Passive raft vs traditional (strip/trench) costs assesment

[SuperJohnG]

I've been back and forth on what foundations I would like to use on our plot and I am really keen on a passive insulated raft. I had originally liked the idea, but then went off it as assumed too expensive, then read more and believe it should theoretically should be cheaper/ less hassle but I have never quanitified this. 

 

Hence I am going do a cost vs benefit analysis to see what the actual costs are and from there assess the benefit. E.g it might still be more expensive, but if it means less labour and less delays then still works out better. 

 

Before I start did anyone else do this? and have any existing excel sheets or similar or input they want to add? 

 

My thoughts are it might be a higher upfront cost but it has to work out similar in costs by the time you factor in all the stages required to do a normal type strip foundation with ground bearing slab and then adding insulation, membrane, UFH, screed etc. 

 

I understand every house and site will be different, and ground conditions will have a factor, But I have a pretty large open plot, lots of space to store muck and use later on, it is pretty much flat and there are no major trees or any ground issues which are currently known and it would seem unlikely there is anything going to come out of left field and surprise me. 

[Bitpipe]

Two key questions I'd ask at the outset

 

1) Have you got a ground investigation report or can you approximate what is under your feet  based on immediate neighbours? i.e. are you on clay (how deep) or clay / gravel/ chalk (like we were), mudstone, rock etc.. That will be they key factor in determining the spec (and therefore cost) of your traditional vs insulated raft foundation.

 

2) Are you aiming to achieve a BR standard, better than BR or near passive? That will go towards the benefit column as the insulated design of the raft (vs the pure structural element of a raft vs strip) may be design overkill or something you need to replicate in a traditional format.  Low energy / passive houses obviously need relatively low amounts of heat injected into them in winter and often benefit from heat removed from them in summer. The design of the passive slab facilitiates this, a traditional system may struggle, or need a lot of work and detail to become equivalent.

 

[SuperJohnG]

1 hour ago, Bitpipe said:

Two key questions I'd ask at the outset

 

1) Have you got a ground investigation report or can you approximate what is under your feet  based on immediate neighbours? i.e. are you on clay (how deep) or clay / gravel/ chalk (like we were), mudstone, rock etc.. That will be they key factor in determining the spec (and therefore cost) of your traditional vs insulated raft foundation.

 

2) Are you aiming to achieve a BR standard, better than BR or near passive? That will go towards the benefit column as the insulated design of the raft (vs the pure structural element of a raft vs strip) may be design overkill or something you need to replicate in a traditional format.  Low energy / passive houses obviously need relatively low amounts of heat injected into them in winter and often benefit from heat removed from them in summer. The design of the passive slab facilitiates this, a traditional system may struggle, or need a lot of work and detail to become equivalent.

 

@Bitpipe thanks fro your comments. 

 

1) No ground investigation report as of yet, and there are no neighbours within 400m. I've attached the plot topo fo r reference and an aerial picture just fer referecne sake, hosue will be positioned at the highest point pretty much in the middle. I am envisaging a few hundred mm of topsoil then firm to stiff clay, but obviously this will be confirmed once this is done. My plot sits at the lowest point of some hills hence, the burn running through it.  

 

2) I am aiming for as close to passive as I can get, but I don't know how close that will be. Hence trying to work out the budget. I would ideally like to be well towards passive. The bigger picture partly being here is the ease of the passive raft when compared with normal strip type. I won't have a main contractor and I don't want to have to manage and complete multiple steps to get to the same point. The way I view it is, once we have that passive slab done, we are ready for the kit to go up and then we can crack, rather than getting kit up and still have three/four steps of time consuming steps that take a while. 

Topo.pdf

[Bitpipe]

Nice plot.

 

We created our own 'passive basement' mimicking the design of the MBC passive slab and then had MBC erect a passive standard frame on top - was easy enough to ensure the two insulation envelopes met correctly. However we do not have any heating in the basement slab, relying on UFH in the suspended timber ground floor so don't get all the advantages - mainly the ability to cool the slab in summer. We're also on mains gas vs ASHP which is required to cool the slab.

 

For me, active cooling would be one of the key benefits, especially as low energy houses are much more likely to overheat rather than be cold (even in spring & autumn when the sun is low and comes into rooms directly.

 

Build wise for us it was a pretty continuous process however as MBC did not build the basement, we engineered a break for 4-6 weeks in proceedings to ensure that the final frame measurements could be confirmed before the frame went into production (i.e. basement designed around frame, frame build finalised around built basement). I believe that when MBC do slab and frame this is not required.

 

 

[joe90]

I too initially wanted an insulated raft but S.E. costs fir the design was not cheap. I got an S.E. to design a raft fir the garage (not insulated) and paid £600 for the privilege and ended up with strip foundations a LOT cheaper than the raft (the raft would have held up a block of flats!!!!). Some on here had a passive slab designed as part of the timber construction so the S.E. costs were part of the overall costs. I went for strip foundations copied from here. https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/
which are passive type foundations. My building inspector accepted them with no S.E. Input and I had my own JCB to do the excavation.

 

the only change my builder proposed (which I accepted) was to lower the DPC to the internal floor level, which put the outside DPC below ground level but they inserted a second DPC in the outer skin to cope with “rain bounce”. This was if the DPC is where it is in the diagram it potentially creates a slip plane above the skirting board (crack line).

 

[Jeremy Harris]

It's perhaps worth having a read of the Kore foundations paper that Hilliard Tanner (an SE who's a bit of a passive slab expert) wrote:  KORE-Passive-Slab-Design-Guide.pdf

[Oz07]

Does whatever your cladding the frame with have an impact too? Any kind of masonry don't you end up with a mini strip found around the outside sure one member had here 

[Jeremy Harris]

1 hour ago, Oz07 said:

Does whatever your cladding the frame with have an impact too? Any kind of masonry don't you end up with a mini strip found around the outside sure one member had here 

 

 

The external cladding doesn't have a lot of effect, other than to keep the rain off and slightly increase the decrement delay if it's made from a material with a relatively high heat capacity (like masonry).  There are several ways to deal with the way the external rain screen is built, but adding a strip foundation for it wouldn't have any significant effect on the passive slab performance as it would be outside the insulation envelope.  There are some sections showing some ways to do this in that design guide above.

[SuperJohnG]

12 hours ago, joe90 said:

I too initially wanted an insulated raft but S.E. costs fir the design was not cheap. I got an S.E. to design a raft fir the garage (not insulated) and paid £600 for the privilege and ended up with strip foundations a LOT cheaper than the raft (the raft would have held up a block of flats!!!!). Some on here had a passive slab designed as part of the timber construction so the S.E. costs were part of the overall costs. I went for strip foundations copied from here. https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/
which are passive type foundations. My building inspector accepted them with no S.E. Input and I had my own JCB to do the excavation.

 

the only change my builder proposed (which I accepted) was to lower the DPC to the internal floor level, which put the outside DPC below ground level but they inserted a second DPC in the outer skin to cope with “rain bounce”. This was if the DPC is where it is in the diagram it potentially creates a slip plane above the skirting board (crack line).

 

 

@joe90 This part I'm actually struggling with to understand how I approach it. As I will be timber frame / SIPS.  How many SE's do I need? feel as though my architect needs one, but If i have a kit supplier surely they would do it for kit, Then i would need an SE to witness test digs, but if they didnt understand insualetd raft, then  I would need tanners or something. 

 

Then as I am in Scotland i need an SER certificate which my architect can do, but i would need to pay them again?. It's an area I feel really exposed on and don't feel i can get straight answers currently.

 

 

12 hours ago, Jeremy Harris said:

It's perhaps worth having a read of the Kore foundations paper that Hilliard Tanner (an SE who's a bit of a passive slab expert) wrote:  KORE-Passive-Slab-Design-Guide.pdf

 

That's very helpful thanks @Jeremy Harris

[Bitpipe]

If you use a package, such as MBC, for both elements then there will be SE fees for frame and slab included as part of a single price. You will have access to all the calcs etc to satisfy building control.

 

Their SE will still require a ground investigation but will never come on site, the report should be sufficient - always good to see exactly what they require from the report and use this to get quotes from the GI firms so you don't over/under spec the investigation.

 

As we built a basement, we commissioned our own SE (and contractor) for that. We had the frame designed first and then passed that (inc calcs) to the basement SE so they could appropriately design their part. Our SE commissioned the GI study etc. They did come on site but just for a general look round, not to witness any investigations.

 

Note - once you have achieved planning, your architect (and your architect's SE) should not need to be involved in the detailed design of the frame (or slab) if you're using the supplier to do this. Don't pay twice for the same thing.

 

Our architect had no involvement post planning as they admitted they had little value to add as we were using a frame design package vendor and as a practice they had very little knowledge of low energy builds or basements. But they designed us a nice house and got it through planning so we were happy. 

 

The architect did offer to take our detailed drawings, re-draw and submit to BC for £15k but we politely declined

 

We handled our own BC submission, using the very detailed drawings and calcs from both MBC and the basement SE - there was not much more detail required beyond that and it was easy to organise. We also discharged our own planning conditions (13+) as most of it was pretty obvious statements or providing results of tests done on site via the GI or statements of work from archeology etc.

 

 

 

 

[joe90]

As what @Bitpipe says above about kit and their S.E. From what I have read Tanners are the kids to go to ref insulated slabs, and MBC regarding frames.

[SuperJohnG]

1 hour ago, Bitpipe said:

Note - once you have achieved planning, your architect (and your architect's SE) should not need to be involved in the detailed design of the frame (or slab) if you're using the supplier to do this. Don't pay twice for the same thing.

 

Our architect had no involvement post planning as they admitted they had little value to add as we were using a frame design package vendor and as a practice they had very little knowledge of low energy builds or basements. But they designed us a nice house and got it through planning so we were happy. 

 

The architect did offer to take our detailed drawings, re-draw and submit to BC for £15k but we politely declined

 

 

 

 

 

 

thanks @Bitpipe . This is quite interesting as I have my architect (actually architectural technologist but I'm happier with that) who has done a house design for us which we are very happy with. We have completed stages 1/2 with him and the next stage is for him to submit to planning and get approval. 

 

The next stage was for him to do building warrant (more reasonable costs than yours of £3400+vat, which I am happy with) but I assumed I would absolutely need this stage as it would give me all the section details, call out all the electrical detail etc and handle the building warrant process, but I suppose he would be pulling together the kit suppliers drawings?

[Bitpipe]

18 minutes ago, SuperJohnG said:

thanks @Bitpipe . This is quite interesting as I have my architect (actually architectural technologist but I'm happier with that) who has done a house design for us which we are very happy with. We have completed stages 1/2 with him and the next stage is for him to submit to planning and get approval. 

 

The next stage was for him to do building warrant (more reasonable costs than yours of £3400+vat, which I am happy with) but I assumed I would absolutely need this stage as it would give me all the section details, call out all the electrical detail etc and handle the building warrant process, but I suppose he would be pulling together the kit suppliers drawings?

 

I don't know how it works in Scotland but in England, BC are not interested in seeing electrical or plumbing design - they just want the electrics signed off by a qualified electrician and likewise the certifiable parts of the plumbing.

 

Timber frame vendors, who supply and erect as a package, tend to design in house from your planning drawings, including calcs etc so you need to understand what value your architectural technician is adding.

 

Most electrical & plumbing contractors will do their own competent design and will likely nod & smile at any that you provide, aside from position of fixtures etc.

 

As ever, you should get quotes from potential frame suppliers once you have planning. You should not need detailed drawings to get a quote (we didn't) however they wont give you detailed designs to satisfy BC until you commit & pay deposit. Check what is and isn't included in their package - it can very considerably and some will/won't include slabs, internal non structural walls, erection, crane hire, fall arrest, scaffolding etc.

 

Then you make you can make some effort as to 'normalise' quotes and see which ones offer the best value.

 

Your architect may well continue to add value in this process but don't take it for granted and make sure you know what you're paying for - could be you value their oversight and 'hand holding' which is fine.

 

[SuperJohnG]

13 minutes ago, Bitpipe said:

 

I don't know how it works in Scotland but in England, BC are not interested in seeing electrical or plumbing design - they just want the electrics signed off by a qualified electrician and likewise the certifiable parts of the plumbing.

 

Timber frame vendors, who supply and erect as a package, tend to design in house from your planning drawings, including calcs etc so you need to understand what value your architectural technician is adding.

 

Most electrical & plumbing contractors will do their own competent design and will likely nod & smile at any that you provide, aside from position of fixtures etc.

 

As ever, you should get quotes from potential frame suppliers once you have planning. You should not need detailed drawings to get a quote (we didn't) however they wont give you detailed designs to satisfy BC until you commit & pay deposit. Check what is and isn't included in their package - it can very considerably and some will/won't include slabs, internal non structural walls, erection, crane hire, fall arrest, scaffolding etc.

 

 

 

Your architect may well continue to add value in this process but don't take it for granted and make sure you know what you're paying for - could be you value their oversight and 'hand holding' which is fine.

 

@Bitpipe again thanks for your input.

 

The electrical and plumbing do't concern me too much (unless BC want to see) as it's straightforward, I will do plumbing alongside friend who is a plumber and electrically another friend will complete and sign off.  I suppose it is the fear of the unknown here and what value people add at what stage. I'm happy to pay the right money where value is added and where I need guidance.

 

We actually already have planning, but we are going back with a change of house design which the architect is going to do (I could submit to planning myself, as I have previously and it was straightforward..now I'm questioning this). I supposed I had assumed that I needed the architect to do all these things, but the waters become muddy when you have are specifying a kit and specific supplier. 

 

We are actually out for preliminary pricing from kit suppliers just now based on our initial design drawings.  

[Bitpipe]

2 hours ago, SuperJohnG said:

@Bitpipe again thanks for your input.

 

The electrical and plumbing do't concern me too much (unless BC want to see) as it's straightforward, I will do plumbing alongside friend who is a plumber and electrically another friend will complete and sign off.  I suppose it is the fear of the unknown here and what value people add at what stage. I'm happy to pay the right money where value is added and where I need guidance.

 

We actually already have planning, but we are going back with a change of house design which the architect is going to do (I could submit to planning myself, as I have previously and it was straightforward..now I'm questioning this). I supposed I had assumed that I needed the architect to do all these things, but the waters become muddy when you have are specifying a kit and specific supplier. 

 

We are actually out for preliminary pricing from kit suppliers just now based on our initial design drawings.  

 

You're welcome, just sharing my own experience.

 

I don't think there is a right answer here as it's a function of how tight your budget is, where your confidence level is and how experienced you are (these two do not necessarily go hand in hand ) and how much your own time costs.

 

Some people find it reassuring, and can afford, having the architect alongside them for the whole journey while some never use one at all.

 

Personally, I'd let the architect complete the planning process if you're already along that path and see what's coming back from the frame companies based on the initial design - that should give you the data to see where there may be duplication on the professional services or where the gaps may be.

 

I'd also take a look at the building warrant process and see what you need above and beyond what you'd get from the frame suppliers - may help you decide if the £3500+VAT is money well spent or something you could take care of yourself.

 

 

[SuperJohnG]

On 03/03/2020 at 17:10, Bitpipe said:

 

You're welcome, just sharing my own experience.

 

I don't think there is a right answer here as it's a function of how tight your budget is, where your confidence level is and how experienced you are (these two do not necessarily go hand in hand ) and how much your own time costs.

 

Some people find it reassuring, and can afford, having the architect alongside them for the whole journey while some never use one at all.

 

Personally, I'd let the architect complete the planning process if you're already along that path and see what's coming back from the frame companies based on the initial design - that should give you the data to see where there may be duplication on the professional services or where the gaps may be.

 

I'd also take a look at the building warrant process and see what you need above and beyond what you'd get from the frame suppliers - may help you decide if the £3500+VAT is money well spent or something you could take care of yourself.

 

 

Oh yeah will stick with him for planning for sure (and probably the BW stage too) and just to clarify and be fair,  he is doing a good job and the attention to detail is really good. I just like to know what people are doing and what should be expected and this SE area seems to be a little confusing. 

Edited March 5, 2020 by SuperJohnG
reworded to make sense.

[LA3222]

@SuperJohnG I have SIP with a blockwork outer skin on an insulated raft foundation.

 

The way it worked for me is I first of all had a soil survey done to get a ground bearing capacity.  Architectural Technician did my house design which I gave to SIP company and asked them for the point and line loads.  They used their SE to get this for me (incl in cost of the SIP supply/install package).

 

I then contacted Tanners and provided him with the soil survey report and the line/point loads of the SIP and the house design.  They then produced a foundation plan which included a slab for the TF and a separate ring beam to carry the blockwork skin.

 

Gave this design to Kore and i am now pulling my hair out trying to build it in the worst February on record?‍♂️?

Edited March 4, 2020 by LA3222

[SuperJohnG]

14 hours ago, LA3222 said:

@SuperJohnG I have SIP with a blockwork outer skin on an insulated raft foundation.

 

The way it worked for me is I first of all had a soil survey done to get a ground bearing capacity.  Architectural Technician did my house design which I gave to SIP company and asked them for the point and line loads.  They used their SE to get this for me (incl in cost of the SIP supply/install package).

 

I then contacted Tanners and provided him with the soil survey report and the line/point loads of the SIP and the house design.  They then produced a foundation plan which included a slab for the TF and a separate ring beam to carry the blockwork skin.

 

Gave this design to Kore and i am now pulling my hair out trying to build it in the worst February on record?‍♂️?

 

thanks @LA3222 as you know, I'm following your build intently! Helpful to understand how you went about it. I need an SER certificate here in Scotland. I need to spend a little more time understanding what is required here. I'm envisaging that once I figure it out I'll just let the architect deal with it.

 

I am hoping to do render board (I've assumed less hassle and easier/quicker than blockwork) and siberian larch cladding, so guessing I wouldn't need that external ring beam. what is your finish? any reason you went blockwork? 

[LA3222]

@SuperJohnG I went with blockwork as I wanted a more substantial/solid outer skin.  It's a bit of a wind trap where I am so it's just a psychological comfort thing!

 

The blockwork will be rendered so could have been done with render carrier board.