Summer house floor insulation

[Woland]

Dear all,

I've recently built a 44mm thick wood log summer house - one of those models where the (floating) can be independently fitted inside after the whole building is erected.

In the standard layout, the structure is designed to stand on a flat surface such as an even concrete subfloor. Treated floor joists would then be evenly laid on this subfloor and floarboards would be nailed to them.

However, as I already had a (sloped) slabbed base, I decided to do things a bit differently: I built a 100 to 200mmhigh level brick wall (with one airvent brick every 400mm) and I erected the summer house on that wall. Now for the suspended floor: the joists are sitting on pillars 300mm far apart; each is made of one brick (45mm) plus a suitable length of 75mm treated fence post. Slabs and brickwork are treated with Hyprufe (a kind of bitumen) to prevent ground dampness to reach the slabs and then the lower side of the suspended wood fllor structure. In fact, slabs sit on cement with no damp proof barrier and in humid and rainy days dampness was very evident even if the whole area was already inside the floor-less summer house.

By having the slabbed area well sealed, a suspended floor hanging on the average some 150mm higher, and suitably placed airvent bricks, I am expecting the resulting cavity to be basically well ventilated and virtually dry in all weather conditions.

The wood floor structure (joists and battens) is already there and I am now am planning to install rigid urethane panel cuts between joists, a damp proof barrier over it, a 18mm thick ply and 18mm floor boards in this particular order. The Kingspan urethane panels have some aluminium-like foil on both sides, so they should address heat loss by radiation. This should allow the floor to be both strong and well insulated.

However, I am wondering if I am forgetting something essential and/or doing something wrong, so here I am asking for suggestions/validation/ criticism. Any hint will be appreciated.

Kind regards

W.

[John Rumm]

Do you need to cut the urethane panel? You could lay it over the joists, then ply on top of that.

[Rod]

Most certainly you have forgotten a vital thing. Posting photos so we can all see it. Please. :-)

[George (dicegeorge)]

wouldnt the kingspan compress where its on the joists, wouldnt it be better between the plywood and the floorboards?

with the damp-proof plastic on top of the kingspan?

[The Natural Philosopher]

Just two points.

Glue the slabs in place with foaming polyurethane glue to stop them falling out, and forget the vapor barrier and instead use the aluminium tape over the slabs AND joists.

[John Rumm]

Not especially - it compresses a little when you screw the boards down, but not much. Pretty much the same construction as a "warm deck" flat roof.

I quite possibly might... the only point I was trying to make was cutting it in adds lots of work, and means the joists act as thermal bridges.

prolly better under it I would have thought. Having said that, in big sheets the foil covering will act as a DPC anyway.

[Woland]

Dear all,

thanks for your suggestions and also for the questions that actually preempted the very same ones.

As requested, here are a couple of photos of the summer house:

was thinking of putting a footnote stating that the whole area is guaranteed and certified 100% tradesman free, but in the end I did not want to kick-start yet another row with the professional handimen around...

The first photo in the above web page shows the front (with the air vents at the bottom); the second one details the suspended floor structure.

Following John Rumm's suggestion I run a small but convincing experiment with less than one square foot of Kingspan 50mm thick urethane board. I basically laid it on a T-shaped part of the wood floor structure and jumped on it one-footed with the weight of my 80 kgs (yes, I should shed some 10 kgs, but we all enjoy a bit of food & drink...). Both the lower and the upper sides of the board chunk showed no sign of damage or compression!

Another encouraging fact about rigid urethane foam boards is that Kingspan sell a flooring variant that is basically the same board with a 18mm chipboard glued on one side (minus the vapour-barrier like finishing foils).

So, I would say that I will proceed in the following way with John's suggestion at its core:

- Glasswool between the joists, with stapled galvanised chicken wire to support it (I have the glasswool from a previous extension job)

- Entire Kingspan panels laid on top of the floor structure; no more need to cut them to pieces (it's a messy job that produces fine dust and I'm not 100% certain that breathing the stuff - although wearing a mask - would be without consequences.

- 18 mm ply on top of Kingspan panels. Ply & underlying Kingspan board can be fixed with thin deck screws as suggested by Kingspan for their ply-dressed variant

- Possibly thick black polyethylene damp/proof vapour barrier on ply (provided with the summer house kit, intended to be laid on slabs but I used Hyprufe aquaseal instead)

- Floorboards on top

Hope it will be ready before the cold weather arrives (I'm in the NE of Scotland and it won't take too long for temperatures to reach values below 7C (all paint, glue etc seems not to like lower values).

Cheers

W.

[John Rumm]

Once you have boards over etc, it ought to be stronger still...

Only thing to watch is cost. Don't know what you need to pay for it with the chipboard prebonded, but round these parts I can get 50mm insulation boards at £10/£12 for seconds/new and a couple of sheets of flooring grade chip at £9 for the pair.

;-) now there is a job I hate! Probably not so bad if you use battens to fix the wire, otherwise it is a irritating job.

Are you sure the door will open over that lot?

[Rod]

Looks impressively solid. Thanks for posting - it adds a lot to the description.

[Woland]

Only thing to watch is cost. Don't know what you need to pay for it with the chipboard prebonded, but round these parts I can get 50mm insulation boards at =A310/=A312 for seconds/new and a couple of sheets of flooring grade chip at =A39 for the pair.

-> I already have all the insulation boards and the ply. Having ordered material to insulate floor, walls, and roof one year ago (!) I got a substantial discount from the timber merchant before prices started to go up and up and up...

;-) now there is a job I hate! Probably not so bad if you use battens to fix the wire, otherwise it is a irritating job.

-> I agree, it's not the most enjoyable job to do. However, using the stapler is not this bad. I am helped by the fact that the fence post cuts that support the joists & the rest of the floor structure are wider than the joists themselves, so they provide a minimum of support for the wire net. Standard rockwool is not very itchy these days...

Are you sure the door will open over that lot?

-> Yes, for two reasons. Firstly, the door opens on the outside so even miscalculations wouldn't have produced any serious practical effect. However, there is plenty of room as the door threshold level is some 10 cm above the floor frame level. This is purely accidental as I had to raise the door frame due to an error that the summer house kit providers did: the frame was originally 5cm too short to be aligned with the top of the windows.

Looks impressively solid. Thanks for posting - it adds a lot to the description.

-> Thanks. I spent some time devising that solution and even more building it. However, it is solid and perfectly dry

Cheers

W.

[dennis]

When I did my last shed I made some panels by gluing 18 mm WBP to polystyrene to 6mm WBP for the floor. They are laid on top of joists (18mm on top) and they rest on some concrete fence posts. Warm, rigid and not very expensive.