Riverside Cottage 2

[Tim Lamb]

Right we have covered the windows:-)

Question 2 concerns Posi-joists. Anyone have any reportable experience?

I have been told they creak.

For a similar load and span, I have been told they need a deeper section than timber. But a plumber/wireman dream for easy access.

Heating designer is opposed because you can't cut a slot through the top section to return the underfloor heating pipes.

The cottage will have 3 dormers leaving a small triangular space above the eaves. I have been wondering about the practicality of keeping this space warm (inside the insulation) and using it to return the heating pipes. Sucking of teeth from engineer concerned about multiple air lock sites.

Any thoughts?

[Tim Watts]

I used mine (triangular voids) to contain cable basket trays.

The pipes will run in front and above this space at the expense of a couple of inches of room space - and will have removable panelling for easy access to pipes and wiring.

This also solves your joists problem - run along, then drop down alongside the joist. No holes or notches.

With a sealed system, I don't see any issues (small bits of air tend to get blown through and there should not be any large amounts of air after pressurising. The plumber can add a bleeding valves at the high points to aid filling the system.

[Andy Burns]

With all them 'oles in the metal web, why would you even want to cut slots?

[polygonum]

So glad you asked that, Andy. Read the question last night and scratched my head thinking I must be missing something. Still can't see what...

[Tim Lamb]

Wet underfloor heating (Speedfit) uses aluminium spreader plates nailed to the joist tops with the pipe run trapped in pre-formed channels. The pipe has to go through or over the joist to reach the next joist pair. Going through involves a lot of pipe threading.....

[Andy Burns]

OK, I see. JG seem to recommend fixing the plates to the bottom of the flooring when using engineered joists rather than to the top of the joists ...

[Tim Lamb]

Sorry! Inadequate description. Heating engineer is used to dealing with conventional timber joists and his fitters expect to be able to cut a

15mm depth slot in the joist top to cross into the next joist/pair run. They nail on a *p* clip to retain the pipe.

Because this is a chalet bungalow where the internal walls upstairs are some distance in from the outer walls (joist supports) I am resisting having slots cut of any depth.

My thinking is to take the pipe under the wall and return through or over the Posi- joist. Hence lots of places where a length of pipe is able to trap air.

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[polygonum]

The OP wrote "... to return the underfloor heating pipes" - I did not take that to mean the loops at the end of every joist.

Nevertheless, I cannot see any significant reason other than reducing the threading of the pipe to cut slots rather than holes in the web. And, if the layout is very simple, you could pull the pipe through all the webs at one end, then pull more and more pipe through to form the loops. Wouldn't seem to be that much of a problem.

[Tim Lamb]

Yes. Sometimes because the joist design requires the flooring to be glued down. Fit from below still needs the pipe threading through the joists and the spreader plates have to *trap* the pipe which requires more convoluted folding.

I hope to find out more this weekend; visiting someone with an installed system.

I will end up with a mix of existing timber joists at nominal 600mm spacing and new at 400mm, timber or posi. The nominal 600 is a lie because there are doubles under the dormer cheeks and partition wall as well as extras for the stair well!

Insulation has to be fitted below the spreaders which is easier from above.

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[Tim Lamb]

I've not done it but say the new upstairs lounge, 7m long, is split into

2 heating circuits (say 6 loops) that is 90m of pipe at the start!

[John Rumm]

For a unvented (i.e. pressurised) primary heating circuit, air traps are not really much of an issue. It may make it slight harder to drain down completely, but filling is usually straight forward, even if you do have to do a second round of bleeding and topping up once the system is running.

[Tim Lamb]

This what I was hoping to read:-)

Plenty of compressed air on farm so I could blow it out if needed.

I've not seen a Speedfit manifold but I guess you can manually close off all the circuits and fill them one at a time on full pump pressure.

[John Rumm]

You are normally filling from mains pressure - so that soon displaces the air IME.

[Tim Lamb]

In message , John Rumm writes

Just back from Suffolk! The underfloor heating pipes were all laid on top of insulation and not routed through or over joists.

I asked how they attached the flooring and learned the chipboard was glued at the half lap and simply laid on top of the slotted in heating pipes!

[Tim Watts]

I've heard of chip over insulation (free floating) with solid concrete underneath - but not over joists.

[Tim Lamb]

Our ground floor will largely be chip over pipes slotted into insulation on concrete oversite. The new bit is screed which I will tile.

Any thoughts about fitting insulation to PVC or shiplap clad dormers prior to plasterboard.

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[Tim Watts]

Yes - make sure the chip has direct support points through to the concrete - do not just float it. What happens (every time IME) is that a) Washing machines shake the whole room; b) the insulation compresses by doorways due to constant pummelling; c) Use celotex, not jablite - it's a bit more crush resistant.

They do make special non thermal bridging support blocks for this scenario.

Personally, I prefer the screed and/or the concrete to be on top of insulation - then at least you get a solid stable floor.

[Tim Lamb]

Fortunately we will only have a dishwasher in the kitchen. Tumble dryer and WM are in the utility on screed. I take your point about the doorways as this has been noticed with the existing chip over jablite.

I think we are stuck with re-doing the existing for the old ground floor. To change would mean burying the timber wall panels in screed.