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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Condensation in metal roof‏ https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1471008969 Message started by colinj on Aug 12^th, 2016, 2:36pm

Title: Condensation in metal roof‏
Post by colinj on Aug 12^th, 2016, 2:36pm

Hi all,

I appreciate that the topic has been flogged to death across many forums and I apologise for resurrecting it again, but I have a specific query:

I'm building a workshop which will have a metal profile sheet roof. I really don't have the budget to stretch to Kingspan insulation or similar, but wondered what your views might be on installing a double skin roof with or without insulation material in between the skins?

The reason I ask is that I have loads of profile sheets and wondered about either

a) fixing sheets on the underside of the purlins
or
b) fixing a second layer of sheets on top of the first layer with insulation in between

Does anyone think that either of these would work in terms of combating condensation on the inside of the roof or would I be wasting my time?

Thank you in advance for any comments or alternative suggestions.

Title: Re: Condensation in metal roof‏
Post by woodsmith on Aug 13^th, 2016, 9:41am

Metal roofs are a magnet for condensation and can run with water when it's cold but unless you do an absolutely immaculate job putting another layer under the metal is likely to cause you more problems than it solves.

If you do want to put a second skin under the roof you would need to ensure that you still have ventilation under the metal roof whilst ensuring that absolutely no air can get past the sub roof as this would carry moist air which would condense in the roof void you have created.

There are firms which can spray an insulating foam over the underside of the metal which insulates and prevents condensation but it's not a cheap option.

Title: Re: Condensation in metal roof‏
Post by Lectrician on Aug 14^th, 2016, 4:55pm

All new build industrial units have a second set of purlins above the first skin, insulation between, then a second skin on top.

It is also common for older industrial units to have a second roof added in exactly the same way.

It does stop the condensation.

Title: Re: Condensation in metal roof‏
Post by colinj on Aug 17^th, 2016, 2:52pm

Thanks for the reply.

So, I'm thinking of putting a 2x1 (or similar) on top of the first skin and adding the insulation. Should i consider rockwool or maybe better with polystyrene sheeting for the insulation?

Title: Re: Condensation in metal roof‏
Post by CWatters on Aug 18^th, 2016, 12:28pm

The problem is caused by water vapour in the air coming into contact with the cold roof surface of the roof (or any cold surface) where it condenses. So the cure is to stop that happening by some combination of...

* Insulation and heating so the inner surface is warm and condensation doesn't occur.

* Vapour barrier to prevent water vapour getting through insulation to the cold side.

* Ventilation to extract the water vapour to the outside. In winter the air is frequently less humid outside than in.

Title: Re: Condensation in metal roof‏
Post by thescruff on Aug 19^th, 2016, 9:36am

Commercial building I've worked in have a layer of insulation sandwiched between two sheets (factory made).

Also been on jobs where they sprayed the walls with 50mm + of expanding foam, don't know but would have thought it could be used on roofs

Title: Re: Condensation in metal roof‏
Post by Natedog on Jan 5^th, 2017, 1:16pm

Possibly a bit late, but I had a prefab concrete garage put up a couple of months back, and that has a corrugated metal roof. The inside of the panels has been sprayed with some white stuff, similar looking to car body seam sealer, doesn't set. That's meant to keep the condensation down and so far it seems to be working