how to insulate ceiling of room beneath barn

[vagrantly3893]

We have a room in our home that was converted from barn space. It is a small hall linking the barn area to the general living space. The room is heated. Above the room is a cold barn.

 

There are 140mm beams running across the room ceiling (highlighted blue) with wooden planks laid upon them. Below the beams is 10mm Fermacell gypsum boards. We are planning on placing 140mm mineral wool insulation between the beams.

 

1. Should we also place a 2nd layer of mineral wool e.g 100mm adjacent to the first, covering the beams?

 

2. Do we need to place something above the mineral wool, effectively sealing the mineral wool in to the ceiling. E.g. OSB boards.

 

3. Do we need to install a vapour barrier?

[Iceverge]

I would. 

 

1. Remove the planks. 

2. From the top down use some permanently flexible sealant to seal any gaps around the perimeter and joints of the fermcell. This will help with airtighness and prevent most of the moist air from the house coming into your roof space. 

3.  Then lay 140mm of a good quality mineral wool like Rockwool or RockSilk between the beams. You could then put timbers at a depth of your choosing across the beams at 90deg at closer centres. Say 100mm or 140mm and lay more more mineral wool in-between. 

4. Then screw some OSB on top to allow for storage on top and finish off the sandwich. 

 

 

There's no need for a vapour barrier if you make a good attempt at the airtightness with the sealant. It may cause more issues than it solves. 

 

Theres probably no point in going crazy with the insulation depth either as you've a large thermal bridge with the brick wall adjacent. You could always insulate that too from the top if you were really keen. 

[vagrantly3893]

8 hours ago, Iceverge said:

I would. 

 

1. Remove the planks. 

2. From the top down use some permanently flexible sealant to seal any gaps around the perimeter and joints of the fermcell. This will help with airtighness and prevent most of the moist air from the house coming into your roof space. 

3.  Then lay 140mm of a good quality mineral wool like Rockwool or RockSilk between the beams. You could then put timbers at a depth of your choosing across the beams at 90deg at closer centres. Say 100mm or 140mm and lay more more mineral wool in-between. 

4. Then screw some OSB on top to allow for storage on top and finish off the sandwich. 

 

 

There's no need for a vapour barrier if you make a good attempt at the airtightness with the sealant. It may cause more issues than it solves. 

 

Theres probably no point in going crazy with the insulation depth either as you've a large thermal bridge with the brick wall adjacent. You could always insulate that too from the top if you were really keen. 

 

So ideally we want to prevent any air escaping the heated space through the ceiling (and probably anywhere else), because when the heated air escapes, the air cools and the moisture in the air will condense against the wood once it passes through the insulation?

 

2. Do you have any recommendations for "permanently flexible sealant"?

4. Is it OK that the sides of the sandwich are left open?

[vagrantly3893]

On 07/08/2023 at 00:07, Iceverge said:

I would. 

 

1. Remove the planks. 

2. From the top down use some permanently flexible sealant to seal any gaps around the perimeter and joints of the fermcell. This will help with airtighness and prevent most of the moist air from the house coming into your roof space. 

3.  Then lay 140mm of a good quality mineral wool like Rockwool or RockSilk between the beams. You could then put timbers at a depth of your choosing across the beams at 90deg at closer centres. Say 100mm or 140mm and lay more more mineral wool in-between. 

4. Then screw some OSB on top to allow for storage on top and finish off the sandwich. 

 

 

There's no need for a vapour barrier if you make a good attempt at the airtightness with the sealant. It may cause more issues than it solves. 

 

Theres probably no point in going crazy with the insulation depth either as you've a large thermal bridge with the brick wall adjacent. You could always insulate that too from the top if you were really keen. 

So ideally we want to prevent any air escaping the heated space through the ceiling (and probably anywhere else), because when the heated air escapes, the air cools and the moisture in the air will condense against the wood once it passes through the insulation?

 

2. Do you have any recommendations for "permanently flexible sealant"?

4. Is it OK that the sides of the sandwich are left open?

 

Do we also have the option to just leave the sandwich open, like you would in an unwalkable insulated attic?

Edited August 10, 2023 by vagrantly3893

[Iceverge]

30 minutes ago, vagrantly3893 said:

ideally we want to prevent any air escaping the heated space through the ceiling (and probably anywhere else), because when the heated air escapes, the air cools and the moisture in the air will condense against the wood once it passes through the insulation?

 

Sorry I forgot to reply. 

 

Correct on point one. 

 

32 minutes ago, vagrantly3893 said:

Do you have any recommendations for "permanently flexible sealant"?

 

Probably not the best person to ask but here's a suggestion.

1 hour ago, vagrantly3893 said:

 

 

 

 

 

1 hour ago, vagrantly3893 said:

Is it OK that the sides of the sandwich are left open?

 

Do we also have the option to just leave the sandwich open, like you would in an unwalkable insulated attic

 

 

You can do whatever suits best. If you're regularly trafficking the attic and storing stuff I think I'd close it in as it'll be tidier and less lightly to become pulled out. 

 

 

 

[vagrantly3893]

2 minutes ago, Iceverge said:

 

Sorry I forgot to reply. 

 

Correct on point one. 

 

 

Probably not the best person to ask but here's a suggestion.

 

 

 

 

 

You can do whatever suits best. If you're regularly trafficking the attic and storing stuff I think I'd close it in as it'll be tidier and less lightly to become pulled out. 

 

 

 

 

Yeah I wouldn't mind leaving that part of the barn open as we don't need it for storage and you can't walk there because the roof is low in that area.

 

For the caulk, is it also a good idea to tape over the joints after caulking with don't vapour barrier tape?

[Iceverge]

54 minutes ago, vagrantly3893 said:

 

Yeah I wouldn't mind leaving that part of the barn open as we don't need it for storage and you can't walk there because the roof is low in that area.

 

For the caulk, is it also a good idea to tape over the joints after caulking with don't vapour barrier tape?

 

Again you can tape if you want it won't make much difference. 

 

The main aim of the caulk is to stop as much of the bulk air from escaping from the house into the roof. As you're in an old barn its probably fairly "holey" anyway so I wouldn't sweat it. There's lightly plenty ventilation in the roof so I would stay away from trying to create a vapour Barrier anywhere.  

 

Just buy yourself a couple of 150mm rolls and get going. 

 

[Mike]

On 10/08/2023 at 17:19, vagrantly3893 said:

Do you have any recommendations for "permanently flexible sealant"?

I've used Ilbruck FM330 flexible foam in similar situations.