Retro fitting external insulation on garden room

[cjames1984]

Hi all,

I was just looking for a basic layer guide to externally insulating my existing garden room.
For reference the existing building is made up of 44mm timber.

I was thinking (starting from wall externally) rigid 100mm Rockwall screwed straight to existing timber using screws and discs(no stud wall), breathable membrane, 25mm battens for ventilation and then 19mm shiplap cladding.

Or

75mm foil backed PIR boards again screwed straight to existing timer using screws and disks, battens and cladding. I'm not sure about the membrane layer on this one because of the foil on PIR

I would be left with enough of an overhang on my roof with either install.

I just want to get it right so I minimise any build up of moisture inside the room.

Any advice is greatly appreciated

 

[freddiemercurystwin]

How would you use Rockwool without any timber studs?

If you want to avoid studs then you'd have to go the rigid insulation route, you still need a breather membrane and for the cost it would be daft not to, vertical battens & horizontal cladding presumably?

 

[cjames1984]

Thanks for the response.
My local builders merchant does accoustic rockwool boards rather than the rockwool that comes in a roll.
Yes, vertical batons and horizontal cladding. If I go down the rigid insulation route where does the membrane sit in the layers?

 

[cjames1984]

How would you use Rockwool without any timber studs?

If you want to avoid studs then you'd have to go the rigid insulation route, you still need a breather membrane and for the cost it would be daft not to, vertical battens & horizontal cladding presumably?

Thanks for the response.
My local builders merchant does accoustic rockwool boards rather than the rockwool that comes in a roll.
Yes, vertical batons and horizontal cladding. If I go down the rigid insulation route where does the membrane sit in the layers?

 

[freddiemercurystwin]

Is the acoustic roll you're proposing suitable or will it deform? Seems like an odd choice of insulation.

In any case the breather membrane goes on the outside of the insulation behind the battens.

 

[cjames1984]

Is the acoustic roll you're proposing suitable or will it deform? Seems like an odd choice of insulation.

In any case the breather membrane goes on the outside of the insulation behind the battens.

It was based on some videos I'd seen online.

My original choice was the PIR.

So if I went with the PIR. It would be PIR (foil side facing out, foil tape all joints), membrane on top, batten and then cladding.

If that's correct, my last question is would you recommend the plastic or metal discs for fixing the PIR?

 

[JohnD]

PIR is twice as good as insulator as the same thickness of mineral wool.

 

[^woody^]

The dense Rockwool is called Frontrock or EWI render slab or suchlike.

PIR board is more efficient, but it's not vapour permeable so the options for retrofitting are limited due to higher risk of condensation issues within the frame.

How are you stopping moist warm air from entering the frame from the room?

It's no use highly insulating a garden room wall, if more heat is going to be going out through lots of glazing and glazed or poorly insulated roof.

 

[cjames1984]

The dense Rockwool is called Frontrock or EWI render slab or suchlike.

PIR board is more efficient, but it's not vapour permeable so the options for retrofitting are limited due to higher risk of condensation issues within the frame.

How are you stopping moist warm air from entering the frame from the room?

It's no use highly insulating a garden room wall, if more heat is going to be going out through lots of glazing and glazed or poorly insulated roof.

The warm air into the timber frame was the bit I was trying to work out.

I have an old single block house so am very used to using dehumidifiers and allowing the circulation of fresh air.

As it stands I have insulted the floor with PIR. The same for the roof, PIR boards in-between the rafters and an air gap between the ceiling to allow for circulation of air.
As it stands I have two small windows and french doors.
Long term I'm hoping to swap these out for double glazed UPVC.

With all that information, any advice on how to insulate the walls?

 

[^woody^]

Whats on the internal face? Plasterboard? Are you leaving whatever it is on the internal wall face and working entirely from the outside?

 

[cjames1984]

Whats on the internal face? Plasterboard? Are you leaving whatever it is on the internal wall face and working entirely from the outside?

It's just the timber frame.

I was hoping to work completely outside, firstly to save losing space and secondly to save having to redo all the electrics.

 

[^woody^]

In that case you need a vapour check layer on the frame just after the plasterboard. Foil backed insulation can act as a vapour check.

Search some other posts for timber frame construction. You could full-fill the frame with no gaps or cavity.

It may also be better to also have a thin layer of insulation across the internal face to prevent thermal bridging by the timber frame.

 

[cjames1984]

In that case you need a vapour check layer on the frame just after the plasterboard. Foil backed insulation can act as a vapour check.

Search some other posts for timber frame construction. You could full-fill the frame with no gaps or cavity.

It may also be better to also have a thin layer of insulation across the internal face to prevent thermal bridging by the timber frame.

I'm sorry, I'm a little confused now, there is no plasterboard. The room is build from single layer solid 44mm timber all around (if that makes sense)

I've just googled vapour check later, this makes sense.
I would install that straight onto the existing timber outside to slow the release of hot moist air from inside?

The PIR would be installed butting up to each other with no stud wall to reduce cold spots and continues insulation.

All my wiring is surface mounted in conduit, I would have to remove all this to put any insulation internally, it's something I could consider as it makes sense to reduce how much boost moist air is hitting the timber directly.

Thanks again for all your help so. I'm just hoping to iron out all the details so I don't cause any long term issues.

 

[cjames1984]

I'm sorry, I'm a little confused now, there is no plasterboard. The room is build from single layer solid 44mm timber all around (if that makes sense)

I've just googled vapour check later, this makes sense.
I would install that straight onto the existing timber outside to slow the release of hot moist air from inside?

The PIR would be installed butting up to each other with no stud wall to reduce cold spots and continues insulation.

All my wiring is surface mounted in conduit, I would have to remove all this to put any insulation internally, it's something I could consider as it makes sense to reduce how much boost moist air is hitting the timber directly.

Thanks again for all your help so. I'm just hoping to iron out all the details so I don't cause any long term issues.

I've just being doing some more research, would the vapour check layer on the outside of the external wall not trap the hot warm air from inside in the timber?

I am starting to wonder if I would be better suited to either putting the solid slab rockwool or batton and rockwood roll, then breathable membrane, batten and cladding to help with

I understand this would not retain as much heat as the PIR, but long term I think I would have to add a small amount of insulation on the inside which would help the bridging and hot, moist air escaping into the timber and retain more heat.

 

[^woody^]

I'm a little confused now

Me too.

It may be better to post a photo of the frame, to show what going on there.