Is Polyisocyanurate waterproof?


Old 11-11-01, 01:32 AM
Visiting Guest
Posts: n/a
I am laying a floating plywood floor that will be padded and carpeted over a cement sub-level basement floor. There doesn't "appear" to be a seepage problem, as the old, thin carpet that was directly on the cement didn't have any water damage.
I planned to place cutout strips of 1 inch Polyisocyanurate rigid foam insulation between the 2x4 sleeps AFTER laying a layer of 4 mil plastic vapor barrier over the entire concrete floor (I will lay the VB down first, then secure sleepers to floor, then insert insulation).
The specific insulation that I have available is called Celotex Super TUFF-R Polyisocyanurate foam inslation. It has an R value of 7.5. It also uses two three-ply poly/aluminum foil facers laminated to the core. One facer is colored black. The other facer is radiant barrier quality reflective foil.
However, the lable does not mention at all whether it is waterproof or moisture resistent, nor have I been able to find the information anywhere on the Net. I also had the option to buy a Polystyrene rigid foam as well but with a lower R value of 5. Can anyone confirm if this is still a good choice for the application I intend? Thanks in advance.

Sponsored Links
Old 11-11-01, 05:34 AM
Join Date: Aug 2000
Location: USA
Posts: 15,834
Received 0 Votes on 0 Posts
Celotex insulation

Where did they come up with such a big word? Go to You will find product descriptions and recommended applications. You can, of course, always email them for clarification.
Old 11-11-01, 12:47 PM
Visiting Guest
Posts: n/a
Thanks twelvepole, appreciate the reply. Yeah, I found the Celotex web site last night but even after reviewing the description and recommended uses, it still never explicitly states the waterproof or moisture resistance of it. However, the "Recommended Uses" does make it sound as though it is moisture resistant, just does not state it in those exact words. Thanks again.

Old 11-17-01, 07:23 AM
Join Date: Oct 2001
Location: USA
Posts: 963
Received 3 Votes on 3 Posts
As I understand it, in talking to manufacturer's reps over the years, polyisocyanurate is a type of urethane and will readily absorb moisture. When we use rigid insulation on the exterior of foundation walls on commercial buildings for perimeter insulation, we always use a molded polystyrene because it does not readily absorb moisture. It has an r-value of 5.0 per inch; the one most people are familiar with is the blue Styrofoam brand (not the white beadboard). From what you describe, though, I don't think you will have any problems with the iso. Incidently, at the risk of sounding like a "know it all", iso (and most other rigid insulation) have what is called "thermal drift". What that means is that the r-value is 7.5 at the time of manufacture, but after aging is around 7.0; the r-5 I gave you for polystyrene is an aged r-value.

If you're using 2x4 sleepers, why not use 1-1/2" insulation? hope I helped.

Old 11-19-01, 04:53 PM
Visiting Guest
Posts: n/a

The problem with using any type of foam on the floor is condensation and the mlod/mildew that will form.

See the info I gave Ben Fallensby, (next one up from you).

This sys is easier to install, is only about 1 1/4" thick, no moisture/ mold problems and the radiant barrier is 97% efficient as compared to about 20% for foam.

Thanks for considering my opinion.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

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