Spray foam under pitch roof

[IanMcP]

Hey

 

I have 4 mono pitch roofs. 2 with loft , 2 will be up to the rafters. 
 

trying to find a detail for the make up to allow spray foam - for insulation and air tight at the wall junction. House is ICF so air tight paramount. Rafters are web joists. 18mm deck above with green coat standing seam. 
 

looking at Isothane but they don’t supply and detail just tell me the depth needed for u value. 
 

does anybody have drawings to share please ? 
 

thanks

[JohnMo]

Mine is as follows

 

Pitched roof

Slate

Breather membrane

22mm Sarking boards

256mm posi rafters - under drawn with 100mm battening, full filled with spray foam.

VCL airtighness layer, double side tape and stapled to battens, fully taped and glued to wall

50mm service battens

Plasterboard

 

12 degree roof

Same except, mechanically fixed EPMD membrane with fleece below, breather membrane and 18mm plywood - everything else as above.

[Temp]

Sorry if you already know this but..

 

Right now there is an issue with mortgage lenders refusing mortgages on houses with spray foam in the roof. Its not clear if they have a blanket ban or if they get their surveyor to check it has/hasnt been correctly installed. 

 

https://www.mortgagesolutions.co.uk/news/2021/12/21/rpsa-warns-250000-homes-could-be-unmortgageable-due-to-spray-foam-insulation/

 

Quote

According to an investigation by Residential Property Surveyors Association (RPSA) and Property Care Association (PCA), said that it had “failed to identify any circumstances” where a roof with this insulation could be given a “clean bill of health”.

 

I would think it worth getting an independent nterstitial condensation risk report done on the proposed design just in case its an issue in the future.

 

[saveasteading]

18 minutes ago, Temp said:

refusing mortgages on houses with spray foam

With good reason. Not a good idea. Try finding a recognised builder who does it, and wonder why they don't.

[IanMcP]

Wasn’t aware no. Thank you. 
 

so spray foam is a no then… 

was exploring it for best air tight seal

 

Suggestions welcome 

[JohnMo]

The issue with spray foam, is when it's applied to roof in the incorrect manner.  Ie normal cold roof space, then spraying in the back of the roof tiles, closing all the required ventilation.

 

Mine was applied by a recognised builder.

 

But what ever system you go for, you need to an interstitial condensation analysis.

 

[IanMcP]

Any recommendations ?

[JohnMo]

Whatever the insulation you use you will be installing a Vapour control air tightness layer below it. This is draped on to the wall internally and glued or taped to the wall..  That is is your airtightness sorted.  Then use whatever insulation to the required U value.

[Temp]

8 hours ago, JohnMo said:

The issue with spray foam, is when it's applied to roof in the incorrect manner.  Ie normal cold roof space, then spraying in the back of the roof tiles, closing all the required ventilation.

 

Also spraying onto the underside of non vapour permeable membranes.

 

[JohnMo]

Basically doing a bodge job, while demanding money to do so.

 

Interesting the article refenced specifically states polyurethane foam.  The foam mentioned at the start of the thread is a polyurethane, while something like Icynene isn't.  Its cater oil and water based that open cell and quite soft and flexible compared to polyurethane foam, which is quite rigid.

[SteamyTea]

On 25/08/2022 at 16:32, JohnMo said:

The foam mentioned at the start of the thread is a polyurethane, while something like Icynene isn't.  Its cater oil and water based that open cell and quite soft and flexible compared to polyurethane foam, which is quite rigid.

My sofa cushions are polyurethane foam, they are not rigid.

The dashboard in many cars are polyurethane and is semi rigid, and integral skin.

Water is what makes most PUs foam (blowing agent), melamine is added for fire performance.

PUs have been made from plant based polyol for decades, but most is mineral oil based. 

[JohnMo]

But I wasn't talking about your sofa or your dashboard I was specifically talking about spray foam insulation.  Which isn't seat foam.