Shed Insulation advice please

Vermin will eat through anything if they can smell something to eat on the other side ... even metal !

For the walls & roof you could try 'Actis tri iso super 9', a multi layer foil & wadding system that's only 25 mm thick.

You will need some kind of damp proof membrane for the floor.

Your main problem will be condensation forming in the roof / roof insulation so ventilation is vital.

I am now trying to work out the best way of insulating the shed - the walls and roof can be lined internally with slabs of insulation inside the framing covered by plywood, but does anyone have any advice on the floor? Does this need a similar level of insulation (which would be on the underside), or would draught proofing with a layer of plywood on the floor and removable mats be sufficient?

I know that there are field mice close by, together with larger rodents which pass through, so any underfloor insulation would have to be protected, although I am not sure that fibre glass or other insulation would make a comfortable nest!

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I recently had the pleasure of watching a local builder erect his own cottage in a nearby street. He used what looked to me like blocks foam polyurethane to insulate the area under the floor. I doubt very much rodents could eat that. My one reservation was that I would not like to be in that cottage if it ever caught fire as I believe foam polyurethane can give off lethal smoke.

(kim)

Once you go the insulation route, it can quite complicated. It's not nearly as simple as the inexperienced think. We Canucks have decades of experience with insulation. And Rule One is, you can never have too much insulation. It keeps the heat inside in winter and outside in summer. But insulation also traps warm air, and warm air contains moisture, which has a regrettable tendency to condense when it hits a cool surface. That means there will be condensation in the walls, as well. So Rule Two is, provide adequate ventilation. If you think these two rules conflict, you're right. Insulating a building as much art as science.

Re insulation: The more the better. You will be heating the shed to prevent damp, which attacks the trains and also provides ideal conditions for mildew. That means a minimum year-round temperature of at least 10C, 15C would be better. I don't know the R factors (insulating capacity) in metric terms (see footnote), but I would advise R15 as a minimum, and R20 would be better yet. Your wallet will thank you when it comes time to pay the heating bill. I doubt that even with multiple layers of foil you will have that R factor in only 25mm of insulation, so put in at least two layers of the stuff that Dragon Heat recommends. Insulation is cheap compared to the cost of keeping warm.

Make sure you seal the edges of the insulating material so as to create a continuous membrane before you apply the interior wall cladding. The material recommended by Dragon Heart appears to have a vapour barrier built in. There should be no gaps whatever in this vapour barrier. You do _not_ want water vapour to get inside the wall. The fact that it includes layers of foil means that it will minimize heat loss through radiation, which can be considerable through a wooden wall (wood is transparent to infrared.)

Re: waterproof membranes and ventilation. We call them "vapour barriers" over here, because that's what they are. You _must_ have this on the heated side of the wall. Otherwise, the water vapour carried by the warm air into the insulated spaces will condense on the cold side of the wall, and that's very bad news. In your case, it's worse, because you have a vapour barrier on the outside of the shed already.

It's OK to have an external vapour barrier, but only if there is a vapour barrier on the inside of the wall, too. Actually, the membrane on the outside of the house is more of a wind barrier - it helps prevent the wind from sucking out the warmed air from inside the walls. Thus, this exterior membrane should _not_ be airtight, as otherwise warm, moist air can be trapped inside the walls. If it is airtight, make sure that the wall cavities have openings into the roof spaces, and that the roof is ventilated enough to allow slow but steady percolation of air through it. This will help to keep the inside of the walls dry.

Re: floor treatment. You lose heat through the floor mostly by radiation, so a layer of foil-backed paper or plastic should do, but you'll have to install another layer of sub-flooring over it. You shouldn't put the finish layer directly onto insulating materials.

Re: roof ventilation. There should be gaps between the rafters to allow external air to drift up into the roof. I'm guessing that you have a typical shallow gabled shed roof. If the rafters are merely 3" or 4" deep, you cannot get much insulation in there, but put in as much as you can. Leave a space about

1" between the insulation and the roof skin, this will permit the ventilation that Dragon Heat rightly emphasises. If you have the vertical clearance inside the shed, install an inch or two of styrofoam insulation inside, and cover it with drywall (gypsum board). You'll need to install nailing strips for the drywall, however.

Er, that's about it.

Footnote: The R factor is the inverse of the thermal conductance. IE, the smaller the heat transfer rate, the higher the R factor. In Canada we use RSI as the metric equivalent. I don't know if the EU does so. Divide the R factor by 7 to get the RSI factor.

HTH

That would be the blue stuff that builders use, which is safe also good for scenery. So non wasted

In this case it was pure white so maybe it was foam polystyrene? I remember thinking I wouldn't want that stuff under my floor.

(kim)

It is polystyrene and it's used extensively as underfloor insulation. Most modern properties in the UK are concrete floor and the insulation goes under the screed (if not under the whole slab) so there is no fire or fume hazard in using it. In the floor of a shed is a different matter...

MBQ

To do it properly, you need to remove the existing membrane and fit a new vapour barrier on the *inside* of the insulation, otherwise you will get "interstitial" condensation in the insulation. There is a temperature gradient across the insulation from the warm inside to the cold outside. At some point, part way through the insulation, it is cold enough for condensation to form. The principle is that you fit the vapour barrier on the inside to keep any water vapour on the warm side where it cannot condense and cause damp problems. You will almost ceratinly get condensation on the windows as they will be the coldest surface once you have insulated the shed. Ventilation is the answer to that.

MBQ

I used the 2" thick 4 by 8 ft expanded polystyrene, two layers thereof between the uprights of the frame and then covered with hardboard to prevent flammable contact with the insulation.

Background heat is by a greenhouse tubular heater and a dehumidifier which gets between 2 pints and 1 gallon a week.

Thank you all for your advice.

John