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Garden building insulation advice

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jimmy_s

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Personally I would try and get some form of cavity - could you not core cut a few vents in at low level?

Also its better not to put all the insulation in between the rafters general rule is half between and rest under as the rafter is a cold bridge but might be ok.

I have to go out but if you want to PM me the build ups I can run the interstitial calcs for you as thankfully I dont have to do them manually anymore.

I'm an engineer not an architect so others here may be able to advise better on build ups but I'll try and give you a hand

Jimmy
 

phantombantam

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Thank you. The reason for not venting low level is the building has just been parex rendered at a decent cost and I'd really not like to damage it. I need to see when I'm in from work as to whether I can put any insulation under the rafters as I'm not sure whether I will have the height once the floor is in. I also need to think about how I can get so holes through the blocks at the front and rear where it was blocked in up to the roof boards to allow the air to get through. And I can't use standard round 70 mm soffit vents as my soffits are not that wide.
 

phantombantam

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I think I might be able to get away with 75 mm between rafters and 25 mm below rafters but will need to measure. Increase in cost is fairly small.
If I thought I could drill through the render and block successfully would several of these do?
342C3073-EC41-4F86-92F7-D4260BF4D0DC.png
 

jimmy_s

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I think I might be able to get away with 75 mm between rafters and 25 mm below rafters but will need to measure. Increase in cost is fairly small.
If I thought I could drill through the render and block successfully would several of these do? View attachment 115018
cant see why not - looks like a round weep vent. Maybe get a 30mm core cutter to lessen the chance of damaging the render?
 

phantombantam

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I'm not comfortable with drilling into the render barring my only other part which is my gutter down pipe which will only be small holes. Can you please let me know what you need for calcs and I'll send you a pm when I can get the relevant info. If the render becomes damaged it WILL end in divorce 😖
 
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RobinBHM

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Thanks, I have seen some of his videos. I'm more concerned about the walls to be honest Robin as i think if I have to I can insulate between and below the battens and feel confident of sealing off well and still retain enough ceiling height.
ive seen the one where he states that fitting cross battens before the roof boards would have allowed cross ventilation before fitting the roof but mines on already so I need to look at another method. I've seen one of his where he shows the cross drilling as a solution after the fact.

I really need to know about the walls and it seems like an air gap would probably make the likelihood of condensation worse or at least the consequences of it.
my apologies if I’ve created more confusion! - but it’s worth thinking it through, because getting it wrong can lead to long term issues.

I apologise if I made suggestions of a hybrid roof - I realise these can fail, but I’ve had experience of cold roofs fail.
 

RobinBHM

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One thing I slightly wonder: if the wall insulation is fitted tight to the wall with no gap and runs down to the concrete slab….would any condensation actually matter - one face is masonry, one is foil.
 

RobinBHM

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. As vapour transport can and does happen in both directions
In regards to garden buildings, Ive always struggled to work out what actually happens.

These buildings aren’t like houses, they are generally used occasionally, so my guess is there are days when the inside may be colder than the outside and vica versa.
 

Fitzroy

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In regards to garden buildings, Ive always struggled to work out what actually happens.

These buildings aren’t like houses, they are generally used occasionally, so my guess is there are days when the inside may be colder than the outside and vica versa.
That was a can of thinking worms I was avoiding opening. It struck me a few months back that the building will seldom be is a state of equilibrium unless it’s heated permanently, so what actually happens is uncertain.
 

phantombantam

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I need to order the gear prob Monday or Tuesday at the latest to get it for my window of holiday from work so I'm hoping I have a definite plan by then. It's definitely a minefield. Thanks for all of the info and points of view so far it's definitely helped me understand more about it.
 

RobinBHM

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That was a can of thinking worms I was avoiding opening. It struck me a few months back that the building will seldom be is a state of equilibrium unless it’s heated permanently, so what actually happens is uncertain.
I built my own garden cabin and I couldn’t get my head around it…..so ignored it in the end.

I have to say though, even leaving it for a week or two unheated in the winter I’ve not seen even a hint of condensation on any metal or glass surface, ever. It’s timber frame, featheredge cladding with 100mm insulation in walls and 120mm in warm roof.
 

johnny

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wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them. ;)
As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..

You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .
 
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phantombantam

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wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them. ;)
As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..

You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .
Unfortunately it's fully rendered now so without cutting a hole through the wall and render 😱 extraction isn't really an option.
 

phantombantam

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Can anyone shed (excuse the pun) any more light on my best course of action please? I will need to order the materials probably Tuesday latest to get it in time for my week off work as that's my only opportunity to get consecutive days working on it.
 
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phantombantam

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wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them. ;)
As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..

You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .
the difference being that it's not going to be occupied anywhere near as often as our house.
I want to minimise any issues but also need to balance that with cost and fitting in with what is already there as I don't want to damage render that was quite expensive.

it won't be occupied overnight except very occasionally if kids want to sleep out. So maybe could be used for 2-3 hours a day but not every day. And sometimes longer maybe during school hols weekends
 

Fitzroy

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Phantom. It feels like you’ve reached the end of our advice/thinking. There is no perfect answer to your problem, so it’s decision time for you on the level of compromise. Ventilated but render holes. Hybrid roof and moisture barrier but no ventilation etc.

Personally I’d insulate the roof with a gap above and vent this gap as best as possible at the soffits. Insulate the walls with no cavity, then spend the effort on the vapour barrier. History will judge.
fitz.
 

johnny

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the difference being that it's not going to be occupied anywhere near as often as our house.
I want to minimise any issues but also need to balance that with cost and fitting in with what is already there as I don't want to damage render that was quite expensive.

it won't be occupied overnight except very occasionally if kids want to sleep out. So maybe could be used for 2-3 hours a day but not every day. And sometimes longer maybe during school hols weekends
the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .

Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .

You can either install an extractor in the roof of the shed ,(preferably at the highest point ,) or you can install a dehumidifier which will extract and store the moisture and will require emptying periodically.

These are simple,cheap and very effective options for your situation that will cause minimal installation disturbance .

The other thing you might consider is how hot the inside of the shed will become when the outside temperature rises and the sun is beating down on the timber flat roof. Some form of extraction system will be essential if you intend to work in the shed in comfort during Summer months . Opening the door and windows is unlikely to provide adequate through ventilation
 
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phantombantam

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the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .

Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .

You can either install an extractor in the roof of the shed ,(preferably at the highest point ,) or you can install a dehumidifier which will extract and store the moisture and will require emptying periodically.

These are simple,cheap and very effective options for your situation that will cause minimal installation disturbance .

The other thing you might consider is how hot the inside of the shed will become when the outside temperature rises and the sun is beating down on the timber flat roof. Some form of extraction system will be essential if you intend to work in the shed in comfort during Summer months . Opening the door and windows is unlikely to provide adequate through ventilation
Hi, no petulance here from me. I'm just trying to weigh up the different opinions and advice which seem to be complete opposites.
Hindsight is a marvellous thing and I obviously did not know what I have since been told about a number of things
 

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