Thin wall insulation help...

[AndyB]

Hi, I wish to Insulate part of my outhouse.

Making a hole in the wall I see there is a 75mm between the plaster board and a layer of felt that is directly against feather edge boarding to the outside. Within the 75mm gap is rockwall fibre insulation.

I thought I will take out the rather useless insulation and put in 75mm sheets of Kingspan or something

Can anybody advise me what product would be the most effective, anything must be better than this.

Andy

[Justme]

Within the 75mm gap is rockwall fibre insulation.

I thought I will take out the rather useless insulation and put in 75mm sheets of Kingspan or something

Why? (IE why is it "rather useless" & why change it)

That sounds like a lot of work & cost for minimal gain.

What is the dif in values between the two products? I doubt at 75mm if the gain would be worth the cost / effort.

[AndyB]

Hi Justme, I thought that 75mm of rockwool would not be as good as 75mm of solid insulation. As will be replacing a window in that wall I thought it would be benificial but if you dont think it is worth it I will take your advise

Andy

[desperate]

Hi andy

according to a reference book I use the thermal conductivity in W/mK or gamma is 0.042 for mineral wool and 0.025 for Polyurethane board . Plaster board 0.16,

 Thermal resistance is calculated by dividing its thickness by its thermal conductivity.
U value is calculated by taking the reciprocal of the summed thermal resistances of the various components of the wall

So I think replacing your wool for PU board is gonna reduce the loss throuugh that part of the wall by maybe 25% ish roughly approx, sort of ............you get the picture, it is hard to be precise.

Hope that helps

Desperate

[AndyB]

Thanks for that Desperate, thats quite encouraging I think. Looking at the Kingspan site, http://www.insulateonline.com/index1.htm?products.htm~main
Do you thgink it would be worth getting 75mm Thermawall insulation? Or do all pu boards of a certain thickness have the same W/mK value.

[desperate]

Obviously the thicker the insulation the less heat you will lose, but I think maybe you misunderstood the unit of conductivity it is expressed as watts/meter thickness/degreeKelvin, so it is not dependant on thickness

Resistance is expressed as per thickness/conductivity

Desp

[JohnS]

If you go for Kingspan, make sure that it is a very tight fit against the studs.  Any air gap will significantly reduce the insulation.  The advantage of rockwool is that, if it is it slightly oversize, there are no air gaps.

[KLD]

I'd be a little worried about building a moisture trap in your wall. With the felt fixed directly to the timber stuts, any moisture that does get through the inside leaf of the wall, will be trapped.
You could put in say 40mm Kingspan between stuts, leaving a ventilated (!) gap towards the felt. And then a second layer across the stuts on the inside, either between cross battens or fixing the PB directly though into the stuts.

Klaus

[AndyB]

Hi KLD, are you would you be concerned about trapping moisture as the wall is or once I replace the insulation. The wall has been in its currect state for about 30 years and is perfectly dry.

[KLD]

Andy, That's difficult to predict. Moisture levels of the inside air could rise after you tackeled insulation and draftproofing. for example. And cross boarding is not a big deal, and you'd be free to add as much insulation in the second layer as you like ;-)

Klaus