Help needed to bridge gap in wooden shed.

[cameronjac]

hi

I'm loving my new shed but need a bit of advice...

I'm going to insulate it as we're going to put a pool table in there.

All around the edges, there's a gap for ventilation. I'm going to start insulating etc so I need to sort the gap out. I'm going to put a few vents in some but, to bridge the gap, not sure how to do it...

It's about 4'' high so thinking of maybe 4''x2'' to bridge it and then maybe silicone or expanding foam to cover any gaps?
The only thing is the insulation would be in body of the wall but the 4''x2'' would then form part of the framework. I'll be putting a DPM on before the inner wall(either pallets or OSB) but would it be okay ?
Or maybe ply screwed on from underneath? Could then maybe put insulation on top of the ply.

Sorry, going to be doing it myself so any advice would be appreciated.

Thank you

Joanne

 

[Fitzroy]

Joanne,

sorry essay ahead:

TLDR: Insulate the walls, leave the roof un-insulated, fit OSB slices as soffit boards screwed over the gap from the outside. See how this fairs from a warmth and condensation perspective, and if required insulate the ceiling and add ventilation holes to the soffit boards.

Essay
Membranes, insulation, ventilation, etc are really all about trying to manage two things:
1. Keeping the warmth in.
2. Managing moisture.

Warm air can carry more moisture, so if you make a space nice a warm to hang out in the air in that space will gain moisture. If this warm, moist air cools down this moisture will condense as water. This is why in an old single glazed house you always had condensation on the windows, relative to the rest of the building they are very poorly insulated, so when the warm moist air from the house contacts them the moisture condenses out. The solution is double glazing, improving the insulation properties of the widow to be closer to the rest of the house.

In a building the risk is the nice warm, moist, air migrates into the structure and gets past the insulation. On the other side of the insulation the structure is cold, and air cools and the moisture condenses out, bingo you have an issue. You can solve this in two ways: 1. prevent the moist air penetrating the structure, this is the purpose of the internal vapour barrier. 2. Ventilate the structure on the other side of the insulation, so if you have any moisture it can either run out via gravity or dry-out with the breeze. In reality most structures incorporate both these design elements.

Why do I say all this? Your plan is not totally clear to me, options:
- You could insulate the walls but not the roof. If this case, the wall insulation will help retain some heat, the air inside will be able to hold more moisture, but when it contacts the cold roof it will condense out. I remember a nigh-club at uni where you would be dripped on whilst on the dance floor, urgh....
- You plan to insulate the walls and the roof but shut off all ventilation to the roof. In this case if any moist air gets past the roof insulation you will end up with a rotten roof.

Ideally you would, install insulation in the walls and in-between the roof battens, with a membrane on the inside and a ventilated gap behind the insulation. For the roof I would then screw OSB strips from underneath outside (you're making a soffit), in to the OSB strips I would cut a gap/drill lots of holes and staple insect mesh over the holes, this will provide ventilation behind the roof insulation without making a very cosy home for wasps etc. You are basically trying to build an insulated room within the structure of your shed, the shed walls and roof then protect this room from rain and wind.

As I say this is all in an ideal world and may not be possible, but I hope some of this info helps you put in context some of the advice you may receive and assists you in making an educated decision on what to do. A half way house would be to insulate the walls, leave the roof un-insulated, fit OSB slices as soffit boards screwed over the gap from the outside. You could then see how this fairs from a warmth and condensation perspective, and if required insulate the ceiling and add ventilation holes to the soffit boards.

Cheers

Fitz.

 

[thetyreman]

I'd put some kind of moulding around it, but it looks very out of square where it meets the wall frame...

 

[cameronjac]

Joanne,

sorry essay ahead:

TLDR: Insulate the walls, leave the roof un-insulated, fit OSB slices as soffit boards screwed over the gap from the outside. See how this fairs from a warmth and condensation perspective, and if required insulate the ceiling and add ventilation holes to the soffit boards.

Essay
Membranes, insulation, ventilation, etc are really all about trying to manage two things:
1. Keeping the warmth in.
2. Managing moisture.

Warm air can carry more moisture, so if you make a space nice a warm to hang out in the air in that space will gain moisture. If this warm, moist air cools down this moisture will condense as water. This is why in an old single glazed house you always had condensation on the windows, relative to the rest of the building they are very poorly insulated, so when the warm moist air from the house contacts them the moisture condenses out. The solution is double glazing, improving the insulation properties of the widow to be closer to the rest of the house.

In a building the risk is the nice warm, moist, air migrates into the structure and gets past the insulation. On the other side of the insulation the structure is cold, and air cools and the moisture condenses out, bingo you have an issue. You can solve this in two ways: 1. prevent the moist air penetrating the structure, this is the purpose of the internal vapour barrier. 2. Ventilate the structure on the other side of the insulation, so if you have any moisture it can either run out via gravity or dry-out with the breeze. In reality most structures incorporate both these design elements.

Why do I say all this? Your plan is not totally clear to me, options:
- You could insulate the walls but not the roof. If this case, the wall insulation will help retain some heat, the air inside will be able to hold more moisture, but when it contacts the cold roof it will condense out. I remember a nigh-club at uni where you would be dripped on whilst on the dance floor, urgh....
- You plan to insulate the walls and the roof but shut off all ventilation to the roof. In this case if any moist air gets past the roof insulation you will end up with a rotten roof.

Ideally you would, install insulation in the walls and in-between the roof battens, with a membrane on the inside and a ventilated gap behind the insulation. For the roof I would then screw OSB strips from underneath outside (you're making a soffit), in to the OSB strips I would cut a gap/drill lots of holes and staple insect mesh over the holes, this will provide ventilation behind the roof insulation without making a very cosy home for wasps etc. You are basically trying to build an insulated room within the structure of your shed, the shed walls and roof then protect this room from rain and wind.

As I say this is all in an ideal world and may not be possible, but I hope some of this info helps you put in context some of the advice you may receive and assists you in making an educated decision on what to do. A half way house would be to insulate the walls, leave the roof un-insulated, fit OSB slices as soffit boards screwed over the gap from the outside. You could then see how this fairs from a warmth and condensation perspective, and if required insulate the ceiling and add ventilation holes to the soffit boards.

Cheers

Fitz.

Thanks for your "essay" Fitz. Its much appreciated

ok, I've got Celotex insulation sheets for the insulation which I will make sure is snug and also tape all the joints will aluminium tape so that no warm air can get through to the cold side.

I'm also going to Celotex the ceiling.

the depth I've got with the walls is 64mm. The Celotex is 50mm so there will be a 14mm air gap between the insulation and the outer cladding. I did consider a breathable membrane but, with only 14mm to work with combined with the foil backing of the Celotex, I don't think I will. I'm going to ensure that its watertight by sealing any gaps/holes so hopefully there won't be any ingress. By also ensuring that the insulation is also completely sealed with foil tape, plus a DPM/plastic sheet, there shouldn't be any interstitial condensation either.
ok...Thats the walls...

The gap (in my OP) and the roof....
I've also had 50mm Celotex for the ceiling. I haven't quite worked out the specifics (as yet) . I was thinking OSB boards to hold the Celotex in...I also realise that I need an air gap between the Celotex and the actual roof. I think I may do as you say and drill holes in the OSB I'll put in to bridge the gap. The interior would then be its own unit.....the holes in the OSB (in the gap) would then ventilate the roof gap (between the Celotex and roof) if I kept all that open.
These holes in the OSBs...I will divert some air flow/ventilation into the shed itself rather than just to the shed roof. I was worried about heat escaping but I think I need air to circulate. If I have OSBs on the walls...I'd drill some holes in the inner wall, in the same spot as the holes in the gap OSBs (I've got a disk cutting drill bit adapter) and also not put insulation in that section so that it would act as a vent.

How does that sound?

 

[Fitzroy]

Sounds good. only bit i don't think is a great idea is:

"These holes in the OSBs...I will divert some air flow/ventilation into the shed itself rather than just to the shed roof. I was worried about heat escaping but I think I need air to circulate. If I have OSBs on the walls...I'd drill some holes in the inner wall, in the same spot as the holes in the gap OSBs (I've got a disk cutting drill bit adapter) and also not put insulation in that section so that it would act as a vent."

With the vents at the top of the wall, and warm air being less dense than cold air, you will likely get air from the shed moving out of these vents, and air in the shed being replaced by leakage through the door frame, floor, other gaps etc etc. The risk being you get warm moist air where you really don't want it. I'd suggest not making these vents and if the shed gets too stuffy you can always open the door for a bit. In reality I doubt any of us will get the sort of air tightness that could result in poor air quality, the cost and detail that modern passive houses go to to achieve that sort of performance is staggering.

Regards

Fitz.

 

[cameronjac]

Sounds good. only bit i don't think is a great idea is:

"These holes in the OSBs...I will divert some air flow/ventilation into the shed itself rather than just to the shed roof. I was worried about heat escaping but I think I need air to circulate. If I have OSBs on the walls...I'd drill some holes in the inner wall, in the same spot as the holes in the gap OSBs (I've got a disk cutting drill bit adapter) and also not put insulation in that section so that it would act as a vent."

With the vents at the top of the wall, and warm air being less dense than cold air, you will likely get air from the shed moving out of these vents, and air in the shed being replaced by leakage through the door frame, floor, other gaps etc etc. The risk being you get warm moist air where you really don't want it. I'd suggest not making these vents and if the shed gets too stuffy you can always open the door for a bit. In reality I doubt any of us will get the sort of air tightness that could result in poor air quality, the cost and detail that modern passive houses go to to achieve that sort of performance is staggering.

Regards

Fitz.

Thank you Fitz

OK, so holes in the OSBs to create the ventilation between the boards and the roof but to completely block off and seal the inner shed/room? If we're using it and need to we can open the door but I'm just a tad worried about when we're not using it at all...there'd be no vents so no air circulating ?

Joanne

 

[Fitzroy]

I think you're at the point where you have a plan to implement a solution following the barrier, insulation, vented gap, outer-skin approach as best as the shed will allow. You will not know precisely how the space will truly work until you give it a go.

I expect you will find if you've had the kids in there for a few hours with say a fan heater it will be pretty warm and humid. When the fun is over and the place cools down this humidity will drop out, likely on the inside of the windows. Realistically a few vents won't solve this, you'd need someway of blowing all the moist air out, ie extractor fan or similar, but you're getting into more and more complex solutions.

Overall it will be a great place where lots of fun is had, but it could have a few minor issues that you have to overlook, in the end it is a modified shed not a purpose built, centrally heated extension.

F.

 

[cameronjac]

I think you're at the point where you have a plan to implement a solution following the barrier, insulation, vented gap, outer-skin approach as best as the shed will allow. You will not know precisely how the space will truly work until you give it a go.

I expect you will find if you've had the kids in there for a few hours with say a fan heater it will be pretty warm and humid. When the fun is over and the place cools down this humidity will drop out, likely on the inside of the windows. Realistically a few vents won't solve this, you'd need someway of blowing all the moist air out, ie extractor fan or similar, but you're getting into more and more complex solutions.

Overall it will be a great place where lots of fun is had, but it could have a few minor issues that you have to overlook, in the end it is a modified shed not a purpose built, centrally heated extension.

F.

Yes, you're right.

When I undertake anything I just research and research and research some more to make sure I'm prepared. I don't want to go in guns blazing only to regret not doing something.

Thank you Fitz, you've been most helpful

 

[Fitzroy]

Most welcome on the advice front. If you want to see where over analysis and detail focus take you have a look at my shed build, I fight daily with the 'come on it's just a shed' demons. Good luck with your build, and more importantly have fun!

F.

 

[Stanleymonkey]

Can't see a shed....

Is it behind that barn? : )

 

[cameronjac]

Can't see a shed....

Is it behind that barn? : )

lol!