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DIY Forum >> DIY - Building Questions >> Impermeable vs permeable membranes in construction
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Message started by housecat on Aug 12th, 2018, 9:01pm

Title: Impermeable vs permeable membranes in construction
Post by housecat on Aug 12th, 2018, 9:01pm

We all know that most frame houses these days are built with interiors which are fully lined with poly plastic in order to create a house that is as close as possible to being completely non-permeable by outdoor air. The reason, as far as I know, is for heat retention. This system necessitates some more consideration given to ventilation to evacuate moist air, which will otherwise remain trapped in the building, and if you have a wood-stove it will need outdoor air piped to it, lest it burn all the indoor oxygen in the non-permeable house.

The last building I made I followed the advice of a builder who told me to line the interior with the same material that is typically used on the exterior- 15 or 30lb tar paper. It comes in 3' rolls and is highly permeable- air can flow through the seams and through the walls from outdoor to in and vice versa. The result is a draftier building but because it is permeable, venting is unnecessary, and moulds and mildews will never become a problem as long as the roof holds.

I have been satisfied with the results of using tar paper instead of poly plastic so far but almost nobody will recommend it because it will result in higher heating bills in winter.  I personally like it because it seems like there is less that can go wrong in regards to trapped moisture, and as long as my wood stove is fed in winter, the house is warm and dry year-round.

What are your thoughts on poly plastic vs tar paper or other permeable membranes? Do you think I ought to make the switch to modern building methods? Am I a complete retard? I am curious and still experimenting and learning new methods and principles. I would like to hear more perspectives on this before I start framing and insulating my newest building.

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Edit: https://www.greenbuilt.org/should-we-eliminate-the-term-breathable-from-our-discussion-of-walls-and-buildings/ I found this which sort of settles the debate for me... "Build tight insulate right" seems to be the prevailing opinion.


from https://www.eco-home-essentials.co.uk/positive-pressure-ventilation.html:


"Prior to 2000 older properties didn’t tend to be affected by condensation to the same extent as today. Mainly because they were pretty leaky. By which I mean they weren’t air tight and allowed draughts to get in around windows and doors etc. Plus they wouldn’t have been well insulated.

In other words the leaky fabric of the building did the job of the ventilation system.

Today however, we all want out homes as air tight and well insulated as possible. So we seal the gaps around our window frames and fill the cavities in our walls and our roof-spaces with insulation. Which all helps reduce draughts and maintain that precious heat.

But it also means the moist air in our homes doesn’t get to escape and causes condensation which in turn leads to black mould."

Title: Re: Impermeable vs permeable membranes in construction
Post by londonman on Aug 15th, 2018, 5:48pm

But a properly designed modern house will have a heat recovery system that ventilates the house.

Title: Re: Impermeable vs permeable membranes in construction
Post by CWatters on Aug 16th, 2018, 5:13pm


housecat wrote on Aug 12th, 2018, 9:01pm:
We all know that most frame houses these days are built with interiors which are fully lined with poly plastic in order to create a house that is as close as possible to being completely non-permeable by outdoor air. The reason, as far as I know, is for heat retention.


My understanding is that this vapour barrier is designed to stop warm humid air inside the house getting through the insulation and condensing on the structural timber frame causing it to rot. This is usually called interstitial condensation.

As I recall the problem was once raised on the BBC program "That's Life" in the 1970s when modern timber framed houses first started to be built in larger numbers in the UK. They filmed houses being built with damaged/ripped vapour barriers and black mould forming on the insulation.  

There are alternatives to a vapour barrier that involve using materials with different permeability. eg Less permeable materials are used on the inside and more permeable on the outside. However I think software is needed to calculate if interstitial condensation is a risk.

If not using a vapour barrier Building Control might want to see the results of an interstitial condensation risk analysis. Do they know you aren't using a vapour barrier?




Title: Re: Impermeable vs permeable membranes in construction
Post by Lectrician on Aug 18th, 2018, 9:08am

Foil backed plasterboard is often used instead of the vapour barrier, or in addition to it.  As I understand, it is to prevent the moisture from getting into the wall.

Title: Re: Impermeable vs permeable membranes in construction
Post by housecat on Aug 29th, 2018, 5:17pm

Thanks for the reply's everyone  ;D.  I will be installing a heat exchanger on the newest building. Also I thought I should mention that the building inspector is unaware that I used tar paper instead of poly plastic.

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