basement insulation - condensation behind the batt insulation

natalie · 01-30-2010, 09:42 PM

We have just placed rigid board insulation on the basement walls and built the 2x6 walls in front of the rigid boardsm, we then started putting the batt insulation between the 2x6 walls a few days ago. we are doing this work slowly so we haven't placed the vapour barrier yet. But we noticed that there is condensation between the batt insulation and the rigid board insulation. We are not sure if this is normal, will not happen after vapour barrier is placed or if we are going to have a mold issue in the future.

Mudball · 01-31-2010, 07:50 AM

Something doesnt sound right. If the foam board is installed according to this link...http://www.buildingscience.com/docum...ent-insulation
then there shouldnt be any moisture issues unless there is moisture coming in from another area.
Were the walls always dry prior to the foam board ?
How thick of foam board was installed ?

wnabcptrNH · 01-31-2010, 08:37 AM

Full thermal break includes taping the joints on the foam insulation. Many people are now spray foaming so you get a full thermal break. Be sure you tuck taped all the joints to confirm moisture will not come through on to the batt.

natalie · 01-31-2010, 11:19 AM

Thanks for the reply.
I reviewed the link and wanted to note that this is an existing home, built in the 1950's. We placed the rigid board from the concrete floor to the bottom of the ceiling joists, we could not place the rigid board to 'act as draft stop at the top of wall' as shown in the link drawing, as there was no room to place the rigid board into the header. I hope I'm using the correct terminology. We did place batting and then vapour barrier in the headers.
All joints are taped and the top of the rigid board was spray foamed. The walls were dry when we placed the rigid board, however the concrete walls were damp/cool to touch prior to the placement of the insulating boards.

The board is dry until we place the batt insulation, then condensation occurs.

Could it be that the spray foam on the top and bottom of the board is just not perfectly sealing the area. It seams like I should respray the top and bottom of the boards to make sure that they are completely sealed.