Hi everyone,

first time poster so go easy.

I'm designing a new-build for my family and whilst not seeking to achieve full passive house accreditation I want to maximise efficiency within my budget - I have a couple of queries which I would welcome input on:


I had planned a closed panel timber frame construction with OSB on inside face of studs to create the airtight layer - however, I struggle to see a practical way of continuing the airtight layer at 1st floor joist level as the joists will penetrate. Am I best to use the OSB on the external face of the timber frame as the airtight layer (this would appear much easier to achieve on site but I've read elsewhere that it is best to position the airtight layer on the warm side of insulation if possible?)

2nd query - I'm weighing up 3 different options for the timber frame studs:
a) JJI studs (I-joists used as vertical studs thus reducing the thickness of timber and resultant cold-bridge). Various section sizes available allowing thicker walls / more insulation - http://www.jji-joists.co.uk/index.php/products/jji_studs

b) Space Stud (2 separate studs with steel clips to bind them together with insulation between - effectively creates removes cold-bridge). Wall depth can be adjusted to allow more insulation thickness -http://www.ochiltimber.com/spacestud.html

c) 'Standard' timber frame construction with 140 studs, then OSB lined internally and horizontal battens with additional insulation fitted on inside face.

I'm in the process of costing these 3 options but has anyone adopted any or all methods and how practical / cost-effective did they find a & b versus the more traditional timber frame construction?

For info my proposed wall build up shall be (from outside): 19mm horizontal larch cladding, 25 x 50 vertical battens / ventilated cavity, building paper, 9mm OSB, vertical studs/insulation(tbc), horizontal battens with additional insulation and service run, plasterboard internal finish. If airtight layer is being fitted internally then an additional layer of OSB or ply would be fitted between vertical studs and horizontal battens.

I can provide initial details if it helps and happy to share cost comparisons in due course if it would assist anyone.

I'll be back with a separate query re windows later!

thanks in advance.

thanks timber - I'll have a look at Trada publication

@borpin

I think I agree that outer OSB is the way to go as the airtight layer.

Can I ask why icynene between your studs - is it cost effective?

Re your roof build up - can i ask how you're fixing your battens to / through the PIR? It is another issue I've been toying with but the problem is that up here (Scotland) the roofers traditionally fix slates directly to SW sarking boards so asking them to fix to battens is alien, asking them to fix to battens fixed to PIR is likely to induce a bout of head-scratching to say the least

thanks timber - I'm familiar with the options and I'm using your option 1 as a start point (as I see it as being the cheapest construction) - I'm trying to assess your option 3 as a comparison in terms of cost / ease of installation on site weighed against the obvious U-value improvements it can bring.

the timber frame will be fabricated and erected on site by experienced timber frame joiners (albeit largely experienced in standard timber frame or reverse wall types rather than deep stud construction).
There are no space restrictions on my site which would prevent me increasing the wall thickness.

Anyone have any experiences of practicalities / problems encountered with deep stud construction? (positive or negative)

@hairydude I'm up here as well. If you need to slate that is a different problem as it needs to be proper sarking as OSB 'bounces' too much when slating. I'm using a slate like ceramic tile from NuLok. You may find you would have to put more sarking on top of the battens over the PIR.

On Icynene, it is probably slightly more expensive (but not much) but the price includes any protection (windows etc) fitting, levelled and all waste and protection removed. It is quick (compared to cutting PIR) and minimal waste. You also gain an extremely air tight finish. For all these reasons I will probably go for it.

On 140 studs, very heavy panels. Double studs, double the cost for no gain AFAICS.

I can get 100mm stud walls built on site with all sections glued and nailed - cheaper and better quality.

Some structural engineers go all wobbly when 200 mm studs on 100 mm blocks are mentioned. They get all stressed out about excentric loads. If your SE is happy with that, then great!

If you are getting it framed on site, I would strongly suggest getting a book on timber frame and reading up. I am sure your carpenters will do a good job, but I will bet you that you save the cost of the book at some point on some thing.

Posted By: hairydudeBorpin, you mention fixing sarking boards over the PIR - which would presumably involve fixing into battens over or in the PIR? How are people fixing through or into the PIR in practice? Or are they fixing right through the PIR into the rafters?

Not my detail other than the counter battens will be directly fixed through to the rafters. Then tile battens (well actually a steel channel for nulok) fixed to them.

18mm may be OK, but advice to me previously has been that OSB and slates are not a good combination as the slates already on tend to be 'bounced' off. At any rate, with PIR over my OSB, you would need a layer of something and an open covering like sarking board may well be best (if you have to slate).

Sorry Hairydude, see you had already made the point about direct fixing of tiles/slates. My main concern with OSB in that scenario would be saturation but then I would be concerned about any timber.

Posted By: TimberSome structural engineers go all wobbly when 200 mm studs on 100 mm blocks are mentioned. They get all stressed out about excentric loads. If your SE is happy with that, then great!

If you are getting it framed on site, I would strongly suggest getting a book on timber frame and reading up. I am sure your carpenters will do a good job, but I will bet you that you save the cost of the book at some point on some thing.

Have spoken to my own SE (who is signing off SER) and the manufacturers of space stud - both happy with offset loading of deeper stud - within reason, I don't think they'd allow a 400mm stud over 100mm (although I have had engineers in past who quibbled over 140mm studs on 100mm block - I guess it depends on how cautious they are).

Having designed and managed various site-built timber frames over the years I'm comfortable that the detailing isn't a problem. (in my experience, I have yet to order a factory fabricated timber frame which hasn't needed onsite adjustment due to fabricating inaccuracies so I tend to favour site fabrication).

If I have understood the application correctly i.e. to fix counter battens through insulation into roof joists. Heifix Inskew and Thor Helical spring to mind, with the caveat that I have never used them! As all my insulation is inboard of the OSB I am using spacers instead of counter battens (approx12mm in thickness) to elevate the battens, or steel channel in the case of NuLok, above the surface of the breathable membrane, which is lying directly onto the OSB. In respect of the helical fixings I understand it can be difficult to maintain direction to ensure connecting with the joists but there are others on the forum who can give first hand experience of their use. If I was needing to do this myself I would be tempted to use one of the longer specialist timber framing screws, if I can find any links I will post. Not sure if that helps

Timberfix Plus 250 mm (hex head) Heco Topix up to 300 mm (T40 head), both Screwfix. I have used Timberlok hex head construction screws in the past and they are very easy to drive and keep on line.