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phantombantam

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Hi everyone,

im looking for some advice with insulating my ew garden building. It's single dense concrete block 8 m x 4m with an internal partition wall to separate the shed and garden room area. This leaves approx 6 x 4 m room to insulate.
Here is my basic plan. 80mm tf70 thermafloor on top of DPM on concrete slab floor then 22mm caberfloor tongue and groove floating floor. 50mm pir on walls with 50mm air gap and then 100mm pir between rafters leaving a 50mm gap to the roof boards.
Does this sound ok? just trying to plan in my mind before ordering materials.

slight problem with how to work around internal double block pillars x 2 which are one either side of french doors and two on the opposite long wall.

outside was treated with storm dry before winterand has since been rendered with parex which will also be treated with paraguard so it's watertight
 
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Jameshow

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How about celotex on wall, and foil on the pillars.

Just a thought

Cheers James
 

phantombantam

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What kind of foil please? I'm just trying to avoid cold spots as much as possible but it will be more insulated being double block I'm assuming anyway
 

RobinBHM

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I’m not sure about your cold roof detailing.

it looks to me as though you would struggle to get decent air flow down each joist gap.

Id be tempted to do a hybrid roof - push celetex tight to roof, leave say 5mm gap all round and then foam up with gun set low.

when set, cut foam off flush, foil tape all around joists so foil on boards is joined.

then use a vcl and fully foil taped around perimeter before boarding.

if you are plasterboarding then use foil backed too.
 

RobinBHM

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What kind of foil please? I'm just trying to avoid cold spots as much as possible but it will be more insulated being double block I'm assuming anyway
I would do 50mm on walls and 12mm or 20mm on pillars….if you need it at all.

TBH the u value of a multi quilt is compromised without an air gap
 

Jameshow

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Just a multifoil. You would need to prevent it being crushed for maximum insulation.

Cheers James
 

phantombantam

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The rafters are 6 inch deep. Will be drilling through for cross ventilation and venting the soffits on all 4 sides.
 

phantombantam

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Originally I was going to fit the celotex just between the pillars and leave them exposed and painted with ply or similar between on top of the celotex, but if the benefit of insulating the pillars is minimal I won't bother
 

phantombantam

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Another thing is like advice on from those that know is whether I need to go to expense of the thermafloor tf70? Does it have better compressive strength over standard pir board or is it just a sales tool?
 

phantombantam

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The other thing I can't seem to find definite info on is which order to fit the insulation. Is it floor, ceiling then walls or another way?
 

MARK.B.

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Good looking shed ;):), Are those signs of damp/mold on the ply ceiling or just mucky spots ?. I don't know if there is a right or wrong order to do the insulation, but there are those on here that will guide you
 

RobinBHM

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Originally I was going to fit the celotex just between the pillars and leave them exposed and painted with ply or similar between on top of the celotex, but if the benefit of insulating the pillars is minimal I won't bother
are the walls cavity construction or a single skin?

it looks like it may be single with returns at the door openings -so no cavity acting as a thermal break.

I guess it wont be too bad given that lightweight blocks are quite insulating -I suppose they are standard 3.6N blocks like thermailites.

I would have a slight concern about damp transfer on a single skin in which case you might want to isolate with foil insulation -including the pillars to help prevent any damp or vapour issues. I know the outside is rendered but that isnt 100% guaranteed to not let moisture through.
 

RobinBHM

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The rafters are 6 inch deep. Will be drilling through for cross ventilation and venting the soffits on all 4 sides.
if you are insulating in between the rafter, you still have a hybrid roof, Id be nervous about a cold roof construction in that scenario.

Will you be running cables in the ceiling and having lights? -if so you would have to be 100% sure you have no penetrations through the vcl and insulation if you are having a cold roof.

There seem to be noggins close to the wall, which would really impede ventilation of a cold roof.


lovely structure by the way -thatll be a great garden building.
 

phantombantam

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They are single skin paint grade 7.3n blocks. The pillars are two blocks thick one either side of french doors at front and directly opposite wall. Was originally going to be painted but had to treat after it was up last year. The out side had two coats of storm dry masonry protection cream as it was like a sponge. That's explains the mould patches on the inside of the roof boards I had to wait 4 months to get the proper roof on and there was no window until early April. It's been seated with mould killer and wiped down and since then no more has appeared.
The render went on about 6 weeks ago and will be treated with Paraguard which is stain and water repellent.
Rafters will be cross drilled and all 4 soffits vented. The short ends are not blocked up fully to the roof boards so already get some air flow that way without the soffit vents fitted and the front and back blocks will be drilled at the tops to allow flow front to back of building.

all cabling etc will be surface mounted and run in conduit on the out side of the internal cladding to avoid having to come through the insulation. Cladding will be plywood on 2 walls and also reclaimed pallets slats on 2 walls
 
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phantombantam

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The plan was to insulate the wall all around between pillars with 50 mm PIR board which leaves a gap of 50 mm to the external wall apart from at the pillars where the block is doubled up. Still not sure which order to start in. Was looking at floor, ceiling then walls
 

phantombantam

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are the walls cavity construction or a single skin?

it looks like it may be single with returns at the door openings -so no cavity acting as a thermal break.

I guess it wont be too bad given that lightweight blocks are quite insulating -I suppose they are standard 3.6N blocks like thermailites.

I would have a slight concern about damp transfer on a single skin in which case you might want to isolate with foil insulation -including the pillars to help prevent any damp or vapour issues. I know the outside is rendered but that isnt 100% guaranteed to not let moisture through.
if you are insulating in between the rafter, you still have a hybrid roof, Id be nervous about a cold roof construction in that scenario.

Will you be running cables in the ceiling and having lights? -if so you would have to be 100% sure you have no penetrations through the vcl and insulation if you are having a cold roof.

There seem to be noggins close to the wall, which would really impede ventilation of a cold roof.


lovely structure by the way -thatll be a great garden building.
how will a hybrid roof avoid condensation though if it leaves no gap to the roof boarding and will be closer to the cold side of the roof. I've never done anything like this before and would like to get it right first time
 

RobinBHM

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how will a hybrid roof avoid condensation though if it leaves no gap to the roof boarding and will be closer to the cold side of the roof. I've never done anything like this before and would like to get it right first time
It is to do with interstitial condensation.

the important factor, that is true of both warm roofs and cold roof designs is that warm air laden with moisture must not be able to get from the warm side to the cold side.

And the solution to that is by having a vapour control layer on the warm side.

Ideally a warm roof will have insulation above the joists and a cold roof will have the insulation below the joists -those scenarios mean no thermal bridge and a nice flat area to lay the VCL

with insulation between the rafters its important to have a continuous vapour barrier -which can be achieved by foil taping across every joist connecting all the foil covered insulation sheets together and in your case, I would tape the to the insulation sheets on the walls to ensure no gaps where roof and walls meet.

you need to be careful to avoid penetrating the VCL with electical cable holes, downlighters etc etc.


Insulation companies dont test hybrid roofs and architects wont specify them - they are a compromise and its important to take a belt and braces approach to the VCL
 

phantombantam

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It is to do with interstitial condensation.

the important factor, that is true of both warm roofs and cold roof designs is that warm air laden with moisture must not be able to get from the warm side to the cold side.

And the solution to that is by having a vapour control layer on the warm side.

Ideally a warm roof will have insulation above the joists and a cold roof will have the insulation below the joists -those scenarios mean no thermal bridge and a nice flat area to lay the VCL

with insulation between the rafters its important to have a continuous vapour barrier -which can be achieved by foil taping across every joist connecting all the foil covered insulation sheets together and in your case, I would tape the to the insulation sheets on the walls to ensure no gaps where roof and walls meet.

you need to be careful to avoid penetrating the VCL with electical cable holes, downlighters etc etc.


Insulation companies dont test hybrid roofs and architects wont specify them - they are a compromise and its important to take a belt and braces approach to the VCL
Thanks, yes I was always planning on using a plastic layer as well.

does the rest of my plan sound ok to you?
 
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