Looking for some advice on suitable insulation for a retro-fit garden office build.

[Pete562]

Hi

I am looking for some advice on suitable insulation for a retrofit build project.

I have a existing pre clad timber framed outbuilding that I am looking to convert into a garden office and I am looking for advice on how to install insulation in the roof and walls but in a suitable way so as to prevent any troublesome condensation from building up.

The structure is mainly timber 6m x 3m with a pitched apex roof that is constructed with 18mm OSB boards and covered with 1.5mm EDPM.

The whole structure rests on a concrete base and there is a single layer of bricks running the perimeter. Sitting on top of the single layer of bricks are the walls, which are wooden and consist of 5mm plywood sheeting with 10mm cladding with 95mm x 40mm timber stud supports.

I believe insulating such a structure is what’s referred to as a ‘Cold Roof’ as the insulation will be going on the inside and my understanding is that I need to leave an air gap in the roof of approx 50mm, which needs to be vented to allow a through flow of air in order to avoid condensation.

My question - How do I do this and do the walls also need venting to allow a through flow of air? Note: There is No breathable membrane installed on the outside walls, I do intend to install a vapour barrier on the inside.

I have searched the Internet both high and low but as yet not managed to find any suitable solutions to this retrofit insulation problem, any advice and suggestions would be very much appreciated.

Many thanks

Peter

 

 

[Temp]

9 hours ago, Pete562 said:

I believe insulating such a structure is what’s referred to as a ‘Cold Roof’ as the insulation will be going on the inside and my understanding is that I need to leave an air gap in the roof of approx 50mm, which needs to be vented to allow a through flow of air in order to avoid condensation.

 

My question - How do I do this and do the walls also need venting to allow a through flow of air? Note: There is No breathable membrane installed on the outside walls, I do intend to install a vapour barrier on the inside

 

The issue is humid air inside the office (due to breathing, plants, cups of steaming tea etc) finding it's way through gaps in insulation to cold parts of the timber frame where it condenses causing the frame to rot. 

 

The standard book ways to avoid this are as you say... vapour barrier on the inside and ventilate the cold side of the insulation. The problem with just drilling ventilation holes is letting rain, bugs and mice in. The holes really need a grill on the outside which might be difficult given proximity to the fence etc. The would also make quite a mess of the appearance. Venting the roof at the ridge might also be problematic.

 

I think I'd probably take a risk and not bother to ventilate the cold side. I might paint everything with a wood preserver and when dry fit a Rockwool like insulation between the timber frame and a rigid  PIR on the inside of the frame so sheets are butted together. Tape joints and fit a plastic vapour barrier as well. Do a really good job of the vapour barrier.  Hard to quantify the risk though.

 

Beware, foil covered insulation may well block your mobile signal and WiFi.

 

If you want power in the shed get an electrician to do it. Perhaps tell him to fit a small "Garage CU". Surface mount wiring to avoid big holes in your vapour barrier. 

 

 

 

[Iceverge]

I would be cautious of trapping moisture in the structure, especially at the roof with the membrane, so no to the impermeable layers inboard for me. No matter how perfect a job you do moisture will get inside a structure, the important thing is to allow it to dry. 

 

I would fill between the timbers with a batt insulation and use a variable control layer like Pro Clima Intello plus or Siga Majrex. Tape all penetrations diligently. These should allow all moisture in the cavity, timbers, ply etc to dry inwards only preventing a trapping of moisture.

 

inside this you can put a layer of battens with batt insulation between and then plasterboard/osb. 

 

I would stay away from foil faced PIR in all cases her, especially on the roof. 

 

[Iceverge]

 

Here is a suggested build up. 

 

Worst case as it's the roof with the impermeable EDPM outboard. There is some condensation but note the outside temperature of -5. I suspect you are getting some in frosty weather as is. 

 

 

As soon the outside temperature picks up to something reasonable any condensation dries again. 

 

This model will show that if you put an impermeable layer inside the studs there will be no condensation, BUT it assumes there's no moisture trapped there during construction, no pinholes in the EPDM , no tiny faults in the taping of the membrane etc etc. 

 

You appear to have first fixes the electricity wires, I would pull them out and put them inside the membrane in the 50mm service cavity. If you run the 45*45mm battens horizontally too you will eliminate almost all repeat thermal bridging. 

 

Here's one I'm going to get finished some time soon..........any day now..............

 

150mm Studs @600mm cc with rockwool. Membrane, 45*45mm battens @ 400mm cc . 

 

Lessons learned. Insulate the roof before the walls, and maybe put the service cavity battens at 600mm cc for 15mm plasterboard.