Insulating a shed/garden room?

[SiT]

Afternoon,

I am after a bit of advice regarding a recent building/shed/garden room I had put up -



This is to solve a storage issue I have, its a 14x7 summerhouse/shed combi with a pent DPM rubber roof.

I want to insulate and board it however have had conflicting advice, the guys who put it up and obviously do this for a living said -

Floor - Celotex boards then ply on top and finish (laminate/vinyl)

Walls - vapour barrier first starting from top overlapping down, then rockwool/mineral insulation with ply/osb on top

Roof - foil backed buuble wrap attached to timber joists only leaving an air gap between each if that makes sense then ply onto it making a sandwich in effect.

Looking online and watching some videos there seems to be content suggesting the following which is contrary to above -

Walls - insulate between timbers first then vapour barrier?

Roof - Celotex boards between roof joists, blocking every hole/gap and then expanding foam into any gaps?



Ideally I would like to insulate and plasterboard to give it a nice finish and be more of a 'room' than a shed, it will have electric etc. plumbed in so would be grateful if anyone in the know can advise which way round I should be layering it all? Its not an all singing all dancing garden room/studio but would just like to make the most of the space and be able to enjoy it all year round.

Si

[Equus]

Unfortunately, you have to accept that the building was not designed to be insulated and as a result, doing so will substantially reduce its lifespan.

That said, the best you will sensibly achieve is:

• Battens on the inside face of the frame to create a gap, if you can accept the loss of space.
• Breather membrane (NOT vapour barrier) on the inside of the battens (or existing frame, if you can't find space for battens; the frame itself will create a small gap, of course).
• Celotex Insulation
• Vapour barrier (albeit this will only be necessary/functional if you're doing something in there that generates moisture; the foil surface of the insulation is perfectly adequate in most cases, if you tape it at joints)..
• Plasterboard or cladding as internal finish.

The main problem is that you should really have a ventilated cavity (gap) between the external cladding and the breather membrane, and this is difficult to achieve without going to a lot of trouble. Without it, driven rain penetrates the external cladding then gets trapped on its inside face, between the cladding and the breather membrane/insulation, where it will accelerate the decay of the structure.

[guitarcarfanatic]

As per Equus, insulating that is likely a lot of effort for little reward. It's designed to store things and keep them dry, whilst allowing air in and out as ventilation. I certainly wouldn't want to keep anything electrical in something like that - not without serious modification...

For something like what you describe, people typically construct as per an American timber house - The cheapest method is normally to use an insulated and suspended floor on ground screws/piers, framed walls/OSB sheathed/house wrapped/double battened/clad, and then a pent roof constructed of appropriately sized beams. Doors and windows UPVC double glazed or sliding bifolds. A T&G OSB deck for the roof with EPDM covering. You can then insulate, vapour barrier, plaster and finish the floors.

[ssray]

The kids had a play shed, I insulated it with the silver bubble wrap stuff from eBay, it was white one side
Made a big difference

[Mr Squarekins]

Something like a Tyvek breatheable membrane, but with an air gap applied to the inside of the shed. Then Celotex or Kingspan. Then board. However, you're going to lose maybe 100mm floor space all the way around.

[OutInTheShed]

Equus said:

Unfortunately, you have to accept that the building was not designed to be insulated and as a result, doing so will substantially reduce its lifespan.

That said, the best you will sensibly achieve is:

• Battens on the inside face of the frame to create a gap, if you can accept the loss of space.
• Breather membrane (NOT vapour barrier) on the inside of the battens (or existing frame, if you can't find space for battens; the frame itself will create a small gap, of course).
• Celotex Insulation
• Vapour barrier (albeit this will only be necessary/functional if you're doing something in there that generates moisture; the foil surface of the insulation is perfectly adequate in most cases, if you tape it at joints)..
• Plasterboard or cladding as internal finish.

The main problem is that you should really have a ventilated cavity (gap) between the external cladding and the breather membrane, and this is difficult to achieve without going to a lot of trouble. Without it, driven rain penetrates the external cladding then gets trapped on its inside face, between the cladding and the breather membrane/insulation, where it will accelerate the decay of the structure.

It's a shed not a house.
The fundamental difference with a shed is that they spend a lot of time unheated.
That can mean they are colder inside that out for a big slice of the day.

So unlike a house where the big issue is vapour going from the warm indoors to the cool outdoors, a shed may have problems with vapour coming in.

Options include insulate it and heat it regularly.
Ventilation
Dehumidifier

Depending on local climate and what exactly you want.


Simplest thing is to insulate with celotex or similar, vapour resistant both ways.
It's not a huge space so a 2kW fan heater with soon warm it up.
Use it often and don't leave it shut up all winter. Open it up when the weather's nice.

The outer wooden skin is a matter of nature vs wood preserver. Here in the wet country, nature wins in the end.

[Techno9000]

I built a summerhouse from a kit five years back and the manufacturer supplied insulation (foam board similar to Celotex) for floor and roof.
As we wanted it to be comfortable for three seasons at least, I added insulation to two of the walls (where no windows exist) before overboarding.

This has resulted in something quite comfortable, since the Sun beats down on the insulated faces of ours for much of the day, and has proven to keep temperatures under control. For instance, opening up in one of the afternoons where temperatures are in the high twenties finds the internal temperature no higher. So insulation can be of use for Summertime as well.

[Mazinbrum]

My wife is looking at a garden/hobby room, the rooms we are looking at come fully insulated as an option.

[Equus]

We're getting into techy stuff, now, so if building science bores or confuses you, walk on by...

OutinTheShed said:

It's a shed not a house.
The fundamental difference with a shed is that they spend a lot of time unheated.
That can mean they are colder inside that out for a big slice of the day.

So unlike a house where the big issue is vapour going from the warm indoors to the cool outdoors, a shed may have problems with vapour coming in.

Very unlikely, unless the shed is sited in a very shaded location: air is difficult stuff to warm up by means of radiant heat, whereas with sheds it's usually excessive solar gain that's the problem. There will be brief periods when its warmer and higher RH outside the shed than inside, but not usually long enough to cause problems.

Remember that moisture doesn't become a problem for timber until the timber itself reaches >20% moisture content. If you keep it below that level, it doesn't rot ever. In a properly designed timber structure, it should normally be sitting at around 14%. It takes time for timber to absorb moisture and change its moisture content.

The cyclical periods when it's warmer and higher RH outside than inside the shed tend to be short enough that it doesn't reach problem levels. We're usually talking about a brief period in the morning, when the thermal mass (which is low for a timber frame building, remember) causes a bit of a lag before solar gain takes the internal temperature above the external. Unless the shed is very poorly sited, it doesn't last anything like long enough to allow a 6% gain in the moisture content of the timber.

OutInTheShed said:

Simplest thing is to insulate with celotex or similar, vapour resistant both ways.

But then you're back to trapping moisture from rain penetration between the cladding and the insulation.

as you said:

The outer wooden skin is a matter of nature vs wood preserver. Here in the wet country, nature wins in the end.

But to achieve an 'adequate' lifespan, timber rainscreen cladding relies on being able to dry out quickly between periods of rain, so that the moisture content of the timber doesn't spend protracted periods at >20% moisture level. If you trap moisture from escaping on the inside of the cladding (by not having a ventilated gap behind it), you prevent that from happening not only for the cladding, but for the structural frame as well.

[SiT]

Morning everyone,

Thanks for all the replies, some really useful information there all be it again some is a bit conflicting.

I think it’s important to make it clear this is not a structure or building I intend to live/work in, it is purely a space to spend time (in respect of the ‘summer house’ portion) during the Summer and be able to protect furniture/bbq during the winter. It would be nice to try and keep it a bit more sealed up and warmer throughout the year is all we were thinking.

The shed portion is simply going to be somewhere I can store my tools and ‘man stuff’ as our current house has no garage so I have stuff hidden everywhere - I just want it in one place.

It will have some basic electrics in it (light in both sides and a socket) for simple tasks and also being able to pop a small electric radiator or similar in it.

Realising I haven’t spent £20,000 on a upvc, cedar clad all signing all dancing garden studio, I simply just want to make it a more habitable place for humans/anything that’s stored in there. But at the same time as has been mentioned above I clearly don’t want to do anything which is going to affect the building and reduce its lifespan.

Perhaps someone can confirm I am on the right lines with what I have picked out from the posts -

Floor - this has 40mm depth around the outside so I was planing to lay 25mm celotex boards, tape the joins and then lay 15mm Ply/OSB on top.

Walls - initially I was going to fit a membrane (still confused if vapour or breathable!) to the walls and uprights all the way round, starting at the top and working down overlapping as I go, on the basis that if some water found it’s way in it would run down and out (due to overlap) rather than inside if I had started from the bottom and home up. Then it was my plan to install rock wall between the uprights before boarding over the top - T&G/PLY.

OR

Walls - install celotex into the void between the uprights but with a space between the back face of the celotex board and the inside face of the external cladding to create an air gap, tape any joins and board over the top.

Roof - and this is the one that confuses me the most! The guys that installed the shed said to attach foil backed bubble wrap across the span but only attaching it to the roof joists therefore leaving a gap in the voids which they said would allow the building to breath. They said if I attached it up and over each joist and then into each void between them the building would very quickly start to sweat as no air circulation.

Any clarity on the above would be great, I know it’s not going to be on George Clark’s Amazing Spaces but just want to make the most of it.

Cheers

Si

[Equus]

SiT said:

I think it’s important to make it clear this is not a structure or building I intend to live/work in, it is purely a space to spend time (in respect of the ‘summer house’ portion) during the Summer and be able to protect furniture/bbq during the winter. It would be nice to try and keep it a bit more sealed up and warmer throughout the year is all we were thinking.

Then why do you need to insulate at all?

Your tools don't care what temperature it is: they will be more affected by moisture, whcih you'll reduce by leaving it uninsulated and highly breathable.

At least leave it until there are signs that you jhave a problem that needs fixing.

[SiT]

As I say it would be nice to have a more consistent temperature through the year rather than scorching in the Summer and Baltic in the winter. There will no doubt be occasions in the Autumn where we will no doubt fancy just sitting outside with a bottle of wine and the fire pit going on the patio.

Si

[Equus]

Whatever, the previous advice holds: the structure is not designed to be insulated, so any solution is going to be a compromise.

The specs you have proposed are acceptable, provided you recognise and accept that they will reduce the lifespan of the building.

[SiT]

I am still none the wiser on this! Thinking I may not bother insulating it at all based on the above advice. I did think I might like to board/ply line the summer house part bu then worried about doing more damage/reducing the lifespan of it as already mentioned?

Si

[Harry Flashman]

Interesting responses, especially from Equus. Does this mean that the insulated garden room kits sold everywhere that look like insulated sheds to me, will basically have a short lifespan due to moisture getting trapped?

I want one, but have not yet chosen. This thread could help...

[wong]

I bought a house several years ago which already had a shed in place, which was used as a gym. It had spray foam insulation on the walls and roof. No insulation on the floor, just some foam gym tiles. It also had the eaves fully exposed, and the wind would blow in between the top of the walls and the roof -so why bother insulating? Sounds like this may be the worst possible solution.

[Equus]

Harry Flashman said:

Does this mean that the insulated garden room kits sold everywhere that look like insulated sheds to me, will basically have a short lifespan due to moisture getting trapped?

It depends on their specification and how they insulate them.

Many of the 'home office' type garden rooms that are designed from the outset to be insulated are fine (particularly the SIPs-based ones).

The timber frame company I used to work for (now out of business), used a fairly 'trick' solution, designed by myself, that made it easy to add insulation properly to our uninsulated structures, but most of the timber frame log cabins/sheds that are designed and sold from the outset to be uninsulated are more complicated to do properly and most people just accept that since it's a cheap structure, it will have a limited life.

It's a matter of understanding what each individual element of the wall build-up does (because with a timber frame, each and every element does have a separate purpose and function), then when you understand that, it becomes easy and obvious to assess whether something will work properly or not.

[Magicmushroom666]

Might be enough to just paint the inside white or a nice light colour?, put some boarding up in small little sections to allow shelves etc to be hung but not sealing off the outside skin so it still gets air.

I agree with others that its better for something to be designed for insulation from the outset. I did this with my shed, with the outside skin being on crossed battens all around, and then ventilated top and bottom on the outside and its been great. A lot more expensive to build of course.

[guitarcarfanatic]

Harry Flashman said:

Interesting responses, especially from Equus. Does this mean that the insulated garden room kits sold everywhere that look like insulated sheds to me, will basically have a short lifespan due to moisture getting trapped?

I want one, but have not yet chosen. This thread could help...

Take a look at Oakwood Garden Rooms on Youtube. They have a pretty proven formula and do them all day every day. They have great how to's on YouTube and a big Facebook group to share advice/ideas/problems. Liam is a sound guy by all accounts, if I was paying to have a garden room installed, they would probably get my money.

I built my own (sans insulation as just a workshop) - basically, like a US timber framed house.

[Equus]

guitarcarfanatic said:

Take a look at Oakwood Garden Rooms...

Their spec. looks reasonable except that they are not mentioning counterbattening, yet are showing vertical cladding.

Their spec. correctly identifies that the battens need to provide airflow, but with vertical cladding you obviously have horizontal battens, which means that unless you counterbatten (with vertical battens behind the horizontal ones), the cavity created is not properly ventilated (you'd noramlly detail it to be ventilated top and bottom, to allow air to convect through) and, in addition, any rain that gets past the cladding will sit trapped on top of the battens (if you want to get really anal about detail, even with counterbattens the horizontal battens should be chamfered on their top edge, to encourage water to run off into the cavity formed by the counterbattens).

The fact that they don't mention an internal vapour barrier doesn't really bother me, as there isn't usually a big problem with internal moisture in this sort of building, unless you add cooking or sanitary facilities, calor gas heating, or use it as a gym, but the fact that the cladding ends so close to the ground isn't ideal - normal rule is that you should keep any exposed timber (particularly exposed end grain) at least 150mm. clear of the ground, to reduce splashback from rain.

These are admittedly all details and - as above - most people are willing to just accept that these sorts of buildings won't last forever, but 'if a job's worth doing...'.