Insulating workshop roof - help / advice needed

Hi
I would like some advice on a roof i am currently insulating

I have a pitched roof barn/workshop which i am trying to insulate
The roof is constructed with 6" deep rafters , the rafters are then boarded externally with 18mm plywood which is covered with Tyvek Supro breather membrane and then directly covered with plastic slates onto the tyvek with fixings into the plywood.

i initially decided to fill the voids between rafters with 140mm celotex and then board the inside out with 12mm plywood

The barn is used as non residential and purely for storage but i decided to insulate to keep it very dry so i could store books vehicles and machinery etc and also occasionally working in there without having to heat it significantly. my plan is to have a small dessicant dehumidifer running in there just to keep the atmosphere dry for papers / fabrics etc..

I am looking at whether i will have any condensation issues

Do i need to add a vapour barrier - ie polythene sheet before fitting the internal plywood sheeting ? or would taping the joists with some form of tape to bridge the celotex over the joists do the same. Or could this trap moisture into the roof timbers and boarding

i am presuming any moisture will permeate outwards through the tyvek supro and out through the very small gaps between the slates.

My only other option is to extend the rafters down by 50 mm and install the 140mm celotex with a 50 mm gap between the top of the celotex and the underside of the outer ply and then ventilate through the soffitts and the ridge.
This cold roof ventilation option is obviously more involved with more work and i would prefer not to go this route - not sure if introducing moist air in this void in the winter months will give me other problems.

I understand that my original proposal is not technically a warm roof as i dont have insulation over the rafters and under the exterior boarding.

just trying to assess any condensation risk and minimise any roof timber rot and also maximise the insulation i can achieve.

I would like to add minimal heating at a later date for occasional working in the barn so this needs to be taken into consideration

The barn is a completely open area - vaulted ceiling with an open mezzanine floor at one end -

I would appreciate any advice from roofing / building experts on here.

thank you

 

If you substitute the plywood with a breather membrane, that would satisfy all your requirements, as described!...pinenot

 

Which plywood are you referring to external or internal ? And what type of breather membrane ? I already have a tyvek supro breather membrane under my slates on top of the external plywood.

 

Internal, and Tyvek breather would be fine, fixed with copper/SS staples to underside of rafters. However if your rafter bottoms are not perfectly aligned it will show, but you could put in a fold/pleat at each rafter, giving a pattern if you feel like it...pinenot

 

I am trying to understand - you are saying i should put in a second tyvek layer on the inside of my cellotex insulation and rafters ?

Which way round should this tyvek be and what will be its purpose ?

Can i then fit the internal ply ceiling over the top ?

I am trying to eliminate the risk of condensation and mould/rot forming on the rafters and on the inside face of my external ply ?

please explain

 

As I first stated, substitute i.e. replace the internal ply with the breather membrane I can only assume the ply was originally intended to hold the soft insulation in place, both netting and breather membrane will achieve this, without forming a condensation trap due to their openness (face down for Tyvek) I suggested the breather membrane because of aesthetics, either that or netting would work well...pinenot

 

The internal surface needs to be faced with plywood to enable painting and to enable seeing a surface to the vaulted ceiling. Presume from this I will have to lower my Celotex and vent each rafter run from eave to ridge to form a cold roof ? That is venting above the Celotex and below the external plywood. If rafters are spaced between 300 and 400 mm apart how many 70mm circular soffit vents would I need between each rafter ? This is also a lot of work removing ridge tiles and fitting vents. Cutting back external ply to provide an exit at the ridge and drilling lots of holes to fit eaves vents. Also I will need I presume to sideways drill above and below the Velux windows I have to vent those to the rafter gaps at each side ?

 

I'm somewhat confused,

1: is this a pitched or vaulted ceiling?
2: why is there a need to paint the ceiling??

 

Please excuse the confusion. This is a completely open barn with no horizontal ceilings. It has a 30 degree pitched roof meeting at a central ridge. Sorry I thought that an internal open sloped ceiling meant vaulted ? I want to board it out on the inside after insulating it and then paint the walls and the 2 sloped ceilings

 

I know I'm asking a lot but could you do a sketch with dims.

 

Hi

Can anyone else help with this please - all the information is in my original post with material specifications i am using

I need to knpow if i need to adopt a vented carthedral roof design or unvented or any other design ?

thanks

 

My workshop has a corrugated tin roof.
I insulated it below with 150mm loft insulation. Spray glue was used to make it stick between the timber purloins which are spaced at about 1m.

Then a 30mm kingspan insulating sheet with a white ceiling finish was nailed on below that using white polytops.
I would expect condensation may be forming under the tin after a cold night. But the loft insulation should absorb it and evaporate off later.

Large scale production chicken houses have an under plastic sheet fitted first below the 250mm deep purloins. 250mm of loft insulation is then laid. Then a felt membrane to cater for condensation drips. Then the tin corrugated roof.

 

100mm celotex with 40mm airgap, 500 guage visqueen vcl followed by the ply.

Soffit and ridge vents.

Either that full fill between the rafters including exp foam any small gaps, tape then vcl and sheet...no vents required.

 

Hi

Thanks - i have 2 options as i already have the 140mm cellotex

1. Extend my rafters down 50mm to make them 200 deep and fit the celotex between to the lower edge making a vent above approx 50mm - then install soffitt and ridge vents - then on the inside seal joints and maybe poly sheet prior to boarding out with 12mm ply - Is the poly necessary ?

2. Fit the 140mm celotex between rafters and seal joints and fit poly sheet prior to ply lining -

Which option will ensure a rot free roof ?

I presume the option 2 is more critical to sealing

 

Option 2 in your case is the most practical and the idea being to fully fill any voids with tight fitting insulation hard against your 18mm ply.
The visqueen vcl being critical to arrest the passage of moisture to the underside of your roofing ply.

That under the circumstances would be my prefered method.