I'm in the UK and in a house built pre-1900. In there I have rock wool on top of old rock wool and vermin are finding this very inviting (found out after I bought the house).

Because of the smell and general sloppiness of the insulation I want to go rigid

  1. so I can gut the loft and find points of entry
  2. So it's harder to make bedding out of the insulation if they still get in and
  3. because I want to overboard for storage and potentially convert in the future to a master ensuite.

I've seen this post:

Can I add insulation to my attic and put down plywood so I can walk around more easily?

But it's more for the US market and UK regs tend to be different. Basically I want to adhere to UK building regs for minimum thickness of loft insulation but using rigid foam insulation between rafters and then overboard.

Trying to search online yields results that would make sense to someone familiar with regs and R values etc but I'm just a DIYer unfortunately.

  • What is "rigid insulation"? We don't know what's available where you are. Polystyrene? EPS? Polyiso? Fiber? It's just a matter of achieving the same R-value. You have about R-36 now at ~12 per 100mm.
    – isherwood
    Nov 1, 2021 at 12:56
  • OP would need about 7 inches of XPS insulating panels (R-5 per inch) to match the R-36 he currently has.
    – SteveSh
    Nov 1, 2021 at 13:22
  • Thanks @stevesh so about 175mm in XPS...will have to look in to the product. Do you know any good resources for rigid insulation and building regs...or is there a simple calculation that can be used to figure things out?
    – Scott
    Nov 1, 2021 at 13:49
  • @Scott - Can't help you with the building regs in the UK (I'm on the East Cost of the US). For resources, I would just type "foam board insulation", or "insulation types" into your favorite search engine. Here's a general resource of R values for various types of building & insulating materials: archtoolbox.com/materials-systems/thermal-moisture-protection/….
    – SteveSh
    Nov 1, 2021 at 14:05
  • 1
    Is the rockwool between the rafters or on the floor of the attic?
    – bobflux
    Nov 1, 2021 at 16:54

1 Answer 1


Rockwool has an R-value between 3.0 to 3.3 per inch of insulation, slightly better than fiberglass insulation which has an R-value of about 2.2 to 2.7 per inch of insulation.

Rigid Foam

There are three basic foam insulation board products on the market produced under several different manufacturer names. The basic types of foam board insulation include: expanded polystyrene, extruded polystyrene, and polyisocyanurate unfaced or foil faced.

Expanded polystyrene foam (EPS) is the cheapest and least used foam board product on the market. This product typically has an R value of 3.6 to 4.0 per inch of thickness. Expanded polystyrene insulation is similar to the foam used for packing “peanuts” and it’s typically used for insulated concrete forms also knows as ICF’s. It is also sometimes used on commercial buildings for roof and wall panel insulation which is typically sandwiched between light gauge metal.

Extruded polystyrene foam (XPS) also known as blue board or pink board comes in many different thicknesses and edge profiles. This insulation board is probably one of the most widely used foam board insulation products in the residential construction industry. XPS has an R value of 4.5 to 5.0 per inch of thickness.

This is the product that typically use to insulate basement walls. It’s reasonably priced, light weight and easy to use. This product is also used to insulate the outside of foundation walls and even under slabs.

Polyisocyanurate also known as polyiso is seen in all kinds of commercial building applications and more recently with residential building projects. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved. The foil facing also makes it very easy to seal with good quality foil faced tapes.

Cost = Polyiso is the most expensive of the foam board insulation products however it’s the highest R value.

Spread Foam

If locally available, the spread foam insulation is also worth considering. It comes two types "Open-Cell" and Closed-Cell.

Open-cell spray foam It has a lower R-value per inch than closed-cell foam and is vapor-permeable. The R-value is R-3.5 to R-3.6 per inch. It absorbs and holds water, this means that if used to make a conditioned, unvented attic in a cold climate, contractors should cover the interior face of the foam with a vapor retarder.

Closed-cell spray foam has a far better R-value per inch than the open-cell foam insulation. The R-value of closed-cell foam is R-6.5-7 per inch.

Contractors often use it to insulate under slabs, ceilings, walls, and roofs. It performs better than other insulation types. It provides an excellent air barrier, resists moisture, and retards vapor.

Closed-cell foam also adds to the structural strength of a ceiling, wall or roof because of its density and glue-like adherence. This makes it extremely useful for sealing air leaks at attic side-of-partition top plate and rim joists.



  • Wow, alot of info to digest but really worth the detail. Gives some idea of readings to look in to
    – Scott
    Nov 2, 2021 at 17:44
  • These are mainly excerpted from the two liked articles at the bottom of the text. I suggest checking the availability of each at your local, then read and compare (cost vs efficiency and applicability).
    – r13
    Nov 2, 2021 at 17:59

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