is there to stop it filling with water if the water table rises? It needs to be like a swimming pool in reverse. Perhaps simply not shown or I have missed it.

Presumably there is another detail showing the structure in detail and lots of reinforcement, and waterproofing.

 

The EPS used for basement construction is just like that used for raft foundations - typically 200-300 grade which is extremely dense and not the more flimsy material that's used in floor insulation. The whole system is designed by a structural engineer and so is fit for purpose.

 

In my case, I have a 300mm layer of EPS 200 that supports the weight of a 120m2 basement with 300mm thick walls and a 2 storey timber frame house ontop of that.

 

The corner shapes etc are only required if you want the EPS to act as shuttering also, in my case I just laid a flat slab of EPS 200 blocks, covered with a membrane and let the groundworkers cast their slab upon it as normal. EPS70 was applied to the exterior walls when the basement casting was complete.

[Thorfun]

12 minutes ago, Bitpipe said:

I have exactly the same arrangement as in the original post. We have a MBC frame upon our basement) basement wall supports inner wall of frame, outer rests on basement EPS.

 

Airtightness fabric, which is about 4m wide was used and taped to the airtightness board on the inside of the frame wall and to the concrete of the basement wall.

 

Similar detail repeated between timber floors and into the roof. 

didn't realise you had the twin-wall system.

 

also, you have timber joists between the basement and ground floor so the airtightness fabric wouldn't take as much of a beating as it might with block and beam so would be less likely to be damaged during the joist installation.

[Thorfun]

Just now, Bitpipe said:

In my case, I have a 300mm layer of EPS 200 that supports the weight of a 120m2 basement with 300mm thick walls and a 2 storey timber frame house ontop of that.

200mm EPS300 for me. after calculating the U-value when taking the depth below ground in to consideration I decided that 200mm was enough. we also only 250mm thick walls. it's interesting how similar builds can have such different structural calculations!

 

2 minutes ago, Bitpipe said:

The corner shapes etc are only required if you want the EPS to act as shuttering also, in my case I just laid a flat slab of EPS 200 blocks, covered with a membrane and let the groundworkers cast their slab upon it as normal. EPS70 was applied to the exterior walls when the basement casting was complete.

exactly what we're doing except we've got EPS100 on the walls. don't know why not EPS70 though before you ask. ? 

 

the membrane covering our flat EPS300 is part of the total waterproofing system which wraps the basement underneath and up the walls.

[saveasteading]

Dense EPS under the wall area, and normal grade elsewhere?

How do you finish the outer face of the EPS as shown exposed here?

[Thorfun]

6 minutes ago, saveasteading said:

Dense EPS under the wall area, and normal grade elsewhere?

dense under the slab as designed by the structural engineer and normal outside the walls as they're not taking any compressive loads, again as designed by the structural engineer.

 

7 minutes ago, saveasteading said:

How do you finish the outer face of the EPS as shown exposed here?

I assume you mean the small amount that is above ground? if so, I'm undecided on that one as yet but I believe there are many ways to do it and I'm pretty sure it comes down to personal taste (or planning requirements if they've mandated something).

[saveasteading]

Might be worth asking if you need the same density under all the slab. It can be expensive and some is made to carry fork-lift loading.

 

Yes, here you would traditionally see 3 courses of brick, you have polystyrene. Perhaps a skin of cement board over, painted black?

 

I am assuming that you are ignoring the other stuff shown outside the wall which has an, as yet unknown, purpose.

[Thorfun]

I did ask if EPS 300 really was required as @Bitpipe has EPS200 and the answer was a resounding yes from the SE so we have done as designed.

 

some form of render board might be a possible solution, also some on here have used metal of sorts (iirc) but as that detail is a long way off it's quite a way down in my list of things to research! it is a very long list.... ?

 

our design doesn't have that stuff shown outside on the MBC twin wall system as we're not having a twin-wall system. simple open panel timber frame for us but still sitting on an insulated slab.

[Bitpipe]

The engineer dictates the EPS compressive strength depending on the loads imposed from above. 

 

I've seen aluminium sheeting (as used for gutter forming) used and I have used ubiflex artificial lead flashing to dress the EPS before the rain screen (in this case render on cement board) is applied on timber battens. You can also render directly to EPS, we did that on a below ground section of the basement that opens to a stairwell / mezzanine.

[saveasteading]

Heavy people in that house?

But Ok, not my job or my money, and there may well be a good reason I don't know of.   Multiple cross walls for example.

My career has been based on questioning over-design, and consultants don't tend to know where costs lie.