Insulating a single skin shed

[Tomsbuild29]

I have bought a flatpack Garden room (Fancy shed) and it comes with floor insulation and roof insulation but not wall insulation. 

 

The walls are 42mm thick single skin wood. I'm trying to work out the best way to insulate it. 

 

I'm thinking about potentially insualting the outside off the wall as to not lose any internal space. My first thought would be to insulate and then screw battens through into the walls, then fix cladding to the battens. 

 

Do I need to consider an air gap, and a breather membrane? 

 

Does this sound like the best approach? 

 

Thanks in advance. 

[Redbeard]

Lots of considerations. Is it near a boundary? How close? Do you have Planning permission of is it within Permitted Dev'pt? 

 

Do you need non-flammable cladding? If so, what about a sheet insulation glued to the shed with cement-board over? Forget the battens. You should not necessarily need a membrane, depending on the materials you use, though using one on the outside of the insulation gives you 'belt and braces'. Ensure that all joints and perimeters of the insulation are tight and filled, or you may get thermal by-pass (cold air on the 'warm side'). You do not need an air-gap; indeed (as above) it could be your enemy rather than your friend.

[Iceverge]

On 26/11/2024 at 13:32, Tomsbuild29 said:

I'm thinking about potentially insualting the outside off the wall as to not lose any internal space. My first thought would be to insulate and then screw battens through into the walls, then fix cladding to the battens. 

 

Do I need to consider an air gap, and a breather membrane? 

 

This would work well.

 

With any building, espically a timber one I would be keen to see a robust drainage design so I'm disagreeing with @Redbeard here and saying that a vented cavity is required. 

 

If you're building for longevity and comfort I would attach a membrane to the walls, tape it to the base and the roof membrane if you can. Tape it to the windows and doors too to get a completely airtight building.

 

Something cheap like this will be fine. 

 

Add your insulation over the top. Anything will help but more will be better.  Ideally use 2 x layers of 50mm with the joints staggered, foamed and taped.  Then use 25*75mm strapping as battens held on with 6*160mm screws. 

 

Add your cladding over the top. 

 

Don't forget to add ventilation to the building as with the Airtightness you'll get a lot of moisture build up otherwise. 

[Redbeard]

47 minutes ago, Iceverge said:

With any building, espically a timber one I would be keen to see a robust drainage design so I'm disagreeing with @Redbeard here and saying that a vented cavity is required. 

 

Disagreeing is good! I think a lot will depend on the detailing and quality of work. Ventilated cavity is a belt-and-braces approach, and such approaches are good. Provided there is really good weather protection and a good roof oversail I think my suggestion will work, but @Iceverge's has an extra 'fail-safe'.

 

 

[JohnMo]

On 26/11/2024 at 13:32, Tomsbuild29 said:

walls are 42mm thick single skin wood

So a log style building. Be aware of the contraction, you must not have battens etc joined across multiple logs without a sliding joint. Failure to that leads to the logs being split, and or big gaps forming.

 

https://forum.buildhub.org.uk/topic/31017-summer-house-insulation/

 

 

[Redbeard]

Further variables:

 

Heated or unheated?

 

Is the roof oversail big enough to accommodate 'EWI'. If not you need either to re-roof or 'improvise', and the latter is not necessarily good.

 

 

4 minutes ago, JohnMo said:

So a log style building. Be aware of the contraction,

 

Good point.

 

@Iceverge, thinking about your lay-up. The ventilated cavity is formed by the 25 x 75 battens, yes? What insulation had you in mind, or just any rigid insulant?

[Iceverge]

6 hours ago, JohnMo said:

So a log style building. Be aware of the contraction, you must not have battens etc joined across multiple logs without a sliding joint. Failure to that leads to the logs being split, and or big gaps forming.

 

Good point.

 

If the building was first insulated as well as fitted with an appropriate ventilation system and allowed to "settle" before battens would that suffice?

 

Most contraction and expansion happens in response to variable temperatures and humidities and a nice layer of EWI should eliminate most of this. 

 

 

6 hours ago, Redbeard said:

@Iceverge, thinking about your lay-up. The ventilated cavity is formed by the 25 x 75 battens, yes? What insulation had you in mind, or just any rigid insulant

 

In an ideal world Rockwool Front rock or similar mineral wool. 

 

However as I expect nobody will listen to me,  PIR could be used instead.

 

[Redbeard]

I always shiver slightly when I see PIR used (v rarely) as EWI - a few 'Grand Designs' have had it on TF construction. Some degree of breathability is desirable, and 2-4 sheets of foil will militate against that more than somewhat. Would be interesting to model where the dew-point will be, but we also do not know yet whether it is heated, or really a 'shed', with no heating. The insulation strategy suggests the former, not the latter.

[Temp]

If you are insulating the outside I would go for..

Insulation

Roof membrane

Battens vertical 

Boards

 

Use insect mesh top and bottom to stop mice etc getting up the batten cavity. 

 

I would line the inside with a vapour barrier and plasterboard or OSB if you want to fit shelves.

 

What's the building standing on? If you have an oversized concrete base already I would raise it up on 1 or 2 courses of engineering bricks with a DPC on top. If you like you can extend the battens and cladding down over the brick leaving a >1" gap between cladding and slab. 

[Iceverge]

2 hours ago, Redbeard said:

I always shiver slightly when I see PIR used (v rarely) as EWI - a few 'Grand Designs' have had it on TF construction. Some degree of breathability is desirable, and 2-4 sheets of foil will militate against that more than somewhat. Would be interesting to model where the dew-point will be, but we also do not know yet whether it is heated, or really a 'shed', with no heating. The insulation strategy suggests the former, not the latter.

 

Provided you have adequate internal mechanical ventilation I wouldn't sweat it as moisture won't be trapped in the timber.

 

It's the susceptibility to rodents and insects, the shrinkage of the boards, the fire performance, the performance loss over time, the thermal bridging through the foil, the waste and dust when cutting are the the reasons I that I don't like PIR. 

[JohnMo]

2 hours ago, Iceverge said:

Provided you have adequate internal mechanical ventilation I wouldn't sweat it as moisture won't be trapped in the timber

As a way of illustration, this is our house with MVHR and our insulated summer house. The summer house has a single dMEV fan and it's heated, but temp is allowed to move about much more than the house. As you will see both building follow each other quite closely. The graph is year to date. Only time you have elevated humidity is in the summer, but it's hot not an issue.