Attic Insulation Moisture Concern

Old 01-14-21, 10:05 AM
Thread Starter
Join Date: Oct 2008
Posts: 35
Received 0 Upvotes on 0 Posts
Attic Insulation Moisture Concern

I live in the northeast and have a Cape Cod style home with a walk in attic that you need to access through one of my bedrooms. This room is always either really hot in the summer or cold in the winter. The attic itself is not insulated except for the wall that is against the bedroom and the floor. There also isn't any sheathing to cover that insulation on the wall so I have lots of air just freely flowing from near the molding in the room. I wanted to seal around all the wall joints and then install rigid foam insulation sheet to cover the fiberglass batts to give it a better R value. Will I have a moisture issue if I put up rigid foamboard insulation on the attic side? Should I just put up some wood sheathing? If no issue with foamboard what type should i use?

Old 01-14-21, 10:54 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,009
Received 1,899 Upvotes on 1,705 Posts
Since its an unfinished walk in attic if you put up foam I would assume it needs to be fire rated. Such as Thermax... Foil side toward the attic... Tape all seams with foil tape. Are you saying the floor is not insulated either? I doubt foam would cause any moisture problems since it can dry to the inside. But you would want to be sure you airseal all edges and seams since its the air exchange that results in most moisture problems. 2" would be best, 1 1/2" minimum I'd say.
Old 01-15-21, 08:07 AM
Thread Starter
Join Date: Oct 2008
Posts: 35
Received 0 Upvotes on 0 Posts
The floor of the attic is insulated since it is over the office room below. I was just worried that if the foam couldn't breathe and with the insulation paper on the otherwise it would cause moisture build up.
Old 01-21-21, 11:37 AM
Join Date: Apr 2014
Location: us
Posts: 1,349
Received 105 Upvotes on 99 Posts
Use Attic Air Space as Insulator

I seal from room interior all electric boxes, AC louvers, wall cracks and whatever. Insulation is in room ceilings, not roof.

If any moisture gets into attic want it to dissipate. Generally there are many places attic air exchanges with outside.

In winter even seal off roof louvers to create attic space with air temperature between in and outside.

In winter when room is 72F, outside 0F, attic is 40F, Consider attic air an insulating space.

Room ceiling heat BTU loss to attic is Square foot of ceilings x difference in temperature
500 sq/ft ceiling X 40 F delta T = 20,000 btu loss
500 sq/ft ceiling x 72 F delta T = 36,000 btu loss

Last edited by doughess; 01-21-21 at 01:08 PM.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: