Hello - conservatory self builder in timber - possibly overambitious

[PeelsJohn]

I am still building my conservatory, using hardwood and glass, the project is likely to continue for some time due to lack of same.

[PeelsJohn]

My conservatory has a lean to roof of plywood and EPDM. I am at the stage of fitting the insulation into the roof, and while I can keep an air gap open at the lower side, it will be nigh on impossible to have an opening at the top side for air circulation over the insulation.

If I don't have air flow over it will I be better to have the insulation tight up to the plywood skin?

I feel that with an air gap the air is going to fluctuate in temperature significantly, and there is potential for 'breathing' in plenty of hot moist air and condensing it overnight.

If I have minimal air gap then I will have minimal moisture in there.

I need some advice from someone with some experience of these matters, as all the advice seems to just say leave an air space and ventilate.

[Temp]

I suspect it might be too late but...

 

If you put the insulation above the rafters (rather than between them) it's called a "warm roof construction" and you don't need a 50mm ventilated gap.

 

Otherwise..

 

a) There are ways to vent where the roof meets the wall..   

 

 

OR

b) You can run the rafters (or 50mm battens above the rafters) side to side so you only need vents at the sides of the conservatory.

 

 

 

[PeelsJohn]

Way too late! Any way that wouldn't work as the roof skin is an essential part of the structure's rigidity. How harmful would it be to have the insulation up to the roof skin along side the rafters? Or would it be better to have the air space and some, but less than recommended, ventilation.

[Temp]

1 hour ago, PeelsJohn said:

How harmful would it be to have the insulation up to the roof skin along side the rafters? Or would it be better to have the air space and some, but less than recommended, ventilation.

 

I think some is better than none but unventilated cold roof construction is known to be a risk for condensation and rot. 

 

It might help to fit a vapour barrier below the insulation but you need to make a good job of sealing it to be effective. No big holes in it for downlighters.

 

Sorry if you know this stuff..

 

https://ww3.rics.org/uk/en/journals/built-environment-journal/unventilated-cold-roofs.html

 

Quote

 

A cold roof is one in which the insulation is placed between the roof joists or below the deck. Air and water vapour will enter the roof construction even if you try to prevent this, and as the air cools the amount of moisture that it can support is reduced, potentially leading to 100 per cent relative humidity – or saturation point –and the resultant condensation.

 

By ventilating the roof space, moisture can be removed before it condenses, or be allowed to dry before harm is caused. A good air- and vapour-control layer will reduce the passage of vapour into the roof; but its effectiveness will depend entirely on good-quality work and meticulous attention to detail.

 

Although a ventilated cold roof can theoretically be designed to avoid condensation this is a high-risk strategy, and such a design ideally needs to be avoided. Equally, an unventilated cold roof is simply asking for trouble. BS 5250: 2002 Code of practice for control of condensation in buildings provides guidance and recommendations; although a later version of this code has been produced, this version is referred to in Approved Document B to the Building Regulations.

 

[Temp]

One option might be to convert it to a warm roof construction by adding a thick layer of insulation on top and then a new OSB and waterproof layer but that might have other issues.