DIY Doctor UK DIY Forums

Hi all,

Hoping for some advice on what offers the best insulation properties out odf Celotex PIR Insulation Board vs Closed CEll spray Foam Insulation.

I need to insulate the ceiling in my basement as currently there is a lot of condensation building up, which is in turn causing mould growth etc.The ceiling actually supports my driveway above and is of block and beam construction - therefore no need to worry about movement etc.

The room also benefits from a constant stream of Positive Input Ventilation so is well ventilated too.

I guess I'm just after what offers the best properties to insulate as I'm getting a bit confused with what will offer me the best R-Values.

The closed Cell spray I've been looking at will offer an R Value of 5.5 when sprayed at 1 inch of thickness, but I realise that this is more of an American unit of measurement. The Celotex 75mm board thickness for example offers an R Value of 3.40m2K/W. Is the first measurement an imperial measurement and the second one a metric equivalent? I guess I'm just after what will do a better job of insulating the cold space!

Any thoughts from people more experienced than I?

Thanks a lot

Nick

Hi Nick,

Hope are you doing well. The R values you have provided cannot be compared without converting the units from inches to mm.

If we convert the units we get the following:
- Closed Cell Spray Foam 1 Inch = 25.4mm > R-Value of 0.65m2K/W
- Celotex Foam Board = 75mm > R-Value of 3.4m2K/W

As you can see the numbers are very different and the Celotex insulates much better becuase it is almost 3x times thicker than the foam spray you are comparing it to.

If we compared the same thicknesses we would get the following:
- Closed Cell Spray Foam = 75mm > R-Value of 1.9m2K/W
- Celotex Foam Board = 75mm > R-Value of 3.4m2K/W

So you can see the foam board still performs better. I would actually advise installing foam boards as spray foam can be very difficult to remove if you ever need to.

One major point to consider with foam boards is to make sure the gaps between each foam board is sealed and airtight...It is best to use air tightness or insulation tape around all the edges to ensure the ceiling is completed sealed. This will stop any cold spots forming between the individual foam boards.

Let me know if you need any additional help.