Insulating summerhouse (during build)

[Stringman_uk]

Hi All
I am planning to build a summerhouse/home office so it will need insulating.
I wonder if you kind people could offer me your thoughts on what I am thinking of doing?

Floor support will be timber frame and joists (47x100) on concrete piles.
I was intending to put baton in the joists at the bottom to create a 75mm gap for pir insulation.
Then lay a dpa membrane on top of that. Then lay the flooring on top. Mrs wants T&G would you advise some ply first?

Walls will be 3x2 studs. T&G cladding outside.
My theory was to build the frame, wrap in tyvex or similar and then fix the cladding to the studwork.
Again fit batons to the stud so there is a small gap between the tyvec/cladding and the pir.
Pir flush to the studs .
Then would I need another membrane? If so what.
then a inside finishing layer probably tongue and groove or plasterboard.

Does that sound logical?
Thanks in advance

 

[motorbiking]

You don't need all this membrane. PIR comes with a silver foil vapour barrier. If you have the chance to create an insulated concrete slab for the floor (compact, sand, DPM, polystyrene, concrete etc) then you can reduce the floor insulation.

75mm PIR will be fine. In my own I used 100mm but that meant adding spacers before the plasterboard went on. I had a 100mm of poly in the floor, so only needed 50mm of PIR. Heating requirement for this unit was about 300w.

 

[noseall]

You don't need all this membrane. PIR comes with a silver foil vapour barrier

As long as you robustly foil tape all the joints - correct.

 

[^woody^]

Timber shed type floors with an air void beneath feel particularly cold under foot, more so when sitting at a desk, so consider insulation between and over floor joists.

Timber sheds dont need any internal vapour check membranes for moisture control, but would need good detailing to stop draughts and noise. So for that purpose alone a membrane layer may be easier to deal with draughts and noise, and if that is the purpose should be suitably taped and sealed to floor, wall and ceiling.

 

[Stringman_uk]

Thanks for the replies.

You don't need all this membrane. PIR comes with a silver foil vapour barrier. If you have the chance to create an insulated concrete slab for the floor (compact, sand, DPM, polystyrene, concrete etc) then you can reduce the floor insulation.

75mm PIR will be fine. In my own I used 100mm but that meant adding spacers before the plasterboard went on. I had a 100mm of poly in the floor, so only needed 50mm of PIR. Heating requirement for this unit was about 300w.

Thanks Motorbike,
I am a little confused, as when i was reading up on the base/foundation, I got the impression that it wasnt a good idea for the frame and joists to sit on a concrete pad due to damp and water pooling.
As the ground is a bit uneven my plan was to dig down 100mm and fill with compacted mot1 for each pile then use old 450mm pavers on to to level it up.

On your pictures you show the pir visible on both the inside and outside which side did you leave the gap ( or does it matter) The ply panel at the end on the last picture confused me a bit as it seems like you are lining the inside with ply but I couldnt see any in the inside pic.

 

[Stringman_uk]

Timber shed type floors with an air void beneath feel particularly cold under foot, more so when sitting at a desk, so consider insulation between and over floor joists.

Timber sheds dont need any internal vapour check membranes for moisture control, but would need good detailing to stop draughts and noise. So for that purpose alone a membrane layer may be easier to deal with draughts and noise, and if that is the purpose should be suitably taped and sealed to floor, wall and ceiling.

Woody, In my post I said I would be putting 75mm pir between the joists for insulation.
So I dont really need an internal membrane, but as you say for draft protection it is a good idea.

 

[Stringman_uk]

Thanks for all the replies.
Maybe I am being a bit thick here but I saw the outer membrane ( tyvek or similar) and a way of waterproofing the frame and the internals.
Hence my reason for planning to wrap the outside in tyvek. Clad straight onto the tyvek outside. Put in pir with a 25mm gap behind and then finish the wall off on the inside.

 

[robinbanks]

Not sure what you're asking, or what you're proposing for wall build up. Try describing it from inner to outer as a bulleted list

When motorbiking advocated a pad floor I don't think the intention was then to joist on top of it; the concrete is the floor on which you lay a covering

 

[noseall]

Timber sheds dont need any internal vapour check membranes for moisture control,

What, even those that are occupied (garden office etc)?

 

[motorbiking]

Thanks for the replies.

Thanks Motorbike,
I am a little confused, as when i was reading up on the base/foundation, I got the impression that it wasnt a good idea for the frame and joists to sit on a concrete pad due to damp and water pooling.
As the ground is a bit uneven my plan was to dig down 100mm and fill with compacted mot1 for each pile then use old 450mm pavers on to to level it up.

On your pictures you show the pir visible on both the inside and outside which side did you leave the gap ( or does it matter) The ply panel at the end on the last picture confused me a bit as it seems like you are lining the inside with ply but I couldnt see any in the inside pic.

I over did the foundations for mine a bit, but I had some breeze block planters to recycle. I went down about 400mm, use the smashed up breeze blocks, added sand, then DPM with a big overlap so I could run it up the walls to about 700mm. I then added 100mm of polystyrene. Poored my concrete, then attached the DPM to the outside of the frame. In effect a DPM tank.

Thanks for all the replies.
Maybe I am being a bit thick here but I saw the outer membrane ( tyvek or similar) and a way of waterproofing the frame and the internals.
Hence my reason for planning to wrap the outside in tyvek. Clad straight onto the tyvek outside. Put in pir with a 25mm gap behind and then finish the wall off on the inside.

One thing I learned is 100mm timber is not 100mm dried. It comes in at about 95mm. You want your PIR flush to the outside and any air gap between PIR and plasterboard. It will only be about 20cm.

I used marine(ish) ply for the hidden sides with 3 coats of wood stain. I then only clad the visible sides and used non-treated T&G, which I treated myself as the treated stuff was 3 times the price. With the overhanging roof and another 3 coats of woodstain it hasn't had any problems in 6 years. Though I don't live there any more.

 

[mrrusty]

Have a look at my project over at Screwfix forum https://community.screwfix.com/threads/garden-room.256842/

Have you considered SIPs - the thinner ones designed for garden rooms are very competitive and fast to construct. They will be slightly more expensive than framing out yourself, but not much, and you end up with a very solid well insulated structure. You do have to make sure your base is very well laid out because they are not very forgiving for tolerance.

 

[Stringman_uk]

Not sure what you're asking, or what you're proposing for wall build up. Try describing it from inner to outer as a bulleted list

When motorbiking advocated a pad floor I don't think the intention was then to joist on top of it; the concrete is the floor on which you lay a covering

Robin,
Thanks for the reply
Concrete pad isnt really viable for me.
So my plans for the wall are as follows outside to inside

T&G cladding
Tyvex fixed to frame studs
PIR insulation between the studs
waterproof membrane ( for draft/noise insulation)
T&G cladding

 

[Stringman_uk]

I over did the foundations for mine a bit, but I had some breeze block planters to recycle. I went down about 400mm, use the smashed up breeze blocks, added sand, then DPM with a big overlap so I could run it up the walls to about 700mm. I then added 100mm of polystyrene. Poored my concrete, then attached the DPM to the outside of the frame. In effect a DPM tank.

One thing I learned is 100mm timber is not 100mm dried. It comes in at about 95mm. You want your PIR flush to the outside and any air gap between PIR and plasterboard. It will only be about 20cm.

I was always lead to believe that the gap should be on the outside (on the cold side) of the wall ?

I used marine(ish) ply for the hidden sides with 3 coats of wood stain. I then only clad the visible sides and used non-treated T&G, which I treated myself as the treated stuff was 3 times the price. With the overhanging roof and another 3 coats of woodstain it hasn't had any problems in 6 years. Though I don't live there any more.

 

[motorbiking]

I thought he was going with cladding not weather board?

 

[Stringman_uk]

I thought he was going with cladding not weather board?

it will be cladding, the picture just showed the gap on the outside
as an example