Is it better to use PIR board under rafters for insulation to allow for easier removal and larger air gap, even if it means losing internal space?
Asked by Cátia Barros 
4 months ago
Last Updated: July 26, 2024
Hey there, DIYers! I’m new to the group so please be gentle. I’m getting ready for an insulation project and I have a question about using PIR board between or under rafters.
Originally, I was thinking of putting 50mm between the rafters (I know it’s not a lot for the R-value, but it’s a start and definitely better than what’s there now). However, I also want to be able to check the roof and rafters for leaks periodically.
So, if I only do under the board (under the rafter, really), it would be much easier to install, remove, and there would be a larger air gap. It all sounds pretty good to me. Besides losing some internal space, is there any reason why this might be a bad idea?
Thanks for any thoughts! 😁
In the photo, the house is incorrectly insulated in the attic – the floor, not the rafters, should be insulated if the attic is not heated.
Thank you, . It’s going to be a partially converted room in the roof, with flooring and usage, but not as the primary space, so we want to ensure some level of heat retention.
I believe the fonz may be under the influence..
Hey ! You didn’t mention where you’re located. The insulation needs in northern Wisconsin are different from those in Southern California. What exactly do you mean by an “almost room”? Are you talking about storage or a spare bedroom? Rooms that will be occupied need certain things that aren’t always found in an attic, such as windows for ventilation and emergency exits in case of a fire. Any foam in an occupied space must be covered with drywall, as most foams produce toxic smoke when burned. Make sure to check the construction of your attic floor structure, as the ceiling joists below may not be suitable for supporting a room on top. You don’t want those 1500 pounds of books suddenly shifting downstairs! Just something to think about.
Hey John, appreciate your input. The specific issues you mentioned have already been addressed. What I was wondering about is if there are any downsides to using overboard PIR insulation as opposed to between rafter PIR insulation, aside from the loss of space. Any insights on that would be welcome.