davidc

Thanks @JSHarris , I didn't know about the cavity insect barrier. Yet another row in my check list spreadsheet !

Ah, maybe i never saw it in 708 anyway as the caveat i was eluding to is in the notes of VAT431NB and makes it plain: "you cannot claim VAT back on any materials used on the construction of rooms above or attached to a detached garage". Guess a taller garage and founds could be reclaimed but sans off all of the materials and labour needed for the upper floor. I could say the high garage roof space is required for parking camper van with roof rack for cycles (Penny-farthing) if questioned ? I could do that @scottishjohn but as i'm not certain yet for sure that the garage will be replaced with a taller one then i'd be beefing up the founds for no tangible reason if it didn't go ahead i guess. But if @Ferdinand is right on the modest difference in cost then it becomes much less of an issue.

Thanks. History is that i phoned HM Revenue and Customs last year and was left with the impression that a newly built garage (built at the same same time as zero rated new main dwelling) also attracts the exemption bestowing it with zero VAT rating. But where the detached garage has an ancillary amenity, as in my case insulated workshop above, then it probably looses the exemption. Quickly (?) reading through Notice 708 i'm struggling to find where this is spelt out and it is possible i may have been misinformed. Even my plan of retaining garage during build process in any form may be problematic as 708 also states that, for a new main dwelling to qualify for zero rating, "before construction starts, any pre-existing building is demolished completely to ground level". I may be digressing into VAT perhaps too far for a foundation post and perhaps should pose the VAT quandary to the VAT section. Either way i should stress i'm not at this stage definitely going to proceed with a taller garage replacement but what i didn't want to do is have a slab put down in one particular way if a small change/consideration will save time or costs if replacing with a taller garage with room above at a later date.

Is a timber clad & timber frame wall build up any more likely to suffer from a rodent problem than a wall build up that incorporates a leaf of block work ?

I am having to move a simple lightweight detached timber garage from its current location to a few meters distant in order to make way for new house build. A simple shuttered slab will suffice (that is what it now rests on). In the future i would like to have the option of replacing the current garage with a taller one with the same or very similar footprint that accommodates an attic workshop above. I don't want the expense of providing more substantial garage founds at the same time as the house build than the very simple one i have in mind. Neither do i want to include it on the building warrant either as the zero VAT eligibility rests on no outbuilding being involved - something that, even if i pay for the garage foundation work entirely separately with VAT paid in full, could mire the main house VAT status in complications. I wondered, if to accommodate any future taller (and ever so slightly larger footprint) garage, i could go ahead with simple slab to get the garage out the way and (at a later time) a deeper built-up perimeter around the simple slab to support the new taller garage that will satisfy building control in a way where the proposed new simple slab doesn't have to be ripped up again ? And if so does anything need to be done differently when planning the initial slab that in the future can accommodate something around it that will support the taller garage structure ? I'm in Scotland i should add.

Thanks all. That's one less thing to worry about !

Has anybody experienced any reluctance from warranty providers (hence also mortgage lenders) to cover EPS type passive slab foundations ?

Their response was "the Zell will still have better anti slump because of its composition and it also goes in less dense as per above figures (on average)". Which, on my reading, implies the opposite of what i had assumed/expected !

I am trying to decide between these two for wall and roof insulation. I asked one supplier (who offers both types - Steico Zell and Steico Floc) for a comparison and the response was that wood fibre (at density ~40kg/m^3) is more resistant to slumping than cellulose (at ~60kg/m^3) but as they both have similar heat capacities then the cellulose, on account of the higher density, will have better decrement delay. Frustratingly they only have airflow resistance figures available for the cellulose version and not the wood fibre. Does any one else have any thing more to add on how the two compare ?

Just before i pursue the passive slab idea further i wondered if anybody here has experience of a traditional slab but using countermeasures for the thermal bridging (floor to wall) such as perinsul or similar ?

I see, thanks - that is helpful to know. As well as possible tape degradation i wondered if any settlement or slight timber movement might become a factor - less likely in modern engineered frames i guess ?

Has anybody any evidence or inclination as to how well air tightness endures over periods of years say in a TF with taped up smartply on the interior with blown cellulose or woodfibre insulation (and less importantly medite vent boarding on the outer) ?

I'm not ! Yes, SER is as Scottish thing as I understand it.

@the_r_sole yes, i am intent on using my own engineer..... but it worried me a little that the TF company were implying that if i had been using their engineer then it they would not have signed it off.

Thanks everyone for the input. I'd rather the TF supplier voiced any qualms about the foundations, if they hold any, right from the start - i appreciate that it is the engineer and local BC who's view is paramount but i just would like to feel assured i'm not storing up any extra potential issues for the day the TF company turn up on site. Ideally, as noted, it would be one main contractor doing both but i'm not sure i can afford that. Thanks @jamieled , i'd love to have a look. I'm up at the site every couple of weeks or so. Would like to get underway before the frost if possible so seeing your groundworks and founds will be helpful.

Thanks for everyone's input. I suppose because I have only seen the airtight layer implemented as membrane with tapes and on the inside never sealed sheathing on the outside I wondered if there was some fundamental reason why the latter would necessarily be inferior to the former. If the vapour control caveat is taken into account and the sheathing and sarking joints are carefully sealed it sounds like it could be advantageous.

I am wondering if I should commission a Soil Survey as a preliminary step in my Scottish self-build programme. Is it a forgone conclusion that the engineer will require one ? Or not that simple ? It has been suggested that irrespective of whether it is required as part of the regulatory or engineers process it may in the end save more money than it costs through being able to design the founds to the specific ground conditions rather than over engineering them to cover a spectrum of possible ground types. It has been suggested that if i can get a contractor to dig the trial pits (digger already needed to move garage out of way of new house) and log the strata and take photographs myself this would be enough to give to the engineer. As i have no idea how to classify the strata i'm wondering it it might be better to just commission somebody to take of care of the the survey as a whole. I would be grateful for any thoughts.

My preferred build type is TF (perhaps SIP but less likley now) and I am trying to decide between more conventional foundation systems versus passive slab type. As I want the TF suppliers (who are also erecting) to be comfortable with the proposed foundations i have asked a couple of them for thoughts on the matter and received responses along the lines of : "The details of this are much more suited to a European build where the insulation is all external, and the masonry all internal, allowing for continuity of the insulation. In the UK, the detail for transfering the load of the insulated frame sits on the slab, and then you creaste a step detail to avoid cold bridging. It isn’t neat because the slab design is unsuitable for typical UK construction detailing. Our engineer’s won’t sign it off as it is essentially a floating raft without any tie in to the ground formations." and "Any engineer requiring to sign off a SERS certificate may have issues given load bearing partitions are normally supported off foundations walls too." So i'm left wondering which method will be best route to take. Can any add their own view/experience ?

I am trying to asses the pros and cons of having the air tight barrier inside or outside the main wall/insulation (that is for TF, SIP and similar build types). And also if it is better to opt for tapes and membranes or to rely on sheathing boards and mastic. As I understand it a benefits of having the layer outside (i.e. close to the exterior) of the insulation is that draughts are less of a factor undermining the thermal efficiency of the insulation (perhaps less of a factor for blown high density type) and the amount of airtight barrier punctures (or potential punctures) for services is reduced as the layer is not in close proximity to most of the service runs. At least one package provider I have looked at uses mastic and external boarding outwith both the TF wall and roof to form a continuous airtight layer and I wondered how this is viewed as up to now I have only encountered the tape and membrane solution placed on the inside of the wall and roof (with a service void inside).

I see, thanks. So @ProDave , do you feel that you may otherwise have been more ready to recommend the wood fibre sheathing if there had been a gap been that and whatever the final render sits on ? I had never seen the weberhaus system before, I wonder how they prevent the possible blistering (assuming they do).

Might there be any compelling reasons not to consider one of the external wood fibre sheathing products for use on SIP constructions such as the one offered by EWI Pro? I have posed a question about wood fibre on timber frame but wondered if there might be particular considerations for SIP ?

Thank you all. Regarding the decrement delay question I had assumed that part of the overheating problem and hence delay issue was due to (solar) radiation rather than thermal conduction - that is why I thought the isolated blockwork leaf may have made a difference for the walls at least in that regards. Looking at some older posts on here it seems I might not have been alone in that (possibly flawed) thinking.

Thank you all. I wonder what the discrepancy is between the two calculated decrement delay values for the same build up. If I decide on the SIP route I could perhaps look at augmenting the 200mm SIPs with some external sheathing to lengthen the delay and bolster the U-value also.

By using SIP (both walls and roof) will I end up with a short-ish decrement delay on account of the insulation layer being polyurethane ? I am comparing SIP against TF and am assuming that U values and air tightness (with a good TF supplier and installer) will be comparable for purposes of comfort factor.

Thanks. So would having a 100mm relatively lightweight block work leaf inserted between cladding and timber frame aid the decrement delay appreciably ?