A Timber framed house with external wall insulation

[ProDave]

A lot of people ask me about the detail of how my house is built so I thought it worth a thread to explain things.

First off, I didn't want an "ordinary" timber framed house with a cavity then a brick or block outer skin. That outer skin just costs a lot of money and adds nothing to the insulation of the house, it's just an expensive rain shield.

I still wanted the traditional Scottish look of a white rendered finish but I want all elements of the wall make up to add to the insulation and air tightness of the property.

The solution is a timber frame, clad with 100mm thick wood fibre external wall insulation boards (I used Pavatex, but other makes are available), and the render goes straight onto the wood fibre board.

Here's a picture to make it clear:

In that picture I only have a few of the fixing screws in place. A lot more were added and then driven fully home. The board is fixed to the frame with long screws with big plastic spreaders to stop the screws pulling through the board.

There are a few twists to the frame however. First thing you will notice is that it is not an "ordinary" timber frame.  For a start it's built with much thicker timbers than normal to allow more insulation in the walls. But secondly people keep telling me i have put the frame up "inside out"  The OSB racking layer is on the inside of the frame. That's done for vapour permeability reasons with the least vapour permeable layer on the inside.

With this build method you can either fill the frame with blown in insulation from the inside once the wood fibre cladding is fitted, or in my case I have chosen to use Frametherm 35 as it's less than half the cost of blown in insulation but gives the same U value.  So I am fitting the insulation from the outside as I fit the wood fibre cladding. Insulating only that bit of frame I expect to get clad in that day as I don't want the insulation left exposed to get wet if it rains.

The render is a lime based system from baumit.com. It has 3 layers, a base coat that is mixed from dry powder, then a primer that is painted on, then the top coat comes pre mixed in tubs.  A fibreglass mesh gets pressed in to the base coat before it is dry.

Overall benefits of this approach Vs an ordinary timber frame with blockwork outer skin:

• Simpler foundations (no need for provision to support the outer brick or block wall)
• More insulation for a given wall thickness
• More of the job can be DIY done, perfect for self builders.
• And an unexpected one, because there is no cavity, there is no need to pepper the wall with weep ventilators, so you get a clean render finish with no "warts"

And here is what the finished and rendered front of the house looks like.

 

 

 

 

Edited May 20, 2016 by ProDave

[Tennentslager]

Looks great Dave, very crisp.

In fact it's almost like one of those CGI things you get on a marketing brochure for a new developments.

The render won't go all green or stained like some stuff I've seen around Glasgow?

[ProDave]

It's been very wet these last 2 days. The adjacent houses with traditional cement based render clearly absorb water to a certain extend and go darker and "wet" looking in places.  Mine, the water just runs off and no change of colour.

I just want 2 more dry days in succession to finish cladding the next wall, the west facing gable end.

One of my design requirements was no external woodwork to keep painting or otherwise treating. You will see I have timber fascias behind the guttering, but eventually they will be over clad with aluminium painted the same colour as the windows.

[Oz07]

Makes me wonder why big boys haven't caught on to this. Traditional foundation systems, no delays or price gouging fr brick shortages, Aircrete block shortages (the latest scam) and completely eliminating the need for the trade which is in shortest supply at the moment!

[jack]

That's looking amazing Dave. Very, very sharp!

[DeeJunFan]

Looks really nice.

I was getting quotes for the wood fibre board when I was considering the warm roof option and it really struck me as a great product.  Something everyone understands (wood) but can insulate and take render.  

Really nice build!

[ProDave]

In case it's not obvious, the roof is a warm roof, hung from a big ridge beam. That too is clad with the same 100mm wood fibre board in place of the normal (in Scotland) sarking board. And the roof will be insulated in the same way as the walls but this time from the inside so we will have a vaulted warm roof.

[hrc]

Hi, I'm in the process of deciding build methods and have asked for budgets from Kingspan (Bentleys) and MBC. I'm now looking at 'stick built' on-site: you say above that you could have blown in cellulose from the inside after fitting the cladding, I presume that this then needs to have all the holes sealed to get back to the airtightness,

I was looking at 140mm SIPs with 160mm (2 x 80mm) Gutex woodfibre boards - on the premise that with an external layer of OSB I could screw in anywhere and not miss!

 

[ProDave]

Yes if you opt for the blown in insulation you have to drill a hole in each segment of wall and the plug gets put back and sealed with air tightness tape.

What thickness is the OSB layer of a SIP panel? there would be a fair weight with 160mm of wood fibre and the render, would that really hold well enough just into OSB? I am much more comfortable knowing mine is screwed into the timber frame.

Also I have ended up with two different makes of the plastic caps that spread the load. One of them takes a fair bit of grunt to pull the cap flush into the wood fibre board, I would be concerned that you may strip the fixing into the OSB before the cap has pulled in flush. 

I would want the SIP panel manufacturer to agree in writing that this is okay before comitting yourself to it.

[Jeremy Harris]

6 minutes ago, ProDave said:

Yes if you opt for the blown in insulation you have to drill a hole in each segment of wall and the plug gets put back and sealed with air tightness tape.

What thickness is the OSB layer of a SIP panel? there would be a fair weight with 160mm of wood fibre and the render, would that really hold well enough just into OSB? I am much more comfortable knowing mine is screwed into the timber frame.

Also I have ended up with two different makes of the plastic caps that spread the load. One of them takes a fair bit of grunt to pull the cap flush into the wod fibre board, I would be concerned that you may strip the fixing into the OSB before the cap has pulled in flush. 

I would want the SIP panel manufacturer to agree in writing that this is okay before comitting yourself to it.

There was a Grand Designs house a few years ago in Herefordshire had steico wood fibre fitted to SIPs and then rendered.  This was the rather bizarre house built by Border Oak with a massive internal oak frame that didn't do anything but look nice!

[hrc]

I am about to raise this with Bentleys (and a couple of other points). I was considering the EPS external insulation (ala passive slab) would be higher externally and thus give some support to the external woodfibre, which would 'overfly' all the wall/eaves joints and prevent cold bridging. The external cladding, depends on planning approval, would then 'join' with a rendered protection at low/ground level which would protect the EPS

H

 

[ProDave]

Another wall has been rendered, and it's really starting to look like a house now.

More on my blog at www.willowburn.net, look for the blog entry "More cladding and rendering"

[Gone West]

That looks very smart. I like the sharp look, excellent!

[PeterW]

Like the lack of boxy soffit/fascia - was that fairly easy to achieve..?

 

[ProDave]

 

54 minutes ago, PeterW said:

Like the lack of boxy soffit/fascia - was that fairly easy to achieve..?

 

Yes it was. It was a design feature that I wanted to keep it simple and in some ways more traditional looking (most of the old croift houses don't have a boxy soffit or fascia)

The wooden fascias you see now are going to be over clad in aluminium painted the same colour as the windows. That will ensure no maintenance and provide a drip bead at the front edge. Knowing they were going to be covered, I just used lengths of 8 by 2 constructional timber left over from framing up the roof.

Originally I had planned to uses some posh rainwater goods, but chose the cheap plastic in the end. Having done so I like the black on white look.

Surprisingly the most "difficult" detail was the offset bends where the downpipe joins the gutter. The downpipe is offset from the wall by a fixed distance by it's brackets. Two standard 45 degree bends gave it way too much offset from the wall. But I couldn't just take it straight down, that would have looked silly.

In the end I had to choose the make of 45 degree bends carefully (the ones I ended up with came from TP) and then some careful work with a hacksaw to shorten them to make a much smaller offset. The finished thing looks good, just a shame you can't use off the shelf stuff.

[PeterW]

Thanks - looks really neat

Have you got any photos close up..? 

[ProDave]

4 minutes ago, PeterW said:

Thanks - looks really neat

Have you got any photos close up..? 

Which bit? I can go and take some more photo's.

[PeterW]

Just the edge detail onto the rafters or from the end if possible..? Would be interested in how it looks in profile. 

[ProDave]

here you go

It was quite a tricky detail to work out. The rafters end at the edge of the timber frame. The roof is clad in wood fibre board, as are the walls so the two boards meet and the entire frame is wrapped in wood fibre boarding with no bits of the frame prortruding to form a cold bridge.. The 8 X 2 is then screwed with 200mm long screws through the wood fibre board into the rafters and timber frame. Then the render goes on up to the "soffit" board.

As it stands there is a poor detail there, in that wind driven water could get blown in above ther render if a gap opened up between the render and the fascia. That will be solved when the ali cladding goes on as that will project below the bottom of the fascia, then return up forming a drip bead to ensure water drips off in front of the render.

You can also see my shortened drainpipe offset detail.

[PeterW]

Brilliant thank you ! Who's are the verge covers...?

[ProDave]

Standard Marley verge covers. If I had got my way with the planners we would have used a concrete tile which has verge tiles available where the concrete tile wraps over the end. But this tile was all we could get passed the planners and they are flat tiles and the plasic end cloaks are the only option.

The tiles are Marley Edgemere Riven, just about close enough in finish to "slate" to be about the only concrete tile the planners would allow.

[Stones]

Looking good Dave, could you take a closer shot of the alu window cladding / render junction? I'm interested to see how this looks close up given the window installation instructions about ensuring there is a drainage channel for the aluminium cladding.

[ProDave]

Sory for the delay. Just back from the days work on site with some pictures.

This is one of the windows on the back wall, yet to be rendered.  You can see there is a deliberate gap between the side of the window frame and the wood fibre cladding so the gap between the timber window frame and aluminium cladding is kept clear.  At the bottom I have shown a bit of the render stop bead that is being used. This ensures the render does not go down the gap between the window frame and the wood fibre board,  and risk blocking the gap between the window frame and ali cladding..  So if any water did get between the timber window frame and the aluminium cladding, it could escape and run down onto the window cill (which is wider than the window) Though quite why it would come out of the side rather than just run down to the bottom I don't know.

The gap between the wooden part of the window frame and the wood fibre board is filled with compriband expanding foam tape.

And this is one of the font windows, now rendered, showing the finish achieved using that stop bead.

I have mentioned before this may not be the "official" way of doing it, but I had to work with the tradesmen available, and the guy doing the rendering (only one up here who does this system) said the "official" way is an impossible detail. So this is a "possible" detail that still preserves the essence of ensuring the gap between the window frame and ali cladding is not compromised.

It does make for a very crisp finished detail.

 

 

[Stones]

Thanks, that's very helpful. I've been discussing this with my builder and the joiner who will be putting the windows in, and I think this is how we are going to do ours as well.

[Stones]

How much wider than the window is the cill?