Sprayfoam

Jim1000

hello folks,

I am renovating an old bungalow. Rafters are 6 inches in new section which will be a dormer bedroom. They are 4 inches in old section which may be used for more than storage in the future.

I have gotten a few sprayfoam quotes. One contractor says vent cards are necessary which are 2 inches in depth. This would mean only 4 inches of insulation in the dormer room.
Another contractor says that really the cards are necessary only on the old black felt and as I have new breathable felt it's better to spray the rafters full for more insulation.

Not sure what to believe at this stage and wonder if anyone had a full rafter spray done and if it has any negatives.

Thank You

Jim1000

anyone know what is the standard insulation thickness of this foam?

embracingLife

I don't know anything about it. Maybe you could get a better answer if you post this to the Construction and Planning forum, people there maybe more about this topic etc

eamon11

i would say spray direct onto felt and you will have more insulation.

Jim1000

ok I will post on the construction forum. Thanks

BryanF

eamon11 wrote: »

i would say spray direct onto felt and you will have more insulation.

and more rot!

Do not spray directly on to the felt. You need to ventilate the roof space between insulation and felt.

Jim1000

Thanks Lads,

Bryan,

I dont know much but my limited logic follows as so...

The foam is breathable, the felt is breathable, so that will work out ok.
By putting in a pvc card, any moisture from below which can travel up through the foam will hit this card barrier and will take the easiest route which is through the timber.
Rot?
Perhaps with a vapour barrier on the underside of the insulation this can be minimized.
The 2nd problem I see with the cards is condensation on the underside in cold weather. This moisture, again, can only escape through the timbers.

Am I missing something here?

Many thanks for your valued response.

BryanF

Jim
check the data sheet for the breathable felt, generally in order for it to work effectively with insulation up against, it there should be double cross battens above - have you done this? Very few tradesmen seem to do this of their own initiative

Is this the basf stuff your going for ? Without more info, I would stick to the vent space requirement under the felt

Regarding the cards, they typically have gaps in them.

installers/ tradesmen are Generally very helpful, but Before you go any further get the foam data sheet and ring the product manufacturer as apposed to taking advice from an installer. They will set you straight, and in my experience often give conflicting advice to installers.

Separately A fully taped and sealed vapour barrier is essential in the inside.

Best of luck with your project