Wall and roof insulation - Pre-work guidance sought

[FunkyJive]

Untitled

• FunkyJive
• 4 Nov 2011
• 1

Before I install roof insulation boards and seal and line the walls of a timber garden building, I need to be sure I'm doing the right thing before I start. This is my first significant DIY build project so informed guidance is appreciated.

Following advice from a previous post I've installed foil-covered 50mm Eurotherm boards between the 60mm wall support timbers (external wall 22mm T&G), leaving a few mm between the T&G wall and boards, and also the vertical timbers standing a few mm proud internally. I've uploaded an image of the building internals as it is at the moment.

Ahead of my questions, I propose to do the following...

Walls...

1. Vapour-seal all gaps with 200mm foil tape over the timbers to the cavity boards.

2. Attach 4" x 1"s horizontally at right-angles to the wall timbers, running the full length of the walls. These will also provide convenient behind-wall cabling channels at both floor and bench heights once fitted.

3. Tack a visqueen vapour barrier covering the horizontal timbers and then Gyproc soundblok plasterboards on top.

Question:
Should (can) I completely seal off the cavity between the plasterboard and the cavity boards, or should I ventilate it internally to avoid a vapour trap?


T&G Roof...

1. Fill the cavities with 40mm foil-faced cavity boards (T&G board profiles affording vertical-running ventilation grooves above, and aeration at the apex where support beams run through the end walls with small clearance gaps).

2. Tack a visqueen vapour barrier over the support beams and cavity boards, running out over the top of the walls to allow natural air-flow between the soffits and T&G grooves to the apex.

3. WBP ply sheet on top of the visqueen for the ceiling.

Questions:
Is this roof ventilation scheme likely to be sufficient, or will it be better to completely seal off the roof from the soffits? The top of the cavity boards will be 3-5mm below the T&G roof timbers.

Conversely, will ventilation actually need to be increased by laying insulation boards on top of the existing roof timbers (not prefered due to reduced headroom)?


Thank you in advance.

FJ

 

[mnemonix]

It sounds like you're really going to town on this shed! A drawing or sketch of your wall, roof, eaves etc details would help as it's difficult to work out exactly what's going on here.

In principle you are trying to stop moisture vapour from the warm air inside penetrating the building fabric and condensing in there where it can cause damage, while ventilating any space on the cold side of the insulation to remove any moist air and condensate that forms there. Your foil backed insulation and vapour barrier - (not sure you really need both!) achive the former, your ventilation gaps the latter, but I can't work out where this is exactly in your roof.

 

[FunkyJive]

@mnemonix

Thank you for your reply.

You may think I'm going to excessive lengths to line and protect the building, but I never do things by halves and, acknowledging my lack of familiarity on this first significant building job, would rather not push risks and perhaps go OTT to be as sure as I can of something intended to last for the longer term. I will also be spending a lot of evening and weekend hours in the finished furnished building.

I've provided a sectional drawing which will hopefully clarify what I'm proposing to do as the prefered option. To conserve head-clearance (I'm a tall bloke) I would like to take advantage of the space between the roof support studs and tuck the celotex in there (as shown). The question here is whether the 3-5mm clearance between the celotex and the T&G roof will offer a sufficient breather gap - given the strong visqueen vapour barrier against condensation from within the room? I'm proposing to run the barrier down and over the top of the side walls (as shown), ensuring no opportunity for vapour entry into the small roof void from within the room. Also, should the felt ever develop a tear/leak (heaven forbid !), then any ingress would drain to the outside without damaging the decorated interior or electrical equipment.

My other question was whether I should seal off the 1" void between the gyproc wall and the thermal boards, or allow some small natural convection top and bottom into the room to avoid a vapour trap and possible mold? If of significant thermal benefit then I could seal the wall void entirely with visqueen, but if of little benefit then not worth the effort as it would avoid the need to seal socket cables which would run along the voids and emerge through the gyproc.

I don't really have a handle on how beneficial sealing such a wall void is likely to be for thermal improvement. Also, will the small ceiling void offer enough natural ventilation or would it be better to seal off the roof area entirely and simply add a further layer of roofing felt to reduce the possibility of undiscovered damage and rain penetration?

One final question... I'll be fitting double-glazing to the four window positions, though they come with a trickle-vent option. Is this likely to improve or worsen matters for this type of building and limited space? Again, I don't really have an idea either way.

Hopefully the image is clear enough (obviously click-expandable), and thank you again for your valued advice.

FJ

Workshop insulation Diagram

• FunkyJive
• 9 Nov 2011
• 1

Proposed workshop insulation and lining scheme.

[/img]

 

[Perry525]

Water vapour will be created by you, breathing and sweating.
I expect the inside of your shed will be warmer than the cold outside.

As such the small amount of water vapour created will be attracted to the nearest coldest surface, usually the windows.

If there are holes/cracks between the warm wet air inside the insulation and the cold walls/roof/floor then the water vapour will find its way to the cold surface and condense with the risk of mould and wood rot.

Installing three inch thick polystyrene sheets (or thicker) over the inside of the shed, will mean that the surface temperatures of the polystyrene will not result in condensation as they will be at the same temperature as the adjacent air.

Just avoid any holes or gaps for the water vapour to slip through.

 

[ratman123]

Your foil backed insulation and vapour barrier - (not sure you really need both!)

You absolutely do need a VCL with foil backed insulation between the studs.

I have seen tests where without a VCL, the gaps between the insulation and studs* act as cold bridges, and condensation occurs, though in fairness it is a low amount, it's still not worth it for the cost of a plastic sheet.



*Even if you fit the foam panels tight, you will either have a few tiny gaps, or some may develop as the timber shrinks.

 

[FunkyJive]

Thanks Perry.

However, my problem is not so much about understanding thermals and causes of condensation, but more about the actual extent of possible issues understood from a more familiar perspective.

This is my first DIY build project and don't have prior familiarity so, for example, if I should read "You really need to seal all holes and gaps so humidity doesn't creep through", does this mean that I should tape over every staple (etc) and every teeny hole, or would this be unnecessarily stupid and pointless? I believe this is advisable for the ceiling/roof, but what about ventilation gaps (etc)?

The building will be much more than a shed but will be a decorated/carpeted office/studio environment, so thick polystyrene over the walls as may be used in a typical shed (I guess you meant in addition to the Eurotherm I've already installed) would be incompatible with the finished result - and therefore my choice of higher-specified cavity boards throughout.

Hopefully my illustration is enough for further comments so that I can confidently move on.

All the best,

FJ

 

[FunkyJive]

You absolutely do need a VCL with foil backed insulation between the studs.

I have seen tests where without a VCL, the gaps between the insulation and studs* act as cold bridges, and condensation occurs, though in fairness it is a low amount, it's still not worth it for the cost of a plastic sheet

Thanks Ratman for your information.

I bought a good roll of Visqueen for this project and other things (4m x 25m) so a non-consideration... Wall VCL is in

I did apply 200mm foil tape bridging the wall studs and around all edges of the foiled boards, but clearly not enough to avoid possible problems in future should the foil become stressed and breached with movement and beam shrinkage.

All the best,

FJ

 

[Perry525]

From 2016 all new build homes must be of Passive house standard, that means as near as possible to air tight.
From 2020 all new offices and factories must be built to the same standard.
In practice all new commercial buildings are being built to this standard now as owners realise how difficult it will be to find new tenants in future.
I think that it makes sense to follow the new standard now....if you are building for comfort and low running costs....something you may not be aware of. Holes equal heat loss......equal higher heating and cooling costs.........lower comfort.