Built my shed but have questions about insulation

Joined
12 Sep 2011
Messages
82
Reaction score
1
Location
London
Country
United Kingdom
Hi guys,

Pictures of the shed I finished building at last: http://imgur.com/a/IU8gI

Floor joists and roof joists are 6x2, studs are 4x2. All wood is C24. Cladding is 27mm x 144mm weatherboard. All pressure treated. Flooring is 18mm marine plywood and roofing is 12mm marine plywood with felt on top.

Size is 4.8mx3m (a bit less than 15sqm). Height is 2450mm.

My follow up question, after I built it and checked out how to insulate a workshop instead of a shed, I read about the need for the breathable mebrane and vapour barrier for constant-temperature buildings.

Basically:

Cladding -> 25mm Air Gap -> Membrane -> 75mm Insulation -> Vapour Barrier

Since I already built it, I can't do the trick with the batons to keep the air gap between the cladding and the breathable mebrane. I was reading about stapling the membrane to the joists, but that can't be too accurate. Any advice how to do this post-build without taking the whole thing apart again?

Follow up questions:

1) My roof is ventilated, so I dont need a breathable membrane in the roof. Do I literally just put 100mm Celotex, leaving a 50mm gap above for the air to flow, and then use a vapour barrier around the whole shed?

2) Since my cladding is weatherboard and thus not air tight, does that mean it also doesn't need a breathable membrane but a bigger air gap? Not sure how this works...

3) I understand the breathable membrane is suppose to somehow protect the insulation from outside moisture and the VB protects from inside moisture / vapour. If you install an airtight VB, and your shed is windowless... And it's heated but you don't visit every day in order to open the door... How do you get rid of the trapped vapour? Are you suppose to add vents? Wont the vents help the heat escape faster too?
 
Sponsored Links
Bit of a ball dropper this, but you will get away with a lot as it is just a shed (although that depends on the quality of work you want to achieve).

Breather membranes do not create breathability, they just allow it whilst providing water shedding properties. Breather membranes are water resistant not water proof (so water held against them can still wick through). This is why you have an air gap between the back of the boards and the membrane, water gets through the cladding boards and the gap allows it to drain. Bit pointless post fitting a membrane between the joists and behind the boards, as you are trying to stop the frame getting wet and providing drainage, fitting it this way doesn't provide this.

But you don't NEED a breather membrane, you need to stop water getting in where possible, and think where it can go if it does get past the cladding. A breather membrane is just a robust easy way of dealing with this, of course for a 'proper' building you would have a hard time justifying not having it.

The advantage of feather-edge lapped boards is that they are fairly 'air leaky' and this helps keeps things dry. Without a cavity the biggest risk is water penetrating the cladding, and then running down and sitting on the flat sole plate and the ends of the studs. Is the bottom cladding board kicked out or is it flat against the sole plate, if the latter pull it out with shims so that water that sits on the sole plate has somewhere to escape to, as well as increasing air flow. (EDIT I am assuming you are not full filling the space between the studs, leave an air gap behind the cladding).


Normally water penetration happens at butt joints, or through wind driven rain. Looks like your boards are full width and you have no butt joints, and wind driven rain is unlikely at that height and location. Also corners, you should consider fitting some L shaped corner pieces for added protection, as without this water is likely to get in at the lapped corner joints.

Is the door fitted, or just temporary? The internal VCL would lap around the door with the external breather membrane, this provides weather-tightness around the frame. If you look around the door threshold you can see how water could work it's way in behind the lapped boards at that junction without a membrane. Think about that area.

Mastic is the fix of last resort.

1). Yes, and have fun fitting it :p
2). As above wall of text.
3). You should not need to worry about internal ventilation to control the humidity, you won't be creating nearly as much moisture as a house and there will be some air leakage around the door. At worst if you do a really good job with the VCL's and get the door well fitted, it might fell a little stuffy if you work in there for long periods, but then you can either open the door or maybe consider fitting a small closable grill to it if you want something secure..
 
I see, makes sense.

But if a breather is just for water resistance, are there any insulation slabs that include it on one side? If I loosen the last board, I will also get insects. This means I can't use wool glass since insects love that stuff, and make nests in it.

Also polystyrene will catch fire too quickly.. so I am left with Celotex or Kingspan?
 
Insect meshes?

If the bottom board is tight, you really want to kick it out to allow some path for water getting onto the sole plate to dry, this is the most vulnerable part of your construction as you have built it.

Insulation can come with membranes attached, but this protects the insulation and it is the studding you should be worried about getting wet, which is why you wrap the building in a membrane before battening and cladding.

Polystyrene can come with fire retardant treatment, still melts in a fire and acts as fuel once the whole thing goes up, but is hard to ignite in the first place so will be 'safe'.

Rigid foam is less of a fire issue and a better insulate. I would take some 25mm deep battens as packers, attach these to the side of the studs against the cladding, and install insulation up against that (the battens insure you maintain an air gap).
 
Sponsored Links
My studding is pressure treated and C24, so it's not that big a deal if it gets wet as long as there is air for it to dry.

So far I have the following actions I need to take:

1) Loosen the last weatherboard
2) Add some 25mm battens to the cladding to put the insulation on
3) Breather membrane (Do I need this between the studs, to protect the insulation?)
4) Put some insulation
5) Add a VB on the inside
6) Ply sheet everything

I am afraid of the insulation developing black mould or getting wet, will this be a problem for things like Kingspan KP10 / TW55?

Something else:

I made a mistake of putting the floor boards on the outer joist frame, so I can't lift it easily to put insulation underneath.. and my internal height is pretty low (2m internal height). I am 1.90m myself so I can't sacrifice much height. I was thinking of cutting the boards from the inside or something.. any ideas?
 
My studding is pressure treated and C24, so it's not that big a deal if it gets wet as long as there is air for it to dry.

Mostly correct but be aware if it is off the shelf timber it will be treated for use class 1 or 2, which is to protect against wetting during construction or from minor short term leaks. If moisture is held permanently on the sole plate it will rot, the main risk is water penetrating at joints and having no outlet.

I am afraid of the insulation developing black mould or getting wet, will this be a problem for things like Kingspan KP10 / TW55?

These are typically foil faced so no point putting in a breather membrane to protect the insulation, remember to foam fill around the panel (and you battens go all around like a frame, not just the sides, to stop the foam expanding into the cavity). You won't cut them to fit exactly so just rely on the foam and cut them with room for foam.

I made a mistake of putting the floor boards on the outer joist frame, so I can't lift it easily to put insulation underneath.. and my internal height is pretty low (2m internal height). I am 1.90m myself so I can't sacrifice much height. I was thinking of cutting the boards from the inside or something.. any ideas?

Where is the ventilation for the joists, and is there a groundsheet to keep the void dry?

Possibly fill the void with polystyrene beads, it is a bit experimental and depends upon the answers to the above.

Otherwise yes you will have to cut the floorboards with a circular saw carefully and re-do.
 
Floor:
The floor joists do not touch the concrete slab everywhere so it has small holes all over for ventilation. There are two Damp Proof Membranes, one below the concrete slab (only up to a certain point) which was added before I bought the house, and one I put on top of the slab plus the extra brick-built bit we added which the joists rest on top of. The joists are 150mm deep (same as the roof joists).

There are three sets of flooring boards. I only need to cut the two squares on either side in the middle line, they are placed something like this

[_____][_____]
[__][_____][__] < This row, the squares on either side need to be cut.
[_____][_____]

The one in the center I can just unscrew.

Then I can just slide the battens and insulation under?

Battens:
Ok, but if I add battens all over then it will also create a little pool at the bottom of the stud, which will have the undesired effect you mentioned. Do I skip the bottom batten and just make sure the bottom is tight?
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top