Owner-Builder looking to build our family home!

[JamesP]

I stick built and have enclosed my wall build up. Hand drawn! The t&g Pavatherm is a great product. Air tightness, insulation and attention to detail most essential. Minimal heating required.

 

Floor, Wall, Roof Build up Jan 2018..pdf

[IanR]

13 hours ago, AccidentalSkydiver said:

I'd picked 37x37mm off top of my head without consulting my tape measure...

 

I'm not sure I'd use a twin stud, unless I was benefiting from its ability to allow a simpler foundation/raft detail, while removing all thermal bridges at this junction. Using a larger 89 x 37 section, at least for the inner stud, allows all loads to run down through the inner stud to the foundation, so that the outer stud can sit over the perimeter insulation of a slab and remove any thermal bridges to the floor and sole plate.

 

If you are going to use smaller sections, that require both sections to take the vertical loads, the an I-Joist seems simpler to me.

 

There's not much to choose between twin stud or I-Joist build up. I-Joist is easier to stick-build on site and has a slightly lower timber fraction, but requires a slightly more complex raft detail to eliminate all thermal bridges.

 

I've never seen it done on a twin stud structure, perhaps because the outer studs are often not load bearing, but an I-Joist structure can allow a very simple build up, combining the racking strength and air tight layer in the outer sheathing, using T&G Egger DHF board, and then a foil-backed plasterboard on the inside as the vapour barrier, with the blown cellulose in between. No membranes required for air-tightness or vapour, although a breather membrane externally for protection. To get very high air-tightness though, a non-curing butyl adhesive is required in the T&G joints.

 

The T&G DHF board and blown cellulose insulation can achieve sub 0.2 ACH, without membranes, if the detailing is done well. Sub 0.1 ACH on mine.

 

This simple build up does mean no plumbing in the external walls (cabling is OK), unless you add a service void. But, that means no OSB, other than where you want plumbing.

 

I'm not sure structural performance should lead the choice of framing type. While it needs to be considered, I would prioritise thermal performance (energy loss) v. cost of the whole envelope: walls, roof and floor. Structural performance leads in to cost, as does the labour required for an over-complicated build up.

 

21 hours ago, AccidentalSkydiver said:

Because I'm an awkward git? 😁 I'm 100% going to be framing traditionally, 

 

Ah, but as a Chartered Engineer, once you have all the required data, you will re-calibrate your view. There is a better way...

 

Edited to add:
If you are looking at JJ I-Joists, James Jones & Sons will pre-cut all the holes for MVHR ducts etc., if you plan ahead.

Edited March 22, 2022 by IanR

[Iceverge]

@IanR as I remember you built within an existing barn. Would you need extra racking strength on a stand alone build? Is the external egger your racking? Is it possible to test and fix air leaks as you build?

 

@AccidentalSkydiver compare your build to @IanRs for thermal performance cost and importantly build time. Even using solid timbers , a small thermal break such as a crossbattened 45mm service cavity creates enough of a thermal break to shift the focus heat loss towards other areas such as external thresholds or ventilation ducting. 

 

[IanR]

2 hours ago, Iceverge said:

@IanR as I remember you built within an existing barn. Would you need extra racking strength on a stand alone build? Is the external egger your racking? Is it possible to test and fix air leaks as you build?

 

My timber-frame, built within the original steel portal frame of the shed I converted, derives no strength from the steel frame. ie. the two are not structurally connected at any point. The build up I have is a typical Cullen Timber Design buildup they use for new build, PassivHaus builds. The Egger DHF is the racking strength layer, as well as the air-tightness layer.

 

The first opportunity to air-test would be when all external sheathing is complete, sole-plate sealed to floor, doors and windows in and no plasterboard (and therefore no insulation) installed, so loads of opportunity to fix any leaks. The air-tightness then improves further with the installation of the blown cellulose fibre.

[Iceverge]

I'm nearly converted, just the money hoarding miser on my shoulder needs to be convinced.  I really fell down a rabbit hole in excitement last night! 

 

I've been scouring the internet for prices of I joists and the closest I could come to a definite answer was from the US, a 2*12 TJI is about twice the cost of dimensional lumber there. Any better guide this side of the Atlantic? 

 

How about the Egger DHF? A polish website suggested it was roughly similar to OSB, is that correct? 

 

Reading back I see you used a raft @IanR.  Soil dependant, a strip foundation is still cheapest near me. There's no reason an I joist/beam wall couldn't overhang a hefty upstand to the floor slab creating a superb wall floor detail with a strip foundation. 

 

How much faff is there is sealing the external woodfiber board, is it a no go in wet and windy conditions?

Did you have any issues with the dense pack cellulose billowing out the plasterboard?

Apart from the foil backing was there anything special about the plasterboard, was it a double layer or extra thick?

Did you skim over the patched up cellulose holes or tape and joint? Any tips on doing this speedily as it would seem to be more difficult than just taping an airtight membrane. 

 

I'm aware I've queried some of this before. Thanks for you indulgence!  

 

 

 

 

[Simplysimon]

1 hour ago, Iceverge said:

How about the Egger DHF? A polish website suggested it was roughly similar to OSB, is that correct? 

this https://mdfosb.com/en/products/medite-vent is the same as egger

this for internal osb with integrated vcl https://mdfosb.com/en/products/smartply-propassiv

and this shows after being blown https://www.lathamtimber.co.uk/products/panels/osb/smartply-osb/propassiv

@Patrick and myself used this system. service cavity on top of propassive

[IanR]

3 hours ago, Iceverge said:

I've been scouring the internet for prices of I joists

 

Somehow you need to get a bulk price direct from the manufacture, ie. James Jones & Sons or equivalent. You'd be buying an artic load, so you won't be paying BM prices.

 

If you went this route, would you pay for a frame design, or do this yourself? With my frame Designed and Engineered by Cullen Timber Design, they send the cutting files electronically to JJ&Sons, and cutting is automated from their files. It's got to be the cheapest way of buying the I-Joist package, but I never saw this price as it was wrapped up in an erected price from Touchwood Homes. I never saw the price for the Egger DHF board either.

 

3 hours ago, Iceverge said:

Reading back I see you used a raft @IanR.  Soil dependant, a strip foundation is still cheapest near me. There's no reason an I joist/beam wall couldn't overhang a hefty upstand to the floor slab creating a superb wall floor detail with a strip foundation. 

 

I've only ever seen both flanges of the I-Joist supported when used for wall framing. When you say "overhang an upstand" does that mean the outboard flange being unsupported, or sitting on insulation? I don't believe you could do that.

 

3 hours ago, Iceverge said:

How much faff is there is sealing the external woodfiber board, is it a no go in wet and windy conditions?

 

Every T&G joint needs a generous squeeze of adhesive, not difficult to do, but there's a lot of joints.  The difficult detailing is where boards come together at corners, or the free edges. Making sure the the boards in the corners have a continuous surface on the underlying frame to be sealed on to is more robust that trying to seal to edges of boards coming together at a corner.

 

Wet boards may stop the glue sticking, you'd need to take advice on that as I've not got experience of it.

 

3 hours ago, Iceverge said:

Did you have any issues with the dense pack cellulose billowing out the plasterboard?

 

Not at all. But, joists we're on not more than 400mm centres, on the external walls and the roof, and PB was screwed every 150mm.

 

3 hours ago, Iceverge said:

Apart from the foil backing was there anything special about the plasterboard, was it a double layer or extra thick?

Did you skim over the patched up cellulose holes or tape and joint? Any tips on doing this speedily as it would seem to be more difficult than just taping an airtight membrane. 

 

Nothing special about the PB. Just 12mm foil backed. I put the 100mm PB discs I cut out, back in to the hole once the insulation is installed, screwing the discs on to a small length of batten I bridged the hole with. There's a lot to do, but it's surprising how quick you get. I had the PB skimmed.

Edited March 23, 2022 by IanR

[jamieled]

To give you a rough idea, I paid £70 ex VAT per 12 m length of 47 x 300mm JJI in 2019. Buying direct from James Jones will be difficult. A single lorry load is probably too little for them to bother about. They have a list of distributors on their website, I ended up using Pasquill as they were local to us in Inverness. If you need any additional engineered timber it might work out cheaper going through a distributor as they can get you everything you need in one go minimising delivery costs and might be more likely to give you decent prices with the volume.

 

Buying smaller lengths or getting the whole thing pre-cut would be advantageous. Although we had a relatively spacious site, moving 12m lengths with a forkilft wasn't straightforward. Having said that, it would only work well if the frame was properly designed - our wasn't and while I got the roof JJI's pre-cut we ended up with a lot of wastage due to the incorrect positioning of a velux on a drawing.

[Iceverge]

6 hours ago, IanR said:

If you went this route,

 

 

Our house is built, from hundreds of tonnes of concrete, oh if I had my time again. Hopefully I'll gather the cash for some other smaller projects in the future though. 

 

6 hours ago, IanR said:

I've only ever seen both flanges of the I-Joist supported when used for wall framing. When you say "overhang an upstand" does that mean the outboard flange being unsupported, or sitting on insulation? I don't believe you could do that.

https://passivehouseplus.ie/magazine/new-build/the-west-midlands-eco-house-with-no-energy-bills

 

Thanks for your reply. 

 

I'll keep pondering. 

 

 

 

[IanR]

7 hours ago, Iceverge said:

https://passivehouseplus.ie/magazine/new-build/the-west-midlands-eco-house-with-no-energy-bills

 

Is that really an EPS upstand? Does look like it is performing at the same level as the EPS under the slab.

 

 

 

It doesn't look like it would work (to me), but if that's a true reflection of the built house then my views on the structural capability of I-Joists are too conservative.

 

I'd like to see their internal plaster after a coupe of years and see if there's much cracking from any settling of the frame.

[Iceverge]

6 hours ago, IanR said:

Is that really an EPS upstand?

 Looks like it. 

 

6 hours ago, IanR said:

I'd like to see their internal plaster after a coupe of years

 

True. 

[TerryE]

9 hours ago, IanR said:

It doesn't look like it would work (to me), but if that's a true reflection of the built house then my views on the structural capability of I-Joists are too conservative.

 

I'd like to see their internal plaster after a coupe of years and see if there's much cracking from any settling of the frame.

 

Have a look at any of the MBC warmslab videos and you will see that they use a similar technique.  It is extremely effective at stopping any material bridge at the floor TF junction.  Typically EPS 300 is used for the last 60cm of the base and upstand.  If you look you will see that the inner leaf of the Larson strut cassettes is the load bearing one and it is standing directly on the sole plate and warm slab, so isn't going anywhere soon.

  

[IanR]

7 minutes ago, TerryE said:

Have a look at any of the MBC warmslab videos and you will see that they use a similar technique. 

 

Absolutely I'm familiar with typical twin-stud and Larsen truss wall build up, but MBC's twin stud uses 89 x 37 section size. As a typical framing section, it's logical the vertical loads can be carried through just one side of the twin-stud, allowing the other to overhang a non-loadbearing insulated upstand.

 

For the I-Joist in the image, the flanges are 45 x 47, and finger-jointed. To my knowledge this isn't a typical framing section, and with the outer flange sitting over EPS 100, it's not going to be taking any load.

 

For this reason, the detail I'm familiar with for a thermal bridge free slab detail with I-Joists is a little more complicated than required for a twin-stud and looks like this:

 

 

A simpler, but not quite thermal bridge free detail is:

 

 

Where the EPS L former width is reduced at the top to get the concrete slab under the outer flange, to support it.

 

Don't get me wrong, I like the detail shown with the unsupported outer flange of the I-Joist, and an SE must have been happy to sign it off, but it doesn't follow I-Joist principles I'm familiar with. In fact, I needed to provide additional calcs for BC to convince them the outer ring-beam in the first image above could take the loads that would come through the outer flange, with "just" 100mm of EPS beneath.