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Tally Ho!

Posted by Crofter , 25 November 2014 · 2,579 views

So, this project has been rumbling along for the best part of a year to get to this point, but here we are at last. Site has been surveyed, design finalised, and planning permission application is ready to submit. Dare I say even the budget may be looking like it might be sufficient.

The design brief was to produce a cheap, simple, but comfortable small house, primarily designed with the holiday let market in mind. It represents something of an under-development of the site, however the purpose is to generate income, not to create a house to sell on the open market. With this in mind, I have elected to go down a slightly unusual build route.

Thanks to advice from 'ProDave' of this forum, I have chosen to design the house so as to fit into the definition of a 'portable building'. This grants the building exemption from Building Control regulations. It does carry certain design restrictions- other than the overall size limits, the building must be some sort of 'box' that could conceivably be detached from its founds and towed or craned to another location. The main advantage of going down this route is that, once I have secured planning permission, I am at liberty to built it pretty much as I please.

Now, this might have tempted me to go 'cheap and nasty' and cut as many corners as possible, ending up with a flimsy, cold, damp, hovel. This is not an enticing prospect for a holiday let, and is not my intention: I want the final product to be virtually indistinguishable from a 'normal' house, both inside and out. I have planned in an airtightness strategy and a 2-300mm insulation layer throughout. Glazed area has been very carefully considered, with the intention of specifying a relatively small number of higher quality windows and doors. The internal finish will be clean and elegant, satisfying the requirements of low maintenance and durability which will be essential for a high end holiday rental property. All holiday lets need a woodburning stove (or so I am told), so I have tracked down a 3kw unit with 88.9% efficiency and external air supply. Simple electric panel heaters will provide such heating as is required. As this building will be lived in only part time, and perhaps at short notice, I need a heating solution which was primarily simple to use and fast in response time.

Externally, the house will have vertical timber cladding, board-on-board, and black sheet metal roofing. By keeping the weight down, I can reduce the structural weight. The house will be built around a timber 'ring beam'- size and materials yet to be finalised, but probably made from pressure-treated wood laminated up to 5x12, or alternatively made from naturally durable larch. This ring beam will be supported on around ten piers, these being made from lengths of RSJ set into the ground vertically. Large bolts will lock the beam onto each pier, making removal of the building potentially simple.
On top of the ring beam, a studwork superstructure will be built up, comprising a pair of uprights joined with strips of OSB, built as a ladder or Larsen-truss type frame. This creates the void for insulation (300mm where possible, but 200mm where necessary) whilst reducing thermal bridging.

The flooring will be built up using 300mm engineered timber I-beams. A 400mm deep beam is used to form the ridge, with one mid-way support hidden within a partition wall. Further engineered beams will make up the rafters. Engineered beams have been chosen to minimise thermal bridging and structure weight. I may end up making up the beams myself (with the input of my SE) but that will depend on costs.

The airtightness layer will be formed at the vapour control barrier, with a 25mm service void on top. This will allow me to minimise the penetrations of the airtight barrier. Once the VCB is in place, I will conduct a DIY leak-test on the whole building, whilst I can still find and fix the leaks. Only once I am satisfied will I proceed with the plasterboard lining.

The floorplan has evolved over many iterations, and in the end I caved in and moved away from the simple rectangle that I had been hoping to stick to. Adding a small lean-to porch makes a better use of space overall and, in my wife's words, will stop it from looking too much like a shed! The final design has a footprint of just a whisker over 50m2, with about 40m2 usable internally. The porch will need to be capable of being unbolted from the structure to keep the overall width within portable building limits.

So, that is the general plan. If anybody sees any glaring errors in my approach, let me know! I have done a lot of reading about condensation, insulation, thermal bridging, etc. The last thing I want to do is put time, money, and effort into a building that falls down or is not pleasant to live in.

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Interesting. Can't say I've ever come across the concept of using a 'portable building' for what is planned as a 'normal' property.
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It is me that has introduced the forum to the concept of "portable building". It stems from a Scottish building regulation bulletin that sets out exactly what they consider to be a "mobile home" and they make it clear that it does not need to be on wheels, it just needs to be capable of being "transported".

I had a conversation with our local building control as we briefly considered this approach, but gave up on the idea as we wanted an upstairs, and this portable building exemption only allows for single storey buildings. but during that conversation they confirmed that a modular building, capable of being craned onto a low loader qualified as portable.

The other bit of useful information they gave me is the drainage system would still come under building control so you would normally install that and have it inspected before you then install the portable building.

For anyone reading that wants to know more, there's a thread about portable buildings on the main forum http://www.ebuild.co...table +building
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I've corresponded with Building Control over this, and have been told that the building need only be capable of being moved around on a site, not necessarily onto the public road. I was concerned about this because the public road that serves my site is single track, so a 6m wide building would not fit down it.
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One more thought. Rather than plasterboard, consider Fermocel for lining the internal walls. It's a lot tougher than plasterboard, and less likely to suffer from any movement should you decide to move the building. Another thing that helps "prove" it's portability.
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Thanks for that, Dave. Looks like good stuff but at five times the cost of plasterboard!
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Yup, re Fermadcell you won't need to plaster and it will make any wall mounted units/cupboards transportable. There's quite a good discussion on the pro's and cons on GBF and to adovcates said it was a wash on total cost.
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OTOH I have a plasterer in the family who will do me mates rates. Was about the only trade I was planning on having. Budget people, budget...

Moving on from plasterboard, any comments on general construction methods? The walls will go:
Cladding-battened void-membrane-sheathing-insulation/studs-osb-vcb-battened void-finished inner wall surface.

The VCB will be the airtightness layer.
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Hi Crofter, I think you will have to consider the porch very carefully. It doesn't look like you will be able to fit it within the 6m limitation as you say and you cannot break it down into more than two sections and you are suggesting 3 sections. The porch will be _very_ expensive as well to build and the planners will probably want it to have a gable. Stick a shed nearby for keeping all the wet weather gear and wellies. Not so convenient in the bad weather but will help budget massively. Add the porch on in a few years...

I also would be tempted to avoid a larsen truss structure on a building like this as you are already short on space and the walls will be disproportionately large - I'd go for 2x4 stud walls and use board insulation between the studs and sheath over it with external insulation board before cladding. You will claim a lot more internal space and it will help avoid bridging issues.
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Thanks for the suggestions. I'll try to explain my thinking thus far.
The building only splits into two sections: the main part, which is about 9x5m, and the porch, which is about 1.5x2.8m, plus the tongue of roofing which will stick out to meet the main house.
I totally agree that the porch will put up the costs and will add complexity. And heat loss. I know this, and it was a tricky decision.
Previous versions of the layout tried to incorporate the porch area into the house, which meant lengthening the whole building, which added a lot of extra floor space where it wasn't really doing any good- basically, the gap between the sofa and tv just got a bit bigger, and the patio door got wider, which is more expensive in itself. At the end of the day, the wife gets the final say...
I'm reasonably confident that the planners won't mind the monopitch porch- our neighbours build their house last year with this sort of shape on the same elevation as ours will have, and some of the older houses around here look like that too. I did initially sketch up a pitched roof with gable on the porch but it looked clumsy.

Why do you suggest adding the insulation on the outside? I could do that, but the main ring beam then moves away from the perimeter. If you were ever to crane the building off the ground, you would need massive spreader bars to stop the outer layers of the walls from being crushed.

Is there an insulating sheathing material I can use on the outer walls?
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Didn't realise you were having the main building as one, that makes more sense. However are you not still going to be over width with the building at 5 and the porch 1.5 + your overhangs for soffits/gutters etc eating into your space?

Insulating the outer sheathing board helps keep the entire structure warm. Play around with some U value calculators online and see how different build ups compare then price them out. I think you will find that a 200mm wall with a truss or I beam style filled with rockwool or similar will perform a lot worse than a 100mm stud wall with board insulation between studs and say 50mm clad over the outside. When you are getting into the 300+mm depths they make more sense but you don't really have the space to spare for those depths in the walls. Underfloor and ceiling yes, but walls no. You can use a wood fibre board outside but I wouldn't bother, 9mm OSB3 for racking and 50mm celotex for insulation will give better performance behind your cladding.

Realistically you aren't going to be craning the building anywhere so it can all be academic, yes you can use beams and spreader bars to lift it without damaging the outer layers. You don't need to buy or make them!
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26 Nov 2014 12:40 PM
Could you consider placing the porch outside the thermal envelope, or additionally even leave it open at the front? The main function I'd imagine is to provide an area to shed and store wet, mucky clobber and a bit of air movement might be as effective in drying things as heating, the added bonus being you keep this dirt and moisture out of the building itself.

This would allow the wall sections to be thinner, giving more space inside or allowing the external dimensions to be smaller, making it more subservient to the main building.

I would personally look for ways to connect the porch roof level with or below the eaves line of the main roof as it simplifies the construction and would make detaching the porch in one piece much easier, but I appreciate the need for the building to be "portable" is merely to satisfy certain paper exercises.
Good luck with the build! Would love a project like this some day!
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When I had my discussion with building control about the dimensions of a portable building, I was told it was a measurement from the outside face of the walls. Overhanging roof verges, guttering, downpipes etc were not counted.

I would stick to 6 metres total, so shrink your porch a bit, but allow it's mono pitch roof to overhang a bit? I would also keep it outside the thermal envelope and probably not even have a door on it, just somewhere you can run under when it's raining. We have a porch on the present house and you still have to stand in the rain fumbling to find your keys, so it's a complete waste of space and money building it. Not again on the new house.
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Having a 'D'oh!' moment here- I hadn't considered that the 6m width was an overall limit, rather than a per-section one. Just as well that was pointed out to me...

I like the idea of simplifying the wall structure. I guess I got rather fixated with how much cheaper glasswool is than PIR/PUR, and I (mistakenly?) thought that a Larsen truss idea would be the cheapest way of achieving a good U value.
I did deliberately planned on having relatively thick walls, so that as things progressed I wouldn't risk running out of space, or options.

I tried squeezing the porch roof under the eaves overhang, but it's tricky, principally because the top of the door into the house is only just below the eaves- the 300mm thick suspended floor pushes everything up.
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If the main building is 5 metres wide externally, then that still gives you 1 metre for a porch.

A compromise, if you want an enclosed porch (i.e 2 doors to get into the house) is just take a small bite off the corner of the living room / kitchen area. For instance, the porch intruding 600mm into the room to nicely line up with the edge of the kitchen worktop run?

As your building control have told you the building only needs to be "portable" within the site, then I would not bother making the porch a separate detachable module. Just make it one integral part of the 6 metre wide building so if you wanted to, you could pick up the whole thing and crane it to the other side of the site.

As you want to get the maximum insulation for a given wall thickness then solid PIR insulation is the best, but more expensive. BUT you are still talking of a 5 metre wide main building when you are allowed 6 metres under the portable definition, so you still have scope to have thicker walls. Our build is planned to be a 195mm timber frame with blown in insulation, then clad with 100mm insulating board then render. So that's a total wall thickness allowing for render and service voids of about about 370mm and they hope to be achieving a U value of about 0.12

Regarding the porch roof, a little gable end on the front of the porch, like our existing house (see avatar) that would nicely tie into the main roof.
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Crofter, on 26 November 2014 - 04:40 PM, said:

I guess I got rather fixated with how much cheaper glasswool is than PIR/PUR, and I (mistakenly?) thought that a Larsen truss idea would be the cheapest way of achieving a good U value..

Your building can work with more expensive board insulation because you simply don't need all that many boards! The joys of building small. Double check the numbers compared to the larsen trusses, but timber isn't all that cheap, there will be more bridging with 200mm larsen trusses and if you keep your eyes open then you can pickup seconds on insulation boards at very good prices indeed which can be a massive saving on the budget. You should be able to achieve a wall build up of around 250mm by the time you take your cavities into consideration you are likely to be around a U value of around 0.15-0.16 depending on the insulation used. That's easy to improve on by simply thickening the outer board a little bit more, but I would be careful about going too far down that road as you will find that the fixings to hold the counter battens on will start to get expensive.
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Lots to think about. Thanks.... I think.... :D
Maybe I should have mentioned that the porch is more of a utility room- I don't really want the washing machine in the kitchen, seeing as it is also the living room. However I am over the 6m limit so something has to change.

Are there benefits to EWI because it keeps the timber frame warm (and free of condensation)?

My eaves height is causing me a bit of a constriction. I've been trying to keep the cladding length to 2.4m for obvious reasons, but the floor as currently designed is 300mm, and I want the cladding to be flying past all the structural timber underneath the house. Thanks to thick walls, I have managed to pull this off, but if the walls are slimmed down I will have to switch my approach to cladding. I'll post a pic of the cross section to explain what I mean.

EDIT- added a cross section to the original blog post
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Back to the walls- is there a u-value calculator somewhere that lets you combine different products? I'm leaning towards a much simpler wall construction- 100mm stud- with 50mm boards outside and, if I can afford it, inside too. Leads to amazing u-values. Trying to work out how a sandwich of 50mm board outside + 100mm batts between + 50mm board inside would perform.

(Planning application receding into the far distance as I type :/)
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The resistances (= 1/U) add up, so Utot = 1/(1/U1 + 1/U2 + 1/U3 ...). BTW, don't forget the OSB layer. I wouldn't regard insulation board as having any structural strength.
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I wouldn't bother trying to get U values past .15 U or thereabouts for the walls, I'd fill the studs and put the boards outside as well and leave it at that. I think its a good idea to keep the timbers warm and your 50mm boards inside would not help do that. You want a stove so no point in making the building so well insulated it can't be used. Remember it is a small building and you won't need to hit tiny U values to have a very small heating load. You don't have any real materials there to trap heat and release it slowly (like a decent concrete slab or block walls) so I think you need to balance things out. Have it so that you can run the stove and not cook everyone inside the building in 20 minutes. You might want to do some more modelling to show how the house will actually sit on site and how you will hide the piers etc, you might find that a skirt of some kind would look better and it could come up behind the sarking cladding boards and cover the ring beam. Don't discount a shed for the laundry either! It can be made up and clad with all the left over bits from the house :-)
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I thought that you could, in theory, add up the R value of your wall elements, then the inverse of that total is your u-value. Except the online calculators disagree with this method, by a reasonable margin.
What would a 100mm stud wall filled with batts, and an outer 50mm layer of PIR, come to?
Do you just use *really* long screws to fix your battens over the top of the 50mm boards?
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