Possible to insulate shed floor before the shed is erected?

[aires69uk]

And if so how?

I will be getting a brand new shed delivered in a couple of weeks time. It's only a small shed 4' x 4' in size but it's a good quality shed - no OSB or chipboard.

I'm going to insulate the walls inside at leisure but I also want to insulate the floor. I'm thinking about insulating the underneath of the shed floor rather than having a floating floor.

I was thinking about sealing any cracks with silicone sealant, followed by a layer of black plastic sheeting, fit some Knauf rock mineral wool sheets (they're cheap), if necessary any joints in the sheets sealed with aluminium duct tape, another layer of black plastic sheeting, capped off with a covering of plywood to seal it all.

Once the floor goes down on to some breezeblocks and the shed has been erected, the shed will never ever be taken apart again - so this is a one shot deal.

Is what I plan for the shed floor overkill or could it be done any simpler? I want to make it completely watertight so that not one drop of moisture could get in through the floor. I want this shed to last for a very long time indeed so what would any of you guys recommend - I am open to your thoughts.

 

[foxhole]

What is the floor of the shed made from that it has cracks which need sealing? Where is the insulation going , not at all clear from your description?

 

[aires69uk]

I'll be insulating the underside of the base - flip it upside down and use Knauf Rock Mineral Wool Slabs.

I'd then be looking to seal the base with a layer of marine plywood.

But do I need to use any plastic sheeting? Do I need to make the base airtight or do I need to let air circulate?

Poor description using the word 'cracks'. I meant I'd probably fill the joints in the tongue and groove with silicone sealant (on the underside).

 

[ladylola]

Insulation won't make it watertight, indeed the insulation you show would actually soak up water . Water wouldn't be coming up through the floor anyway unless the garden gets flooded. A dpc and airflow under the shed will sort out any damp problems you may encounter.
What will be the purpose of the shed that it will need insulating? Don't forget without any form of heating in the shed the insulation will hold cold air inside on warm days so the shed will actually be colder inside than out at times.

 

[aires69uk]

Insulation won't make it watertight, indeed the insulation you show would actually soak up water . Water wouldn't be coming up through the floor anyway unless the garden gets flooded. A dpc and airflow under the shed will sort out any damp problems you may encounter.
What will be the purpose of the shed that it will need insulating? Don't forget without any form of heating in the shed the insulation will hold cold air inside on warm days so the shed will actually be colder inside than out at times.

I was going to fit an exterior socket to the outside of the house and install one of these in the shed

I just don't want this insulation getting damp - my number one priority is to ensure this insulation does not accumulate moisture.

If I DPM the shed base all it will do is keep it water tight, it will not keep it insulated. While it is a small shed for my tools, I want to fit a cat flap so the cats can go inside in bad weather. I just need to know the best way to insulate the base because once it's down - it's down.

When I attach the plywood to the joists, the inbetween will effectively act like a cavity and the Knauf Rock Mineral Slabs will be the insulation in the cavity.

The shed I have bought is similar to this

What is pictured is 3' x 2' but I asked them to make me a 4' x 4' box shed.

I am aware wood 'breathes' but I really don't know what I'm doing and I genuinely need some advice.

 

[aires69uk]

The local wood yard (just a stone's throw away) said leave out the plastic lining and it should be okay. This was backed up by another wood merchant in the town centre.

(Didn't bother either with this query specifically, had to go and get some flexible glue to bond the oak flooring to the screed floor - you wouldn't believe the cost if I told you!!!!!).