Help with insulation choice

[DigitalM]

My shed is basically a single course of bricks with 2 x 4 frame. A1protect membrane was atached to this this and then the whole thing covered with featherboard cladding. The roof is felt, plywood, then A1protect onto the 2 x 4 frame.






In this respect, it's built like Peter Parfitt's shed from his New Brit Workshop YouTube channel.

I was trying to get my head around the insultations options. Can anyone nudge me in the right direction so that I can start researching the right thing (still struggling with the hot roof / cold roof issue and which applies to mine!).

 

[Jones]

Glass or rockwool slab type are cheap and easy to fit which reduces mistakes and so improves performance. The insulation value is lower than board types but in an intermittently used shed this is not very important,the extra heating costs would be low and only needed occasionally. A warm unvented roof is easier to build, you will need to include a continuous vapour barrier ( plastic sheet) to prevent moisture getting into the insulation, tape all joints and don't poke holes in it, you'll have to surface mount the lights.

 

[DigitalM]

Glass or rockwool slab type are cheap and easy to fit which reduces mistakes and so improves performance. The insulation value is lower than board types but in an intermittently used shed this is not very important,the extra heating costs would be low and only needed occasionally. A warm unvented roof is easier to build, you will need to include a continuous vapour barrier ( plastic sheet) to prevent moisture getting into the insulation, tape all joints and don't poke holes in it, you'll have to surface mount the lights.

Sorry for taking forever with follow up questions ... this is a long term project!

Are you referring her to what I've seen elswhere as a "hybrid" roof? This looks much simpler than creating a vent space between the roof ply and the insulation.

 

[Phil Pascoe]

I had to look - I wasn't sure what "insultation" was.

 

[DigitalM]

Seems to be a common speed-typing error of mine. Probably built up from muscle-memory due to insulting so many people Phil.


Mod edit: Title fixed, but the rest is on you.

 

[Fitzroy]

Sorry for taking forever with follow up questions ... this is a long term project!

Are you referring her to what I've seen elswhere as a "hybrid" roof? This looks much simpler than creating a vent space between the roof ply and the insulation.

Yes, strictly this is a hybrid roof. A warm roof has the structural element of the roof on the warm side of the insulation. The risk with a hybrid roof is that moisture gets through the insulation to the ply wood, which is cold, condenses and then degrades the structural integrity of the roof. With the structural element on the warm side it's much easier to get a continuous vapor control barrier before the insulation and manage this risk.

Regards your roof, the gold standard would be a vented cold roof. Vent the 2x4 cavities at either end (using insect mesh on any vents) then install Kingspan type insulation over the rafters, and your surface of choice over this. You would however loose ceiling height.

The further you step away from the gold standard the more likely you are to have issues with interstitial condensation. However if you occupy the space for a few hours a week and work in relatively low temperatures c.15degC, and vent out any moist air at the end of a session by leaving the door open for 10mins then IMHO your moisture risk is low anyhow.

Fitz.

 

[DigitalM]

Yes, strictly this is a hybrid roof. A warm roof has the structural element of the roof on the warm side of the insulation. The risk with a hybrid roof is that moisture gets through the insulation to the ply wood, which is cold, condenses and then degrades the structural integrity of the roof. With the structural element on the warm side it's much easier to get a continuous vapor control barrier before the insulation and manage this risk.

Regards your roof, the gold standard would be a vented cold roof. Vent the 2x4 cavities at either end (using insect mesh on any vents) then install Kingspan type insulation over the rafters, and your surface of choice over this. You would however loose ceiling height.

The further you step away from the gold standard the more likely you are to have issues with interstitial condensation. However if you occupy the space for a few hours a week and work in relatively low temperatures c.15degC, and vent out any moist air at the end of a session by leaving the door open for 10mins then IMHO your moisture risk is low anyhow.

Fitz.

Thanks Fitz, I'm being more and more tempted toward the sort of cold roof you describe. The rafters are 93mm deep and the wall studs are 87mm deep (no idea why they're different, whatever). If I opt for 50mm PIR all round, that would give a vent space of 43mm on the roof and 37mm on the walls. Do you think that is sufficient on a cold setup, or would you have an alternative suggestion?

I'm starting to look into venting products like fascia vents, anyone with any advice on good choices there I'd be very glad to hear your experience.

 

[Fitzroy]

Thanks Fitz, I'm being more and more tempted toward the sort of cold roof you describe. The rafters are 93mm deep and the wall studs are 87mm deep (no idea why they're different, whatever). If I opt for 50mm PIR all round, that would give a vent space of 43mm on the roof and 37mm on the walls. Do you think that is sufficient on a cold setup, or would you have an alternative suggestion?

I'm starting to look into venting products like fascia vents, anyone with any advice on good choices there I'd be very glad to hear your experience.

I used 100mm Rockwool which is equivalent to about 40mm of PIR board. It made the world of difference as with a small fan heater I can bring the workshop to about 15C in an hour from c. 5C. Then about 2kWHrs to keep it there for the day, 10+ hrs.

Vent spaces sound fine to me. Can’t comment on facia vents as I made my own on my build.

Fitz.

 

[DigitalM]

I've finally completed the cold roof, vented at either end and insect meshed as advised.

I now have the walls to do, are these vented top and bottom too? Seems extreme?! Or can you just leave an airgap without vents for walls?

 

[Molynoox]

I've finally completed the cold roof, vented at either end and insect meshed as advised.

I now have the walls to do, are these vented top and bottom too? Seems extreme?! Or can you just leave an airgap without vents for walls?

Defo don't do that.
Either vent it with an air gap, or don't vent it and push the insulation tight against the cold surface / outer skin to reduce / eliminate the air gap as much as possible.
You can use a moisture barrier too on the inside if you want to go to town.
Nobody is venting the walls on sheds and garden rooms by the way, the roof is the main issue as the warm air rises.
Martin