polyurethane wall insulation

[Anonymous]

I was told today that using polyurethane insulant in a timber frame stud wall was a practice not recommended by the NHBC- condensation problems.
Is this true?

[David Olivier]

I don't think that's a problem but it can be impossible to cut slabs of foam to fit tightly. This gives rise to air circulation and makes the insulation less effective than it should be (especially given the cost of PU foam).

D.

[Nick Grant]

Pete

So is “Low Vapour Construction' the new black, sorry, 'breathing wall'??

Worth pointing out that the low vapour (resistance) construction is a safety net rather than something that will mop up any amount of wet from moist air howling through a leaky construction. I have heard an architect friend say that with low vapour resitance you don't have to get uptight about airtightness.

[David Olivier]

I've seen horrendous condensation in roofs with rockwool and I've read of walls in US & Canada that became so rotten that the window could be pushed out! It's not the specific material(s), it's usually the design of the system into which they fit that's at fault (designing for heat, air & vapour movement.) There was a lecture at the last AGM on condensation horrors in German roofs which were reasonably well-designed and built but had subtle faults in design or construction sequencing … suggests to me that your architect friend is far too casual about airtightness.

In short I've never used PU foam between the studs due to (1) the expense of PU (2) the expense of cutting it (3) the residual small air gaps where the cuts weren't perfect (will it insulate much better than a slab of 90 mm rockwool?). But it is used in tens of millions of dwellings in the USA as a continuous layer of sheathing outside the studs and plywood, and AFAIK they haven't observed condensation yet.

D.

[Anonymous]

It seems to me that any condensation problem is therefore caused by poor application of the internal air/ barrier. I thought that modern external vapour barriers, being vapour permeable, would allow internal areas to dry out?
I was proposing 200mm foil faced PU, in 140mm timber frame studs (60mm to prevent cold bridges) followed by a 'meaty' polythene air barrier, followed by a 100mm dense concrete block, wet plastered.
(externally, 50mm cavity then 150mm sandstone).
I'm also proposing 225mm PU in between the attic trusses.
I realize that detailing will be crucial! and I am looking forward to the aecb 'details' when they get published.

[David Olivier]

Dave

Do you own shares in Kingspan?! It can cost 4X more than e.g. rockwool cavity wall batts and 7X more than RW roll-batts (which may be used in timber-frame). Yet its thermal resistivity is barely 1.5 times higher than RW now that the ozone-damaging gases have been taken out.

If I was advising on this sort of construction I'd probably be looking to improve performance and save money at the same time. In a new building, masses of PU foam may not be the best way to go although on a timber-frame I agree that a layer of it outside the frame can dramatically improve the R-value and may be pretty cost-effective.

David.

[Anonymous]

Doesn't the foil faced PU prevent the external vapour barrier from working?

[David Olivier]

You might not use foil-faced PU or PIfoam outside the frame.

D.

[Anonymous]

I worked my u value out allowing 15% for timber in the framework. I get an average value of 0.12. I dont think you could get that using batts or wool.
The blockwork is for thermal storage and it will simply sit on the floor slab. Its not structural, and this way it is not connected 'thermally' to the foundations.
David. I don't understand your comment. I assume you mean that the PU can transmit vapour (if it is not foil faced) and that the external vapour barrier is placed on top of the insulant?

[David Olivier]

Like Alan I'm now not sure I follow the rationale behind a timber frame wall. If so my comment is obsolete. I've just re-read the relevant posting more fully and your proposal includes a layer of 100 mm dense block and a layer of 150 mm sandstone. Together these make a strong enough cavity wall to hold up a 3- to 4-storey building (or with a 150 mm inner leaf, some solid partitions & concrete intermediate floors, a 20 storey tower!)

It doesn't need a polyethylene membrane as plaster is an air barrier and vapour diffusing out usually condenses on the outer leaf which is damp anyway. Nor do mineral fibre batts in a correctly-designed wall need a 50 mm air gap.

You might like to read the Vales' book “The New Autonomous House” as it goes into masonry construction in more detail incl. their experience with several low-energy buildings & their arguments based on UK experience for & against masonry and timber-frame. Also I presume you've looked at http://www.passiv.de (Passive House site).

You don't usually have or want external vapour barriers in the UK climate as they'll normally cause interstitial condensation. Any membrane there, more commonly in a timber-frame wall is usually intended as a wind barrier to protect the insulation.

HTH.

David.

[Anonymous]

I,ve been debating your responses for several days now. I was convinced that I had devised the ideal construction,so i've been re-considering it.
All i'm doing really, is to build a timber frame house but with additional insulation and added thermal mass.
The hypocaust floor i'm adding is just for a bit of fun.
I can see the advantage of manufactured timber joists (OSB web) for walls and roofs, which would allow large amounts of insulation. Can such a constuction be built quickly and cheaply?
I,m going to visit the passiv house exhibition in Hannover, in May, so I hope to come back with all the answers.

[Author]

Posts