Garden building insulation advice

[RobinBHM]

Thanks, yes I was always planning on using a plastic layer as well.

does the rest of my plan sound ok to you?

I dont know if it makes any difference but Id be slightly tempted to do the wall insulation before the floor dpm and floor -only because if any moisture gets through the wall -it will run down to the slab and not the floor.

Am I right in saying you are having on walls: a 50mm wall cavity, then 50mm celetex then plywood?

I think on a solid external wall, it might be better to put 50mm insulation tight to the wall, with all gaps filled and the joints taped and if you want cover with VCL. Then fix 50 x 25mm battens flat and screw in place through the insulation -then fix plywood to the battens.

If you google "internal insulation of solid walls" I think this is the recommended method.

eg:
Wall insulation types: solid and cavity wall insulation


I would definitely insulate the pillars as they made of Hi 7s -they will be cold.

it might be a good idea to put a vertical dpc where the doors are -a length of 100mm dpc would so, folded so it creates a say 70mm x 30mm corner -tape in place at the doors, put insulation in place then clad with plywood -afterwards cut off any dpc poling out in front of the plywood. Why do that? -well if you get any driving rain that gets through between door and frame, it will hit the dpc and run down and cant make the ply damp.

 

[phantombantam]

The reason I was going to do the 50 mm cavity was that kingspans installation guide say minimum 50 mm air gap if solid wall is less than 200 mm thick. Mine is 100 mm plus the coat of render

 

[phantombantam]

The door and window was fully sealed and foamed in before the render was done. Its what and how to insulate at the pillars next to the door frame that I'm struggling with as the others I could just use the 50mm around but can't do that at the door

 

[phantombantam]

This although a cavity wall would suggest that the walls should be done first. And they show both methods for fitting with either battens on first the PIR and then PIR first to walls. Is there a definite way that gives least chance of interstitial condensation ? And if masonry is breathable like concrete block rendered is should any moisture that tracks through disperse more quickly than it can accumulate?

 

[phantombantam]

I feel like I'm overthinking it but don't want to have to revisit later as the insulation is going to set me back quite a bit

 

[phantombantam]

Was thinking if I can get some 25 mm PIR for the pillars I could fit that in without being too close to the edge of the door frame ?

 

[phantombantam]

If the VCL is so important why do all installation methods have you screwing through it to mount battens or your cladding??

 

[Fitzroy]

I feel your pain, I have a shed I built that I want to insulate, different construction to yours but I've been through much of the same thought processes. My main take-aways from all the thinking are:
- You need to design for insulation, retrofitting, or fitting to a building after the fact is going to be sub-optimal and lead to compromises.
- Most buildings designed for insulation incorporate 'belt and braces; ie stop the transmission of moist air and provide ventilation if the moisture air passes, so small penetrations (screws etc) should not be an issue.

On the flip side I've settled myself that
- It's a shed not my main residence, I'll do my best but if it breaks/goes mouldy it's a pain but it's not my families health at risk.
- It's a shed that will not be occupied 24/7 so it'll have many more hours in an ambient state, rather than a humid state, hopefully allowing it to dry out if I get it wrong.

Hopefully any compromises / miss judgements will be so minor that they will not create an issue.

My building is a framed wall construction rather than a solid walls. but a very similar roof. My plan is to:
- 100mm Insulate between the rafters (150mm), vent the 50mm space above them at the soffits and drill vent holes through the noggins.
- 100mm Insulate between the 100mm wall studs, no air gap, rear ventilation compromised by OSB on the exterior of the wall frames.
- Door and window returns will have no insulation around them, but they are wood not stone, so hopefully no condensation.
- Internal vapour barrier with taped joints.
- 12mm ply wood internal walls, screwed in place (through the vapor barrier, ho-hum)
- All electrics and lights surface mounted to avoid large penetrations.
- I am insulating in rockwool not PIR board, to provide more sound deadening.

PS: Please do not digress from the OPs questions, if you have comment that solely pertains to my build feel free to 'throw rocks at me' on my build thread. I'm not saying what I am doing is right or wrong, just trying to provide support and where I got to with my compromises.

Very messy build pic, but shows the construction.

Regards

Fitz.

 

[phantombantam]

What's your plans for the floor? I'm considering order of fitting with walls and can't make my mind up whether to do floor or walls first. Robin made a valid point about the DPM being inside so condensation will run behind if there is any and not down onto top of floor but I want/need my floor DPM to go against the wall and my existing damp proof course above the bottom course of bricks

 

[Fitzroy]

What's your plans for the floor? I'm considering order of fitting with walls and can't make my mind up whether to do floor or walls first. Robin made a valid point about the DPM being inside so condensation will run behind if there is any and not down onto top of floor but I want/need my floor DPM to go against the wall and my existing damp proof course above the bottom course of bricks

I’ve probably cocked that up as well. I insulated the floor whilst building. And the final floor is laid. Take a look at my build thread for pictures.

 

[phantombantam]

Ah right I didn't see the link on your previous reply at first very nice looking shed though

 

[phantombantam]

Is there any benefit in batenning out walls first so that the wall insulation cavity and roof cavity share the ventilation from the soffits I wonder? Then the warm part of the room is further away from the cold wall?

 

[Fitzroy]

Is there any benefit in batenning out walls first so that the wall insulation cavity and roof cavity share the ventilation from the soffits I wonder? Then the warm part of the room is further away from the cold wall?

Building an insulated box inside a rain proof structure, with a well ventilated and insect proof cavity between the two is pretty much what you’re aiming for. It’s just that the air gap and battens steal space from the living area. You would want to ventilate the bottom of the wall also to prevent a stagnant area.

This is actually how Aberdeen houses are built (less the insulation). There is a granite shell, then an air gap and then a timber framed lathe and plaster secondary box inside. The gap behind the lathe is vented outside at the bottom through 6” holes in the bottom stones and connected to the vented roof space at the top. This is excellent for managing penetrating moisture from wind blown rain and allows internal moist air to be carried away. However without insulation it results in a cold house or a huge heating bill. The problems arise when debris bridges the gap and moisture wicks to the internal box, resulting in damp patches.

In our case the bridges we are concerned about are thermal bridges bringing internal cold spots where condensation can occur.

Fitz.

 

[phantombantam]

Yeah I can't ventilate the bottom unfortunately as the render is now on and don't want to disturb it. So maybe better to insulate direct against the wall and seal the roof void off then. My head is battered with this

 

[phantombantam]

Although I am pretty confident of the water tightness of the block and render. So not concerned of penetrating damp. If there's no more likelihood of interstitial condensation on the walls with it against the wall than spaced off then i would prefer to keep the internal space.

 

[RobinBHM]

I feel like I'm overthinking it but don't want to have to revisit later as the insulation is going to set me back quite a bit

You are right to think about it, I’ve seen the damage interstitial condensation causes.

this is my concern as I’m not sure you will have enough air flow.

the best source of info is Steve roofer on youtube


this guy does a hybrid roof which is criticised by Steve roofer

 

[phantombantam]

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.

 

[phantombantam]

This was the make up before boarding.

 

[jimmy_s]

It would help if the cavity in the walls was getting some ventilation from the ventilated roof and ideally some vents at low level.

I'm going to try and explain how interstitial condensation occurs so hopefully its easier to figure out how best to avoid it. Interstitial condensation will depend a lot on how you are using the building and how much moisture is being generated.

Basically you have a temperature gradient over the wall. The overall vapour pressure depends on the difference in temperatures over the construction and also the water vapour content of the air at each side. The pressures are always trying to equalise so you get a pressure gradient over the constructions. Similarly, you have a temperature difference over the walls and roof etc and this produces temp gradients. The lower the temperature the less moisture the air can hold and you get a corresponding dew point temp. Without going into what causes this basically the temp at each interface has a corresponding saturated vapour pressure - this is the maximum corresponding water vapour pressure that can be held without condensation occurring. So, without a vapour control layer you can end up with a vapour pressure at an interface that is higher than the saturation pressure and you will have condensation.

So, the things that you can do to help overcome issues:

Incorporate vapour control layer on warm side
Not insulate it to death
Ventilate the room - with trickle vent etc - allows some outside air in so vapour pressures can even out (diffusion)
Not heat the it excessively all the time as this will encourage a higher vapour pressure to form inside
Dont fill it with loads of damp stuff - again raises vapour pressure

You ideally would want a ventilated cavity wall as the outer leaf works like a rain jacket to cast water away - anything that gets through will hopefully diffuse away by the ventilation. If it cant get away you might end up with problems as it can get driven in the other direction into the structure. As vapour transport can and does happen in both directions.

I hope this helps


Jimmy

 

[phantombantam]

It would help if the cavity in the walls was getting some ventilation from the ventilated roof and ideally some vents at low level.

I'm going to try and explain how interstitial condensation occurs so hopefully its easier to figure out how best to avoid it. Interstitial condensation will depend a lot on how you are using the building and how much moisture is being generated.

Basically you have a temperature gradient over the wall. The overall vapour pressure depends on the difference in temperatures over the construction and also the water vapour content of the air at each side. The pressures are always trying to equalise so you get a pressure gradient over the constructions. Similarly, you have a temperature difference over the walls and roof etc and this produces temp gradients. The lower the temperature the less moisture the air can hold and you get a corresponding dew point temp. Without going into what causes this basically the temp at each interface has a corresponding saturated vapour pressure - this is the maximum corresponding water vapour pressure that can be held without condensation occurring. So, without a vapour control layer you can end up with a vapour pressure at an interface that is higher than the saturation pressure and you will have condensation.

So, the things that you can do to help overcome issues:

Incorporate vapour control layer on warm side
Not insulate it to death
Ventilate the room - with trickle vent etc - allows some outside air in so vapour pressures can even out (diffusion)
Not heat the it excessively all the time as this will encourage a higher vapour pressure to form inside
Dont fill it with loads of damp stuff - again raises vapour pressure

You ideally would want a ventilated cavity wall as the outer leaf works like a rain jacket to cast water away - anything that gets through will hopefully diffuse away by the ventilation. If it cant get away you might end up with problems as it can get driven in the other direction into the structure. As vapour transport can and does happen in both directions.

I hope this helps


Jimmy

I do have a double trickle vent above the french doors. My end window has two openers which can be locked on night vent. There is no way I can vent at lower level that I can think of . I'll be using PIR boards taped and foamed with vapour layer inside before cladding. Kingspan states a minimum 50 mm gap if wall thickness is less than 200 mm and mine is 100 mm block plus render coat.
if I was to try to vent a 50 mm cavity behind the insulation with the soffit venitilation I am going to install along with cross drilling the roof rafters will it be sufficient?

also you say to not over insulate. So plan was 50 mm PIR on walls with or without cavity depending on best advice from here.
100mm in between rafters all panels joined across with tape after being foamed in at edges and joins. Then VCL inside. Floor 75 mm on top of DPM.
does that sound like too much. It will not be occupied constantly or at night unless the kids have an odd sleepover in there very occasionally. Was planning on using a small oil filled radiator or solar to introduce a little heat in autumn/winter.

can you advise on that specifically please?

And thanks for the explanation too it does help me to understand it more clearly