Rigid insulation OVER concrete slab?
My question. I have a cabin in the White Mts in NH with a concrete slab that sits on a ledge. No rigid insulation under the slab so it stays rather cool in summer. Occasionally we get real hot days and we get condensation on the wide pine floor that sits directly on top of the slab. I was thinking if I was to put down some rigid insulation under the pine floor and ABOVE the slab and tape the seams carefully, I would be able to place some plywood on top and perhap renail the wide pine boards down on the plywood. Hence I would have a themal barrier to keep the mosit air from the cold slab/floor.
I cannot get insulation UNDER the slab as it is obviously down now for better or worse. I realize this is not the BEST method but what am I missing? I currently am able to get rid of the moisture with a dehumidifier but I would like to fix this “permanently ” or at least the best I can without blowing up the house and starting over.
Does this approach make sense or am I our in left field? You comments are always welcome and appreciated. I tried to find something similar out there but didn’t have much luck so apologies if this has been answered previously. I keep getting hit with the SPAM blocker for some reason.
Thanks!
Michael
Replies
Extra flooring
That is a good suggestion. Perhaps a couple of 1/2" sheets, staggered seams, as opposed to a 3/4 " sheet? I have 9'6" ceilings so I would have plenty of head room . I could also lay the pine boards perpendicular to the plywood seams as well. Boards are about 10" wide (nice pine floors). Perhaps some adhesive between the 2 pw panels and let the plywood "float" from the insulation and concrete. I suppose I would need to leave 1/4-1/2 in for expansion at the edges? I would make sure it was taped and still had rigid insulation under the ply gaps to make sure the thermal barrier stayed intact.
Michael
I'd be concerned with condensation under the foam
The slab is porous, so as it sits on the slab, how does the moisture get out?
I might try dricor or something similar instead of the foam. http://www.dricore.com/en/eIndex.aspx
Why does the moisture need to get out?
The moisture (should there be any) will leave via the force of gravity. That is, unless there's some sort of hydraulic pressure acting that we don't know about. In which case its probably a very good idea to use xps foam that will act as a vapor barrier.
Also, I wouldn't worry one tiny bit about the foam being compressed. The wide plank in conjuction with some 3/4" ply will spread out any live load as to be insignificant to the foams ability to support.
If you want to be extra sure, you can get a bag of H clips that would gap the plywood edges and support eachother if a plan seam ended up on a plywood seam.
http://www.strongtie.com/products/connectors/PSCL-PSCA.asp
I like most of the suggestions.
I like all of the suggestions. I believe i will try and implement most of them. For a point of clarification, this mositure occurs only on real hot and humid days. It materializes on top of the surface of the floor that sits directly on the slab. If the water came up via hydraulic force or from underneath, I would have many more problems, especially after spring thaw and heavy rains. It only occurs during these HHH days. You can actually see a slight haze on the floor . If I place the XPS on the slab, tape and seal the seams and then 3/4 " ply on top with H clips, I think that would be the ticket. While currently the cool floors are actually comforting in the summer months (like air conditioning) it would be a good idea to isultae this floor. Wide pine floor planks stay drier and more stable and the temperature would be more even in the house.
As far as the mateial to place on the slab, what are the additional advantages of XPS vs. that regular rigid foam board ?
Great insight gentlement. My thanks to you all!!
Michael
XPS and Polyurathane foam panels are equally good for your application. Polyurathane has slightly more R value, but for your application I don't think it's as important
have you completed the project?
Hi Michael,
I found your post and I'm very curious if you've completed the project.
Did you end up going with 'floating' rigid over the slab, 'floating' plywood sheets connected with H-Clips over the rigid, and then the wood floors on top of the plywood?
Did the h-clips work well?
We are about to start a similar project, but we are converting an attached garage (without rigid insulation underneath) to a heated living space.
We'd love to float the rigid and flooring over the slab, but it seems that there would be issues with bending in the plywood and the flooring therefor being slightly wavy.
Which did you chose?
(XPS or Polyurathane foam panels?
Any direction from anyone would be appreciated!
THANKS SO MUCH!
Terah
(FYI: We have low ceiings)
TB
Is your slab flat? level?
Does it have pitch to a drain or to the garage door meant for draining water?
If flat, the foam with two different direction layers of plywood screwed and glued together with the joints staggered.......probably will work fine with either fastened or floating engineered floor. If there is slope or fall, then as the prior poster - user - recommends, furring that flattens out the floor, fastened to the slab is better.
In either case, fully tape a 5-6' pc. of visqueen to the floor for a week. If moisture appears between the slab and plastic, you have a moisture problem to think about. If dry, have at it. I would still lay down some 6 mill visqueen or heavier over the whole floor, overlap and tape all seams and run up the wall a bit-seal that as well-caulked to the wall.
I finished my garage into an office. Since the floor was sloped and because I wanted to insulate the floor I put down furring strips of varying thickness and insulated between them with foam. I think I ended up with foam varying from about 1/2 inch thick to 2-1/2 inches thick with osb on top. This worked out weel. The floor is not cold and I have had no problems.
Condensation on floor with temp below dew point
You have probably done something by now........
If not, cover the floor with 2 inch thick sheets of polystyrene or similar closed cell insulation, then lay one inch thick sheets of glued t&g OSB over the polystyrene.
This will give you a warm floor the surface of which will be at room air temperature, if you add underlay and carpet you will really notice the difference, warm feet.
It's probably too late but checkout Platon by Certainteed. More commonly used as foundation waterproofing the manufacture claims it can be used over basement floors. Then you add plywood. Checkout the site:http://www.certainteed.com/resources/Platon%20Flooring%20Brochure%2040-98-02D.pdf If anyone has used or does let me know how it works
Steve