There are several approaches to insulating an agricultural building: underlaid boards, built-up systems, composite panels and spray foam.
With this technique dense styrofoam insulation boards are laid over the purlin and the cladding fixed over the top with long screws through the insulation.   If retro fitting the boards can fitted either between, or over the inside face of, the purlins and rails.
The boards themselves come in two flavours: tongue and groove or more commonly plain edged.   The edges of plain boards have to be supported with T-bar while T&G panels lock together by themselves.
Dow Corning make a T&G board suitable for agricultural use: ROOFMATE RL/X.   This is available in thicknesses varying from 25mm to 120mm and is a very fetching shade of blue.   The boards have no facing foils - the blue goes right through the material so that damage is not noticeable.   They are not cheap but very strong and will stand pressure washing - I would choose these if I were building for myself.   An alternative is "POLYFOAM AGRIBOARD" available in a lovely shell pink but otherwise very similiar. Pink or blue- you pays yer money and you takes yer choice but our environmental compliance unit would like you to know that Agriboard is blown with steam instead of CFC's.
Please note that although T&G boards lock together edge to edge you must check the maximum span for the desired thickness of insulation.   for example: with a metal roof with a 6' (1825mm) purlin spacing 25mm boards will not self support so for thin layers of insulation you might be better with the cheaper plain edged panels as you will need T bar anyway.
There is more choice when it comes to plain edge boards; Kingspan and Celotex make good products but there are others so shop around.   Mostly these boards have white or silver foil faces, white is obviously preferable but be aware that the foil is easily punctured and ripped when used on walls exposing a yellowing foam core, which while it doesn't affect its function does look terrible
Plain edged boards must be laid with T-bars to support their edges.   As you may have guessed this is a galvanised steel bar shaped like a capital "T", the upright is called the "stalk" and the horizontal the "table".   You need the correct size depending on the application, I mention this so you will understand when trying to buy them.
We have known people use the much cheaper expanded polystyrene products (you know - like polystyrene roof tiles), these are deigned to be installed and protected with plasterboards, plywood etc and are not suitable for this purpose because of their fragility.
The principal of this system is to lay an inner steel sheet with a white face, then a spacer bar, fill the void with fibreglass insulation and then fit a top sheet to the spacer bar.   The depth of the spacer bar determines the insulation level acheived
It is often used in industrial buildings and most of the cost lies in the top and bottom sheet and spacer, therefore it is more economical with deeper fills where higher U values are sought.
There is one big drawback to using these systems in agricultural application and that is R A T S, and mice, birds etc - probably even bloody badgers if the fill is deep enough.
King of insulation these have a white faced steel liner and a plastisol coated top sheet bonded to a dense foam core.   These are undoubtedly the best choice but unfortunately you have to be a king to afford them.
A foam material literally sprayed on the internal surface of the building - sheets, steel,purlins - the lot gets covered.   This is mostly of use for retro fitting to an old building as it has the beneficial side effect of glueing everything together, water and draught proofing as it goes.   The big downside is that it just doesn't look clean especially once it has started to yellow nicely and harbour a bit of dirt.
On a practical note you should consult an electrician before insulating surface mounted wiring as it may lead to overheating.
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