Attic insulation – R-21 and radiant barrier
Hi I know this topic has been asked a lot but I was hoping to get some info I could not really find in other posts.
I live in Queens NY – climate zone 4, I have a tall pitched non-vented attic. The attic is really a 4th floor with a smaller crawl space attic on top and closed un-vented spaces on the side these spaces get really hot and heating up the room. BTW I have no duct work in the un-vented attic spaces but we do use it for storage. The house is from 1945 – used to have a stone roof but the previous owner replaces it with a shingle roof maybe 8 years ago (only the shingles and roof paper not the sheathing were replaced) and I am in the process of redoing the insulation, the roof joists are 2×6 so I am using r-21 faced batts, not wanting to go with spray foam for various other reasons
My issue is the house is in direct sunlight all day and gets really hot just from the radiant heat. I wanted to use a radiant barrier since the sun is so strong but since its an un-vented roof I am trying to figure the best route. I tested the following scenarios today and these were my results
Outdoor temps today got up to 82 degrees, these were the inside temps:
1 – bare underside of roof – roof got up to 107 degrees !!!
2 – old foil faced R7 insulation – 88 degrees
3 – new R-21 kraft insulation – 83/84 degrees
4 – New R-21 with 1/2″ air gap and Radiant barrier – 88 degrees ??
5 – Radiant barrier alone – with 6″ air gap – 91 degrees
As you can see the Radiant barrier worked – by itself, it didnt help when put after the R-21 (even though I left a gap.
What i am proposing and think would give the best effect is to install the radiant barrier first – with 3/4″ gap on the underside of the roof, then install the r-21 faced insulation this way I am insulation 91 degrees instead of 107 degrees. My problem/concern is that the air will be trapped in that 3/4″ air gap between the radiant barrier and the roof sheathing (actual wood slats not osb board) I am concerned that the super hot air will cause damage to the roof as it has no where to go.
Interesting to note – the old R7 foil faced insulation was stapled so there was a 2″ air gap behind it but I really dont understand why since there was no place for that air to go.
Any thoughts and info would be greatly appreciated.