Is polyurethane insulation in the cavity the same as cavity wall insulation?

Andy From Sligo · 13-09-2022 03:30PM #1

Hello all .

Building Survey done on a house found:

"Construction: Walls: External walls are 300mm cavity wall construction (100mm cavity) with 50- 60mm polyurethane insulation"

just to double check this sounds like the cavity just has slabs of polystyrene of 50-60mm in the cavity and that the property has not actually 'cavity wall insulation' ? - IE those small polystyrene round balls that fill up the cavity ?

If it is just slabs of polystyrene can the house still accept cavity wall insulation where its pumped into the wall to make the house warmer, and will there be much of a difference if the house does get cavity wall insulation seeing as the house already has slabs of polystyrene insulation?

House was built in circa 2006

Thank you for your advice.

10-10-20 · 13-09-2022 03:35PM

That's this config, Andy:

The yellow board there is a polyurethane board which is (hopefully) held in place by the wall ties leaving 40 to 50mm of an air-gap.

I have similar in a 2002 house and I asked a question about getting pumped insulation about a year ago and got very good responses here. Let me find that.

10-10-20 · 13-09-2022 03:52PM

Actually the advice which I got was off a guy who pumps walls with beads and he mentioned that he doesn't recommend pumping walls where existing partial-fill insulation boards are present as there can be an issue where the beads are injected into or go behind the insulation (missing the gap) and causing the insulation boards to shift around, ending up with an inconsistent application of insulation in the wall.

If I recall correctly he recommended that closed-cell foam be used instead as I think he said that the pressures were lower and were less likely to knock the insulation out of place.

Perhaps the more knowledgeable here can correct me on the finer elements of this.

Andy From Sligo · 13-09-2022 03:59PM

https://www.boards.ie/discussion/comment/119613320#Comment_119613320

ah right thank you for all your info - yes i was wondering too if its possible with insulation already there because you still have to leave an air gap and how can you do that if the slabs of polystyrene are there and then you pump in the beads , all the air gap space would be taken up by insulation

10-10-20 · 13-09-2022 04:03PM

Yes, but that's the point of using closed-cell pumped insulation - it does away with the air-gap where existing convection is happening, and prevents the transfer of moisture through the new & existing insulation.

You just need to be sure that you're getting the right internal wall insulation treatment for your wall-type and I think that beaded is not the right application.

BryanF · 13-09-2022 10:22PM

https://www.boards.ie/discussion/comment/119613320#Comment_119613320

I’d avoid closed cell foam in old cavities.

10-10-20 · 14-09-2022 07:57AM

https://www.boards.ie/discussion/comment/119615181#Comment_119615181

Thanks for the correction - I recalled the conversation incorrectly.

Andy From Sligo · 14-09-2022 03:00PM

https://www.boards.ie/discussion/comment/119615181#Comment_119615181

so would a house built in 2006 be classed as 'old cavities' ?

BryanF · 17-09-2022 08:08AM

https://www.boards.ie/discussion/comment/119617983#Comment_119617983

Get 3quotes from polystyrene bonded bead installers - explain the cavity 60 of 100min filled with board and ask them will they’re manufacture stand over the installation, (joint warranty)

C. Eastwood · 17-09-2022 10:00PM

https://www.boards.ie/discussion/2058263213/is-polyurethane-insulation-in-the-cavity-the-same-as-cavity-wall-insulation

The existing 50 mm of polyurethane in the cavity with give the wall an approx U value of 0.55 watts/m2/degC/hr.

Adding the 45 mm fill of beads in to the residual cavity will reduce the U value to approx 0.3 watts/m2/degC/hr.

10-10-20 · 17-09-2022 10:20PM

https://www.boards.ie/discussion/comment/119632437#Comment_119632437

This calculator is great. http://www.vesma.com/tutorial/uvalue01/uvalue01.htmW

Here's a standard double-leaf wall config with a 100mm cavity with 50mm iso insulation:

With 50mm EPS (expanded polystyrene, probably the closest match) in the cavity.

Andy From Sligo · 17-09-2022 10:35PM

https://www.boards.ie/discussion/comment/119632437#Comment_119632437

Is that good then or bad? - (That whoosh is the sound of going over my head now!) 😟

Biker1 · 18-09-2022 09:14AM

https://www.boards.ie/discussion/comment/119632488#Comment_119632488

You really need to start with the correct calculations as your internal and external resistance and thermal conductivity do not comply with. ISO 6946. See image. Add in a thermal conductivity of 0.035w/mk for the bonded bead to get the after works u-value.

10-10-20 · 18-09-2022 09:26AM

https://www.boards.ie/discussion/comment/119632516#Comment_119632516

Pretty much any improvement in insulation is good, but you'll be making a ~30% improvement in the thermal resistance by pumping the cavity.

Calculating on the basis of the figures from Vesma: For a 10m2 wall and a 20 degree difference between the interior and exterior, 0.29 watts/m2/degC/hr would be 0.29*10*20 = 58w lost through the existing wall make-up per hour versus 42w per hour for an pumped wall.

10-10-20 · 18-09-2022 09:27AM

https://www.boards.ie/discussion/comment/119633127#Comment_119633127

The wall-ties? Correct, these aren't factored into that online tool.

C. Eastwood · 18-09-2022 09:48PM

https://www.boards.ie/discussion/comment/119632516#Comment_119632516

Thanks Andy,

Lowering the U value is very good.

Lowering the U value of the existing wall from a U value of approx 0.55 watts to approx 0.3 Watts with cavity fill means that you are reducing the heat loss by approx half of what it is at present.

Cavity fill may cause dampness if there are facing bricks on the outer face of the external wall, or if the external rendering (plaster) is not preventing ingress of rainwater in to the existing cavity.

Is polyurethane insulation in the cavity the same as cavity wall insulation?

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