warm roof/deck for garden room

[bendeneo]

Hello all,

I have put up the wood frame for my new garden room, and the roof joists are 6 x 2.

I put a cold deck on my extension a few years back, and that was relatively easy, but this time think that warm deck would be the way to go this time on the garden room.

However, I wondered if anyone had any thoughts otherwise when it comes to garden rooms?

The reason I ask is that I have lots of space between the joists 150mm, so it does make me wonder if I could do some form of composite?

Perhaps I could have 70mm between the joists...... and if so is there a different configuration for the vapour barrier?

I know this may be silly, but thought I would put it out to the experts in case there was another way.

The reason I thought about this is just because it seems mad to whack OSB3 down on the top of the joists, and then create a whole new frame on top to hold the insulation in place before putting ply down on top of that.

I know I shouldnt be thinking about the material costs, but its always worth exploring.

Thanks for any advice!

 

[Xenon]

Foil faced insulation foil down onto joists, fix 12mm ply or OSB3 on top, you then have your deck. Tape joints in foil underide with appropriate tape. Bear in mind this will raise deck height and deeper fascias will be required.

 

[bendeneo]

Thanks Xenon,

So just to be clear, you are saying to:

- Lay the QuinnTherm/Cellotex directly onto the joists.
- Tape edges of insulation together
- Lay the OSB on top of insulation
- Create enclosing edges around the outer edges of the insulation (with 3 x 2 probably as 70mm insulation would fit snugly)
- Felt over the top of it all

I have attached a diagram too.

Is this correct? And if so where should I put the vapour barrier?

Thanks for your help,
View media item 49671

 

[Xenon]

Drawing is correct, and the foil face is the vapour barrier, hence the need to tape the joints.

 

[bendeneo]

Thanks again Xenon.

As I have a great big roll of the stuff, should I use some vapour barrier anyway? If it isnt overkill, where would you put it?

Regards,

 

[freddiemercurystwin]

Your vapour barrier must be installed on the warm side of the insulation ie underneath it. Its purpose is to prevent any moist air entering the insulation where it can condense beneath the deck, within the insulation. Never seen the foil facing used as a VP in this situation. Although low risk, there will be gaps in the VP where the insulation sits on the joists as well as being a PITA to stick all those bits of tape on. Easier to just sling a membrane over the joists first.

 

[bendeneo]

Thanks for this,

As I have it already, then I will do exactly that and throw it over the joists before laying the insulation down on top.

Regards,

 

[bendeneo]

NB - what thickness of Cellotex would you say was acceptable for the roof of such a garden room?

As it is not a living area for a lot of the time (and I'll probably put a bar inside) and I'll just heat it when needed with the Portable Calor Gas Heaters .... and the fact that I have loads of 3 x 2 spare, do we think that 70mm thickness is enough?

Regards,

Ben

 

[freddiemercurystwin]

Sounds OK to me.