Help - insulating a suspended floor

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Hi DIY experts, calling for your collective wisdom.

I am having a timber cabin installed at the end of our garden in the spring. The cabin suppliers will install the cabin itself. I will do the first fit, by constructing a suspended timber base and insulating it.

I have designed the timber base in (I think) a pretty standard way: Timber joists running the longest span (left to right), raised off the ground using threaded bars, all adjusted to be perfectly level and square. I could use timber posts for this, but threaded bars would have longer life with no rotting. I will fit PIR insulation (around 100mm) between the joists, with all joints covered with alu-foil tape so as to create a vapour barrier and air tightness.

Here comes the issue: instead of building up the cabin directly from the base I make, they insist on raising the building on their own bearers, just as you would builda cabin if you were working from a concrete base. This would then leave multiple voids between my insulated base and the finished floor of the cabin. These voids are sealed off as the cabin walls fix into the base itself, so no 'outside' air or gusts of win can get in (leaving no ventilation either~).

My options:
  1. I do nothing. Accept that some cold air will travel between the insulated base and the finished floor - at best it may just be a little colder, at worst this could cause damp since warm air will meet cold air?.
  2. I ask the installers to leave the floor boards up. I can then fit an additional layer of PIR insulation in between, adding to the 100mm already below. All voids would be filled but it would be a bit of a criss-cross of PIR running in different directions and far from air tight?
  3. I ask the cabin installers to avoid using bearers altogether, and build directly onto my base (thus solving all my issues in one easy way, this is obviously my preference)
  4. Or an alternative that i've not actyually thought of myself! ...putting down some kind of airtight permeable membrane?
Below is a picture that hopefully explains the issue?.
 

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I don't get why you need to span the longest length. Shortest length would be stiffer, no?
I don't get the threaded bar either. They won't last outside and are you setting them in concrete?

On YouTube, there are companies who sell self assembly metal cabins and have videos of how to install them.

I would be looking to raise the cabin significantly off grade, so as to discourage vermin. I'd also look to pour a concrete base underneath to both take the load and to stop the undergrowth. Good ventilation under will also stop damp and rot.

But then again, i would probably talk to the vendor of the cabin and try to incorporate his recommendations.

If you fill and tape the insulation, you wont get draughts, though your floor will still be cold.

In summary,

Pour a low grade concrete slab, sit the finished cabin on concrete blocks off the slab, shutter/skirt the outside of the cabin

Job done.

I think.
 
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Thanks BlueLoo - spanning the longest length was actually the suggestion of the cabin vendor, albeit only for the reason of being able to easily fasten down the floor boards which run the other way (don't ask me why). However, I've calculated the spans and it's plenty strong enough given they're supported in the middle too.

Regarding the bars, I thought so myself too. But I've visited many builds done in the area with this technique. Using strong galvanised bars, and raising it high off the ground has worked well and stood the test of time. My only reason in doing it this way is because of access for the equipment to lay concrete. I may live to regret this, and so I think you are right.

My main concern was the 'warm' void below the floor boards, and I take it from your advice that you think this should be OK. The worst is that it would be colder, but I can deal with that. What I can't deal with is rotting floor boards because the vapour from the cabin is stuck in the void below. Thanks again!
 
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How much concrete in total will you need to pour down for encasing the rods, vs making a simple concrete slab base, no rods?
 
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Thanks Akist. At first, I presumed the sloping of the ground would have meant a significant difference. However, now you say it, I think the difference may actually be far less and I will do some rough sums. After all, using blocks to level out might simplify the process further.

One of those examples where it's easy to over-engineer things while overlooking the more obvious traditional methods.

Thanks for all your help
 
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