insulation of old barn roof polyurethane foam vs wool

[Jon H]

 

 

 

 

 

 

Hi i'm doing up this old barn, to be kept as a workshop so no specific building regs applicable as not a habitable  building

 

My plan is to use an insulating lime render for the walls, but i was wondering about the roof we had to put new felt on, so the traditional slate roof has been taken off new felt applied (bitumen for the bats) and eave and ridge vents system included. I plan to keep the original scissor trusses but want to insulate at the rafters. The rafters are 50mm x 50mm the purlins 100x100 i only want to use a maximum of 100mm insulation as i want to leave the trusses completely exposed. My original plan was to counterbatten with 25mm battons  to keep a reasonable gap for ventilation of the rafter then use 50mm wool and a 25mm wood fibre board and then lime render/plaster - so no vapour layer as i wanted it all to be able to breath a bit. However a few people i have spoken to seem surprised I'm not using celotex 100mm strait onto the rafters and then VCL and plasterboard. I guess with a barn of this age i was concerned that using modern plastic products seemed wrong and the principal of using breathable materials important. - Would value some views on the pros and cons of these 

[Iceverge]

I think your instincts are good here. Those timbers will be mush in short order if you trap moisture in there. PIR is not breathable and will trap moisture. 

 

However the predicament is that unless you create an unbroken layer of insulation that the air can't get around it'll all count for naught. 

 

Is it really worth insulting when you've open windows etc? The felt will take care of any drips from the slates. 

 

A beautiful building by the way. Nice to see it being taken care of. 

 

[Jon H]

Hi thanks for your input I am planning to glaze the slit windows just single pane near the external surface with oak frame and getting double glazed aluminium  windows and doors for the rest - i plan to use the space for workshop and heat it, so was keen to get some insulation in the roof, I guess i was hoping that the eaves ventilation and ridge vent would allow for ventilation up between the rafters behind the insulation to keep them dry and whilst a little bit of moisture will come through the lime/woood/wool roof into this space without a VCL it should also pass the other way out - the room should have a generally dry atmosphere no water to the building so just me breathing out moisture and i wont be in there that often, do people think i should use a VCL ??

[Marvin]

What is the floor constructed of?

[Iceverge]

You might be confident that you won't be making moisture in there but I wouldn't rule out the possibility someone else might in future. Most of the unwanted moisture inside wall and roof buildups gets there from bulk air movement, very little through diffusion. 

 

Although you are rightly planning a ventilation gap above the insulation and below the felt I'd be keen to see that area well sealed from the "room air" as it were. When insulated it will make the rafters colder increasing the lightly hood of condensation if moist room air gets up there.  Use a dedicated ventilation elsewhere to make sure the moisture inside the workshop is taken outside. 

 

The trouble is it's difficult to create a good air sealing barrier with those lovely scissor trusses. 

 

After some pondering my suggested roof buildup would be the following. 

 

1. Slates.

2. Slate battens

3. Rafters. 

4.100mm Woodfiber board screwed directly to the rafters. Maybe in 2 layers with joints staggered. 

5. Strips of breather membrane sealed to the side of the trusses with flexible airtight sealant and staples folded over the face of the woodfiber. Then mesh stapled over to provide a key.

6.lime rendered over the woodfiber. 

 

Note only the outer edges of the woodfiber would be covered in membrane . 

 

You could return the membrane to the top of the walls . Mesh and lime plaster over too to provide a good airtight layer. 

 

Just my thoughts as it's tricky to provide a balance of materials that'll flex with the structure and not crack leading to a very air leaky ceiling. 

 

 

 

 

 

Edited December 15, 2021 by Iceverge
Diagram added

[Jon H]

6 hours ago, Marvin said:

What is the floor constructed of?

 

6 hours ago, Iceverge said:

You might be confident that you won't be making moisture in there but I wouldn't rule out the possibility someone else might in future. Most of the unwanted moisture inside wall and roof buildups gets there from bulk air movement, very little through diffusion. 

 

Although you are rightly planning a ventilation gap above the insulation and below the felt I'd be keen to see that area well sealed from the "room air" as it were. When insulated it will make the rafters colder increasing the lightly hood of condensation if moist room air gets up there.  Use a dedicated ventilation elsewhere to make sure the moisture inside the workshop is taken outside. 

 

The trouble is it's difficult to create a good air sealing barrier with those lovely scissor trusses. 

 

After some pondering my suggested roof buildup would be the following. 

 

1. Slates.

2. Slate battens

3. Rafters. 

4.100mm Woodfiber board screwed directly to the rafters. Maybe in 2 layers with joints staggered. 

5. Strips of breather membrane sealed to the side of the trusses with flexible airtight sealant and staples folded over the face of the woodfiber. Then mesh stapled over to provide a key.

6.lime rendered over the woodfiber. 

 

Note only the outer edges of the woodfiber would be covered in membrane . 

 

You could return the membrane to the top of the walls . Mesh and lime plaster over too to provide a good airtight layer. 

 

Just my thoughts as it's tricky to provide a balance of materials that'll flex with the structure and not crack leading to a very air leaky ceiling. 

 

 

 

 

 

 

 

Thanks for the input that is amazingly helpful and I understand the principle i think there are a couple of points 

 

- if i don't conterbatten the rafters (this was a plan to create a bigger air space to improve ventilation behind the insulation and if i used wool would stop the wool from blocking the space ) i could get at least one layer of wood board behind the truss running completely across i think there is a 75mm gap between rafter and truss  - the thickness of the purlins - i know 50mm of board does not have a great U value but might be a compromise i guess as it would avoid the need to make a good seal with the truss,

 

- you mentioned ventilation for the workshop do you mean a PIV system i guess through the wall to try to reduce moisture in the workshop?

 

This is the second floor of the barn the ground floor is further workshop/storage space- the floor of this room is therefore wooden with joists - i plan to put a ceiling in the ground floor and fill the space between floorboards and ceiling with loose fill cellulose as the ground floor wont all be heated.

[Jon H]

I just had another thought if a put 50mm wood board on the rafters behind the truss and apply a thicker insulating lime render - the local product here has a good U value for 50mm - and really work this into the truss with a mesh on the side of the truss stapled to struss and board would this provide a good enough air seal this would then be continuous iwth the render on the walls = the ridge might be difficult as the top of the trusses extend onto the rafter space but i could create a little horizontal ceiling at this height 

[Iceverge]

It's really a question of how well insulated you need it to be. Going beyond a nominal amount on the roof is unlikely to be worth it as you have uninsulated walls. However of you don't take care of airtightness it will be completely impossible to heat. 

 

I think a continuous 50mm woodfiber board on the rafters and some insulating lime render is a very good compromise. If it is sealed to the walls well. 

 

If you have a solid woodfiber board under the rafters 50mm will be fine provided your eaves and ridge ventilation is correct. With this there should be a flow of air above the insulation so if you used mineral wool the air would blow though the insulation making it perform poorly. (Thermal bypass)

 

Again with the floor your aim is to stop air movement without trapping moisture, espic

 

[Iceverge]

It's really a question of how well insulated you need it to be. Going beyond a nominal amount on the roof is unlikely to be worth it as you have uninsulated walls. However of you don't take care of airtightness it will be completely impossible to heat. 

 

 

 

I think a continuous 50mm woodfiber board on the rafters and some insulating lime render is a very good compromise. If it is sealed to the walls well. 

 

 

 

If you have a solid woodfiber board under the rafters 50mm will be fine provided your eaves and ridge ventilation is correct. With this there should be a flow of air above the insulation so if you used mineral wool the air would blow though the insulation making it perform poorly. (Thermal bypass)

 

 

 

Again with the floor your aim is to stop air movement without trapping moisture, especially at the cold side of insulation. If you don't plan on removing the floorboards you could apply an airtight membrane from below between and under the joists pushed up against the bottom of the floorboards. Taped at all joints and returned to the wall and lime plaster sealed. This last bit will be tricky. Then a breather membrane across the bottom of the joists, batten and filled with blown cellulose. 

 

 

 

It might be overkill though. A thin layer of woodfiber, joints taped, above the floorboards with a  flooring floating above would be fine.

 

 

 

With regard to ventilation a pair of hole in the wall vents at opposite sides of the workshop would be fine. Maybe PIV or MEV if you're planning on using it a lot but again probably overkill. 

[Jon H]

thanks again - i am replacing the floorboards so can do whatever seems best I assume that i need to stop moisture getting from warm room down into the cold ceiling in and below the cellulose to condense, so i want to try to vapour seal above the cellulose is that correct ?? What about from below up plasterboard then cellulose then taped osb sheets then my nice reclaimed floorboards above this (it will be very difficult to seal the reclaimed floorboards) all sealed at the wall as best i can 

[Iceverge]

That would work well, you seem to have a good grip on the principles. 

 

Take care to seal to the airtight layer to the outer walls and you're home and dry. You can get dedicated plastic faced OSB for this but regular OSB should suffice. We used 18mm on our window reveals.