Summer house - better to insulate or thicker walls?

Miss_Riot

My partner needs a home office and in my tiny 2 bed, that's not possible with children. So we need a summer house. But in order for it to be warm enough and dry in the winter, it needs to be insulated. Cost wise, are we better to go for a fully spec-ed up one with 18mm thick walls (not frame) and then insulate the walls floor and roof and boarding it, or are we better off going for the thicker timber walls at 2k more expense with less features, and doing minimal insulation? We are having to borrow money to do this so need the most cost effective option, but also it needs to be warm enough for him to work in the winter without it costing a fortune to heat - I am also considering a solar electric heater. The size is below planning regulations. 

grumbler

'Minimal' insulation will cost marginally less than normal. 

What do you mean by 'solar heater'?

Miss_Riot

Sorry, can you explain? Are you saying minimal insulation and higher building costs will be cheaper overall or that the cost difference between high quality insulation and minimal is marginal? 

A solar panel plugged into a large battery bank powering an electric heater. We have been given one by a friend who used to live out of the back of a van but does no longer. 

MikeJXE

Whatever type of shed you build it will be COLD in the depths of winter 

Doozergirl

MikeJXE said:

Whatever type of shed you build it will be COLD in the depths of winter 

If it is a shed.  

There are different type of garden room.   If someone wants something that isn't going to cost a lot to heat then the insulation values need to meet the building regulations (or be as close to them as possible) and for that thermal envelope to be as unbroken as possible. 

No 'shed' is going to do that.  

OP, what products are you looking at and at what cost? 

FreeBear

Doozergirl said:

MikeJXE said:

Whatever type of shed you build it will be COLD in the depths of winter 

If it is a shed.  

There are different type of garden room.   If someone wants something that isn't going to cost a lot to heat then the insulation values need to meet the building regulations (or be as close to them as possible) and for that thermal envelope to be as unbroken as possible. 

No 'shed' is going to do that.  

OP, what products are you looking at and at what cost?

To stay warm in winter without breaking the bank heating, 100-150mm of Celotex/Kingspan all round, plus decent double glazed windows & door. One could start with a basic shed, but it would be better designed as a proper room from the outset.

Get the insulation levels right, and it will be easier to keep cool during the summer.

ThisIsWeird

Tbh, I have no idea how timber rates as an insulator compared to, for example, PIR (rigid, foil-faced insulation board such as Celotex). I imagine timber is 'pretty good', but going even for the thicker timber walls - say 45mm - ain't going to be good enough on its own.

So, since additional insulation will be needed, I think it makes sense to just go for the thinner 18mm timber walls, and line the inside with at least 2", ideally more, of PIR.

I'd couple that 2-3" of wall PIR with 1-2" in the floor and 3" in the ceiling. Provided you don't have any large draughts from windows and doors, that will be cosy, will be easy to heat, and will match the insulation levels of most of the houses in this country.

Yes, you can go 4", 5", 6" and whatevs in the walls, but the improvement over the 3" will be marginal.

Being a 'summer house', so presumably an uncomplicated build - essentially one rectangular room? - this will be dead easy to turn into a well-insulated box, far more so than a house would be with all its nooks and crannies, and window reveals and stuff. Foil-tape all the board joints, and you are well sealed.

What I don't know is how best to approach the construction. Is it ok to simply bond the PIR to the inside face of the timber walls? Will this cause condensation issues? I wouldn't have thought so, as no moisture should get from the inside to the wall-PIR bond. But, you'll need advice on how to manage this.

For the floor, the easiest way would be to float 2" PIR (or similar) on top, and then cover that with, say, laminate or LVT click flooring. Easy. (The floor is the least concerning surface, so perhaps even 1" PIR would do?) For the ceiling, depending on how its constructed, see if you can add the insulation to the outside of the ceiling material to save headroom. Or, if there's flexibility with the construction, add an extra 6" to the wall height as you build it, and then PIR the inside of the ceiling - safer and more effective.

Finish the insides with your personal choice - T&G boards, thick lining paper and paint, whatevs.

FreeBear

ThisIsWeird said:

Tbh, I have no idea how timber rates as an insulator compared to, for example, PIR (rigid, foil-faced insulation board such as Celotex). I imagine timber is 'pretty good', but going even for the thicker timber walls - say 45mm - ain't going to be good enough on its own.

So, since additional insulation will be needed, I think it makes sense to just go for the thinner 18mm timber walls, and line the inside with at least 2", ideally more, of PIR.

Wood is not a very good insulator at ~0.13W/mK compared to PIR at 0.023W/mK (the lower the number, the better). Your 2" (50mm) will be less than half of what would be required for a decent wall, and a third for a roof or floor. Yes, 50mm will provide some insulation, but no where near enough for building regulation compliance. Even although this room isn't going to be used for sleeping, it is worth aiming for BR compliance or better - To do so will reduce heating/cooling costs and make the space much more comfortable.

grumbler

Miss_Riot said:

Sorry, can you explain? Are you saying minimal insulation and higher building costs will be cheaper overall or that the cost difference between high quality insulation and minimal is marginal?

I mean that as you need insulation anyway it will be cheaper to do a good one combined with cheaper timber (that has poor insulating properties, so thickness will make little difference).

A solar panel plugged into a large battery bank powering an electric heater. We have been given one by a friend who used to live out of the back of a van but does no longer.

Don't know how big the panel is, but AFAIK best domestic panels are about 400W max each when new (their efficiency drops with age). And it works only in daytime when heating is needed. I think you need more than 400W to heat your office in winter.

ThisIsWeird

FreeBear said:

ThisIsWeird said:

Tbh, I have no idea how timber rates as an insulator compared to, for example, PIR (rigid, foil-faced insulation board such as Celotex). I imagine timber is 'pretty good', but going even for the thicker timber walls - say 45mm - ain't going to be good enough on its own.

So, since additional insulation will be needed, I think it makes sense to just go for the thinner 18mm timber walls, and line the inside with at least 2", ideally more, of PIR.

Wood is not a very good insulator at ~0.13W/mK compared to PIR at 0.023W/mK (the lower the number, the better). Your 2" (50mm) will be less than half of what would be required for a decent wall, and a third for a roof or floor. Yes, 50mm will provide some insulation, but no where near enough for building regulation compliance. Even although this room isn't going to be used for sleeping, it is worth aiming for BR compliance or better - To do so will reduce heating/cooling costs and make the space much more comfortable.

Thanks. Yes, if you can afford to lose the internal room, then go 4".

But 2" will still give a high level of insulation.

Doozergirl

FreeBear said:

ThisIsWeird said:

Tbh, I have no idea how timber rates as an insulator compared to, for example, PIR (rigid, foil-faced insulation board such as Celotex). I imagine timber is 'pretty good', but going even for the thicker timber walls - say 45mm - ain't going to be good enough on its own.

So, since additional insulation will be needed, I think it makes sense to just go for the thinner 18mm timber walls, and line the inside with at least 2", ideally more, of PIR.

Wood is not a very good insulator at ~0.13W/mK compared to PIR at 0.023W/mK (the lower the number, the better). Your 2" (50mm) will be less than half of what would be required for a decent wall, and a third for a roof or floor. Yes, 50mm will provide some insulation, but no where near enough for building regulation compliance. Even although this room isn't going to be used for sleeping, it is worth aiming for BR compliance or better - To do so will reduce heating/cooling costs and make the space much more comfortable.

This.   Also remember that a small timber building has a huge ratio of surface area to volume; in winter it's surrounded by cold air on the top, sides and bottom and would need more heat than an equivalent room in a house.  

And if it's a slot-together job, it's full of air gaps.  You may as well be outside.  

I took shelter in a garden building at the Chelsea Flower Show during a storm and the woman was trying to tell me that the wood in their £10k timber-only building was insulating when the whole thing was hundreds of pieces slotted together. It would have leaked warm air like a sieve.  

Timber has nothing like thermal mass that will retain some heat, unlike a brick/concrete/stone house (even without insulation) 

People should be thinking of their garden rooms like extensions.  They're not cheap even when they're incapable of standing the test of time so it is worth trying to meet the building regulations - with a building regs completion certificate you create peace of mind and a true extension that adds genuine value to a house. 

You need decent structure, minimal air gaps and proper research into the genuine insulation level.  Simply referring to something as 'fully insulated' is a con.  

There's some info on this website which (as freebear recommends),  properly designs rooms from the outset and can meet regs.  
https://www.gloriousgardenrooms.co.uk/built-to-last/

(It's not my company anymore so I'm not advertising myself!)