More foundation fun - straw bale garden room on clay

[Nick Thomas]

Been a while since my last update - lots of rain. We had a clear few days this weekend though, and I managed to make some progress.

 

 

->

 

 

 

(scuse censorship; imagine it's a starry night sky)

 

That's all the bales up now, although I'm eyeing the left corner a bit unfavourably; I *think* it's OK but there's a bit of an overhang situation going on, and the fourth course particularly is dodgy. Worst case, I'll have to take it down and re-size a few bales, but it might be within trimming range in-situ. Worth a try, anyway. Similar for around the door and window posts - they get vigorously trimmed before the lime plaster goes on, which should improve how it looks significantly. There are still 24 bales in the garage - so I over-ordered by about a third. I've got some wall-shaped projects in mind for them come spring, though, they won't go to waste.

 

Next is the wallplate/roofplate - just another timber box beam round the top of the bales, only harder because it's more than two metres up ^^.  I got the back wall section put together today, and it's up there - weight was a problem so I ended up doing the timber ladder on the ground (40kg?), getting that up, cladding it with the OSB (30kg/side?) then flipping it so that side was on the bottom. A bit sketchy, honestly. The remaining sections are shorter, so I might be able to get more done on the ground for them. Regretting not at least cutting the timbers back when I had access to the baseplate now, of course, but if I'd spent time on that back then, I might still have straw to do now.

 

I've explored the hip roof idea a bit more but increasingly feeling like I just don't have the vertical space for it. So probably a flat roof over winter, keep an eye on it , maybe explore guy ropes if we get any wind forecasts above 80mph. The roof deck *looks* simple enough, timber-wise - 2x5 joists, use firrings to get the fall, T&G OSB on top of that and finish with EPDM. Maybe add insulation under the joists later.

 

More rain is forecast, but the walls are all tarped up and should be fine like that indefinitely. I can build the rest of the bits of wallplate in the half-empty garage and uncover one wall at a time whenever there's a clear moment to put one up.

[Nick Thomas]

Welp, a week of big sighs. Got the two side sections of wallplate on and they demonstrated, rather clearly, that the walls weren't straight enough. Here's the SW corner:

 

 

And the SE corner (fiddling a week later after sorting out the SW one):

 

 

Fortunately, not fatal - I "just" have to take down bales, resize them, and restack until everything's within tolerance, which is about 5cm for quality load-bearing construction (at least, according to the Canadian building code. Might be able to get away with more than that, but I definitely can't get away with half the wallplate hanging in space).

 

The usual way to keep this from happening is to put up some vertical timber braces at each corner and use them to guide you as you go up. Obviously, I didn't bother. Lesson learned 😅.

 

I've sorted out the NW corner, at least in terms of how it kicked out W - it might still be too long, as the SE corner is, but I know it's not *as* bad, at least.

 

Vaguely considering putting some of the strapping horizontally along the wall and compressing laterally, to see if that might straighten it all up, but that might be madness compared to just redoing it. Bleurgh. Chances of getting to the roof next week diminishing rapidly, anyway.

 

 

[Nick Thomas]

Yeah, lateral compression does nothing. Took the wall down, trimmed some bales, put it back up, and now have a wallplate running all the way around:

 

 

The bottom 3 bales on the SW corner could still be narrower, but it's not structurally fatal, and I'm pretty sure I can trim them back any time - so, on to compression:

 

 

this little section of wall is separated from the rest by window and door, so makes a nice test bed. Even very light compression has turned it solid, but the more the better. I'd like to squash by at least 10cm so the straw is 2M high - but if I can get the wallplate parallel with the door header, even better.

 

Wood for the roof has arrived, so I can get on with that once compression is done. I'd like it to have a waterproof roof on well before christmas, so I'm on a bit of a deadline now. Fortunately, the weather seems to be cooperating... all except for temperature. I'd been using D4 wood glue to put the wallplate together, but that says it doesn't work below 10°C. PU glue says it needs 5°C. Today was 3°C, so I ended up using some "sticks like sh*t" that claims to be fine down to -40 or so. Bit of a learning moment, and proof, if proof were needed, that I should've had this finished long ago.

Edited November 26, 2023 by Nick Thomas

[Nick Thomas]

Oh hey. EPDM roof adhesive needs 5°C temperatures as well. That might end up being fun.

[markocosic]

I once made the mistake of using "all weather" resin fix here. Indoors.

 

"Rated down to -25C" it said on the package

 

What I didn't clock was this meant it had all sorts of accelerants in the 2-part resin to get it to cure within 24 hrs at -25C.

 

Using it indoors at +25C...fudge me did I have to fit the floating shelf into some brickwork QUICKLY and shortly thereafter the plastic spiral mixing nozzle as good as caught fire. 😂

[Nick Thomas]

Bit of a winter, eh.

 

Took the chance of a break in the weather this weekend to finish the roofplate and get (some of) a flat roof up there. Pics:

 

 

Ended up working in the dark at the end, which was all sorts of fun. The OSB3 isn't glued&screwed properly yet, and I've got some wood of the structure left to do, but the tarping situation is much improved - two wrapped around, and one laid flat on top of the roof, since it's not quite watertight yet.

 

Now there's space in the garage, I've placed an order for EPDM so should be able to finish the roof properly soon. The two wall-protecting tarps can stay like that until it's time to render, they're great.

 

I've been a bit worried the whole time about how a square roof might look on top of the D-shape of the building, but it's coming across as quite cute in the end - to me, anyway. It's like a little guy with a big hat (and sleeves at the moment, too!)

 

Managed to take a knock to the temple while building the roof though - screwing a short bit of wood below a joist to support another joist, holding it in place with my head to stop it from spinning round, I was getting frustrated and put the drill into reverse. So it spun around and clocked me one. No concussion, but a good reminder to slow down. More time, more people, maybe nails instead of screws for the final piece I need to get up in that way.

 

 

[SteamyTea]

53 minutes ago, Nick Thomas said:

two wrapped around, and one laid flat on top of the roof, since it's not quite watertight yet.

Makes it look liker a crime scene.

 

Tape the joints up with some of this.

 

[Nick Thomas]

Clearly I should've used some plastic sheeting instead

 

 

[Nick Thomas]

OK, EPDM installed -

 

 

Ordered it from Rubber4roofs / Optagon, which turns out to be employee-owed, so that was a nice surprise. Reasonably easy to fit, although I made my life difficult by not having a completely flat roof deck. Just a few bits of trim waiting on a tube of superglue to go up.

 

I guess I can't put it off any longer - time to start tidying the garden and trying to rescue the turf.

[Nick Thomas]

OK, I'm onto internal stuff now. It turns out some of the straw had gotten wet over Dec/Jan - rain soaking through the tarps, I guess - so I've had to dig out to a depth of ⅓rd in some areas. It's not fatal - they'll make nice niches and shelving, which I was planning to have anyway - but definitely not ideal.

 

On the more fun side, I've got lots of rockwool and wood coming tomorrow for the ceiling and floor inside. I've decided to use rockwool to try to get decent acoustic insulation - as with the walls themselves, I'm not super-worried about the thermal side of things.

 

I've got enough for 150mm depth (100+50) in ceiling and floor, but I'm trying myself up in knots about whether I should be trying to get a layer beneath the joists or not. A roof build-up like this:

 

 

is only marginally better thermally than putting all 150mm inside the cavity (U=0.27ish) - but I imagine it's significantly better acoustically? Worth the hassle of (somehow) suspending the ceiling to create the 50mm cavity? I've seen other approaches that use 50mm celotex below the joists and just fixed through with long screws, but that focused on thermal, rather than acoustic, performance; what I read suggested that PIR might as well not be there from a sound perspective.
 

The ceiling itself is going to be 12mm pine matchboard, with a membrane between it and the insulation / underside of the joists... I have lots of vapur-permeable Vent3 roofing membrane left over, but I assume that's inappropriate and it should be something that doesn't let vapour though, instead?

 

I'm building regs exempt so don't *need* to hit U=0.14, but I do have space for ~100mm below the joists if I absolutely needed it. I can't imagine I'd be able to just fix through all that space with 200mm screws though. I've seen metal framing solutions for plasterboard, maybe something like that for timber if it came to it?

[Nick Thomas]

Aha. Resilient bars and acoustic membrane!

[scottishjohn]

On 16/09/2023 at 17:18, saveasteading said:

do you need it though? hard clay is hard clay and any settlement will presumably be even.

not so sure 

my experience of concrete raft houses on old brickworks ,as we had alot of them in manchester ,was that the clay dried out under the slab  over time ,contracted and then left voids ,

this is why on clay ground they want very deep foundations 

 how deep are founds for house --have you dug down deep enough to find where they stop?

[Nick Thomas]

12 hours ago, scottishjohn said:

 how deep are founds for house --have you dug down deep enough to find where they stop?

 

I've not looked at the house foundations - I've been working all the way over on the other side of the garden. I know that they're trench fill, but no more than that.

 

I'm expecting the whole thing to move some, and hoping that either it'll take it in its stride, or it'll be slow enough that I can remediate. A not unlikely scenario would be that one of the piers differentially sinks down enough for a 1M span between piers to become 2.5M at some point, for instance - and if that happened seasonally, it'd be a right pain. I think the wood would be fine if that happened with one pier, and less fine if it happened with two in a row 🤷‍♂️.

 

Ultimately it's small enough that I could just jack it up, re-do the foundations and put it down again - and cheap enough that even if I couldn't do that, I could write it off and turn it into some decent compost before building another one more sensibly 😅. I've bought just about everything I need now and it's around the 4K mark, ignoring various tool purchases.

[Nick Thomas]

Bit of a palaver, but the ceiling is almost done.

 

 

Pretty much the layout I mentioned above, except I decided to do without the membrane; fiddling with the ubakus calculator convinced me that condensation was unlikely enough that it wasn't worth the faffing - it might have been different if I were doing building-regs-compliant levels of insulation, but I just can't see it being 20°C and high-humidity in there while it's -7°C outside ^^.

 

It's not as airtight without the membrane, I suppose; there's the expansion gap all around the perimeter, which I'm planning to tape with something then hide with some sort of moulding, and some(?) air will get through the t&g. No detectable draughts at the moment, though - and the wind was properly howling through before. I've left 50mm between the top of the insulation and the bottom of the roof deck, and that'll stay ventilated.

 

I really should've gone to more effort to get the screws straight and evenly spaced, though - they look a bit amateurish, and I doubt they can be hidden. Pretty minor in the grand scheme of things.

 

Just a few more boards to do - obviously, I left the tricky cuts until last - then I can get started on the floor. With that done, it'll be time for the lime render.

 

[Nick Thomas]

I did finish the ceiling 😅 but I've been ill pretty much the whole time since.

 

 

Dragged myself out today to get on with some floor joists, though. I'll take halfway.

 

 

It's a mix of 2x6 C24, and 2x8 C16 - I misremembered what I had left over from the roof. Oh well. Going for both joist hangers and screws, as I did with the roof joists. Having to dig out some more soil to get them clear; once they're all down and there's no more digging to do, I'll wiggle geotextile membrane underneath, then get the rockwool down on top of that. Should be able to keep it from touching the ground with some of the strapping I've got left over.

 

It's coming up to my last chance to fill the on-their-side blocks as well 😬.

[oliwoodings]

26 minutes ago, Nick Thomas said:

I did finish the ceiling 😅 but I've been ill pretty much the whole time since.

 

 

Dragged myself out today to get on with some floor joists, though. I'll take halfway.

 

 

It's a mix of 2x6 C24, and 2x8 C16 - I misremembered what I had left over from the roof. Oh well. Going for both joist hangers and screws, as I did with the roof joists. Having to dig out some more soil to get them clear; once they're all down and there's no more digging to do, I'll wiggle geotextile membrane underneath, then get the rockwool down on top of that. Should be able to keep it from touching the ground with some of the strapping I've got left over.

 

It's coming up to my last chance to fill the on-their-side blocks as well 😬.

Wouldn't a non permeable dpm be better than permiable geotextile membrane if rockwool is gonna sit against it? I'd be worried about moisture wicking up into the rockwool through the membrane personally.

[Nick Thomas]

3 minutes ago, oliwoodings said:

Wouldn't a non permeable dpm be better than permiable geotextile membrane if rockwool is gonna sit against it? I'd be worried about moisture wicking up into the rockwool through the membrane personally.

 

I had thought about something impermeable, but I'm worried about spills, etc, from above - I'd rather any water was able to drain away, rather than sitting around the joists.

 

The plan is to have an air gap between the rockwool and the geotextile - they shouldn't be in contact. I'm going to have some strapping run perpendicular to the joists, which will support it from below and keep it from touching the ground. It'll also be friction-fitted, but I don't quite trust that to hold it in place.

 

Rockwool does claim to be non-wicking, so it might be overkill, but I just don't like the idea of it being in contact. Originally I was going to have a nice layer of pebbles on top of the geotextile, but with the 2x8 joists it adds up to more digging than I fancy ^^.

[Jilly]

I’m late to the party, but enjoying reading this thread.  I was going to suggest you could use some netting to suspend your rock wool between the roof joists to get the air gap, and/or to hold the stuff in place before you boarded out, but I was too late. I’ve seen it done to hold rock wool insulation under floorboards in an old house.

Edited April 22 by Jilly

[Nick Thomas]

Netting would work 👍. I'm thinking of using the strapping because I happen to have it, not because it's the best thing for the job 😅.

 

Up in the roof, the air gap is above the rockwool, rather than below; I didn't see any details suggesting there should be one between the bottom of the rockwool and the ceiling material. Definitely too late if there should be one though - I wouldn't fancy taking it all down and going again!

[SteamyTea]

5 hours ago, Jilly said:

I was going to suggest you could use some netting to suspend your rock wool between the roof joists to get the air gap

 

5 hours ago, Nick Thomas said:

Rockwool does claim to be non-wicking

If you use netting, the insulation can get air movement though it, called advection in Physics.

I think the non-wicking depends on which product is used, @joe90 used non-wicking, full fill, version on his build.

The above is not that important on a garden shed, but may be on a house.

[Onoff]

How about a layer of big bubble, bubble wrap under the joists, for the Rockwool to sit on? 

 

Air gaps between the bubbles and you could puncture it all over in between the bubbles for "drainage".

 

 

[Jilly]

6 hours ago, SteamyTea said:

 

 

11 hours ago, Nick Thomas said:

Up in the roof, the air gap is above the rockwool, rather than below; I didn't see any details suggesting there should be one between the bottom of the rockwool and the ceiling material. Definitely too late if there should be one though - I wouldn't fancy taking it all down and going again!

Above the insulation is right in a cold roof. 

[ToughButterCup]

Dunno wot you all fink, but this is sooooo much better than following Grand Designs.....

Its not often that I read and re-read many posts as carefully as I do this series. 

[joe90]

10 minutes ago, ToughButterCup said:

Dunno wot you all fink, but this is sooooo much better than following Grand Designs.....

Oh yes, my original idea was straw bale build and i would have if I was much younger. I will be following the other straw bale thread. I wrote to channel 4 years ago suggesting they start a programme called “green designs” but they did not reply 🤷‍♂️

[Nick Thomas]

Hah, modest designs it is. Sometimes I think I put too much detail into these posts, but, it's details all the way down.

 

Had a friend over this week so we tackled the insulation, after filling the concrete blocks and getting some geotextile down.

 

 

The datasheet and some experimental testing confirmed that the rockwool (RWA45) was definitely non-wicking - and surprisingly (to me) hydrophobic too, but we still wanted it off the ground if we could. I'd picked up some chicken wire with the idea of stapling it around the bottom of the joists to support the battens from below.

 

Absolute disaster, couldn't get it fixed properly any which way. In the end, we gave up and went for a friction fit. I'm not super happy with it, but at least it's in. Will be interesting to take a look in a year or so and see if it's stayed in place or slid down to contact the ground; if it has, I expect I'll just pop another 50-100mm on top to make it flush with the top of the joists again.

 

Originally I was going to use some old decking boards I'd saved, but I ended up deciding I wanted a nicer finish, so splashed out on some new redwood PTG floorboards - 150x22mm nominal, trimmed to ~3.6M long so there's no joints in the middle of the floor. Those arrived today; I've got the first seven down and they're looking pretty nice, if I say so myself.

 

 

I was struggling to choose between nails and screws as fixings, before coming across these lovely little things: https://www.screwfix.com/p/tongue-tite-tx-countersunk-thread-cutting-floorboard-screws-3-5mm-x-45mm-200-pack/85991

 

They screw the boards to the joists where the tongue is, so no nail or screw heads on show.

 

 

The groove side of each board is a little springy, since there's only the previous board's tongue holding it down there, but it seems OK - and a lot prettier than the mess I made of the ceiling.

 

I've decided to try for a tung oil finish on both floor and ceiling, so 5L of "pure tung oil" is arriving tomorrow - but I'll just experiment with some offcuts for now, then give the ceiling a go. I expect rendering is going to be messy, so I can sand and treat after that's complete.

 

Tomorrow or Friday I should finish the floor - then I can fir the windows, sort out the doors, and prep the walls for rendering. I'm trying to decide between an NHL2 or simple lime putty render, with a particular eye towards what's going to be closest to foolproof for someone with no practical experience. We only really played with clay plasters on the course, so I'm more out of my depth than usual on this one ^^. Initial investigations suggest the lime putty stuff will be easier to apply but require more aftercare - but it's also half the price, which might swing it, being honest. I need a couple of tonnes of the stuff, it adds up.

 

Edited May 15 by Nick Thomas
add geotextile