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Tell me what you think about the following construction technique. Pour slab on grade – float smooth. Glue rigid insulation on top of that, tape seams. Then vapor barrier, then 3/4 plywood subfloor. Bearing walls would be brought down to concrete.
I’m sure this can work. In fact, I don’t even think it’s an original idea. My biggest question is: how would you fasten the subfloor? Also what would you spec for the insulation? It not only has to insulate, but also support the floor load.
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Replies
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how about this.. slab .. exterior walls & BEARING walls..
then the 6 mil vapor barrier..
then (2 lb/cf )EPS 2 inch to 4 inch Performguard (treated for vermin).. you decide.. no glue..
then one layer of 1/2 cdx.. and a 2d layer of cdx glued and ring-nailed (stagger joints ) then a 3d layer of cdx laid the opposite direction...glued and ring nailed..
set your partitions right on this and your finish floor..it's a 3-layer floating sub-floor...
b but hey, whadda i no
*That seems like it would work. Why no glue on the EPS? I was thinking a squirt would keep the stuff from bumping around.I was envisioning fastening all the way through the EPS into the concrete. That's either a long drill to go into concrete, or the Hilti-hit of death. Is there any reasonable way to fasten down to concrete in this situation? It would save a lot of material. But could you count on the subfloor to be level and remain level? I think your three-layer floating floor would maybe be more likeley to stay true. ??
*Why? Just curious, what is it you are wanting to do?Why not put your foam under the slab? What kind of plumbing arrangements are you picturing in this set up? Going to have a step up at all the entrance doorways? Joe H
*Wow Mike, thats a lot of CDX. You could frame the floor and sheath it with 3/4 T@G and insulate cheaper then that and provide better air flow beneath the floor. I would just go with under the slab insulation. Yr 001, The problem with your way is the corners and edges of your plywood would push down into the rigid board. What type of floor will you be installing for finished product?
*ron..he asked ...i answered..actually did this about twenty years ago in a slab on grade house...except there was no foam.. just the TWO layers of plywood...then sheet vinyl...for a clubhouse.....this was on a bad slab....i'm curious as to why the insul doesn't go under the slab myself... but if you wanted to put a floor down on rigid insul.. that would work.. especially with a high density rigid like ...styro-sm... foamulR.. or 2 lb/cf EPS....
*I know under slab is typical. But you also have a thermal break at the slab perimeter. It just doesn't seem like it should be that difficult, so why not? But maybe it really is a pain... :(The structure I would consider doing this for would be a residential outbuilding I'd like to build for myself. A studio, basically, for me and my wife.Another idea I've been considering would be to have a concrete floor. So concrete slab, rigid insulation, then 2" (?) more concrete.Mike, you said when you did this before you used /two/ layers of plywood. Now you're suggesting /three/. Does that mean that you found two insufficient for some reason?Plumbing: good water in, bad water out... ;)The doorway detail is a little odd, huh? I have to think about that a bit. You never really walk in off grade anyway. Just a bit of fussy detailing, I think.Why? I guess I'm trying to figure out why not. Because I want to and I can... ;)
*no .. two was ok...but doesn't comply with ceramic tile guidelines..suppose you wanted to turn a garage into a living space.. and the slab was level but uninsulated...if you are starting from scratch.. i'd put the insul under the slab...you can do a good edge detail....
*Point that you boys are forgetting is that if you put the VB on the top of the slab as opposed to under it, you may not like the stink from the dampness over the concrete, afterwards.First layer of VB belongs only one place...under the slab.Gabe
*gabe.. i humbly disagree... if the vapor cannot move..there will be no condensation... no moisture..no stink...even if the RH below the vapor barrier is 100%.. unless the vapor barrier is passed there will be no accumulation of moisture...it will be in a state of equilibrium...the stink is a symptom of bacterial growth... it needs 3 legs... moisture.. air.. temp..it only has one leg...moisture...b but i been wrong before.. maybe i will be again
*Mike,In a laboratory maybe but in real life and this is real life the VB over the concrete would be intermitent as a result of nail or screw punctures, gaps, open seams, wear spots, not to mention the obvious, the perimeter which is wide open.Couldn'd begin to tell you about the number of times I openned up a floor in a finished basement that stunk to high heaven because of plastic covering over a concrete floor that wasn't installed OVER a good VB.It normally has the moisture as well as the temperature so the missing leg, as you put it is in the installation.Mike, I know that you would have experienced more than one musty damp basement so you know what's possible and likely.My position remains the same.Well compacted base with good drainage, a quality poly VB, a sand buffer, a good concrete slab which is allowed to dry properly, and then and only then do you lay down a rigid insulation/sleepers cover with plywood and whatever covering you want afterwards.This makes for a dry, comfortable and healthy living space.Gabe
*The vapor barrier issue is interesting. Forget the slab for a minute and think about walls. In a cold climate, the VB is installed on the interior. In a warm climate, just the opposite. It's purpose, in either scenario, is to prevent the water vapor on the warm side from infiltrating the cold side and condensing.A no-no is to put a VB on /both/ the inside /and/ the outside. This gives moisture in the wall cavity nowhere to go. At least one side of the cavity should breath.A slab, though, is different. Here vapor remains a concern, for the same reasons. But in addition, you have to contend with /water/. (Wouldn't the poly under the slab, in a certain sense, more properly be called a water barrier?)Which source of moisture presents the greatest threat? Water from under the slab, or moisture condensing out of the warm interior air onto the slab? I think both.Remember though, that the rigid insulation itself can be considered a barrier, as long as it's taped. So I'd agree with Gabe that the vapor/water barrier belongs under the slab. I'd also tape the rigid insulation. Unlike with a wall, I don't think that the double interior/exterior barrier is a problem, because you're only sandwiching concrete and sand, not something that will rot.Gabe, would you put sleepers and plywood under a finished concrete floor? What would you recommend for the spacing on the sleepers - typical 16"? Or closer to avoid putting too much pressure on the rigid insulation?
*The other alternative, of course, is to put the insulation under the slab, and the vapor barrier on top of that. That solves both the interior warm air moisture and ground water moisture problem at the same time. Maybe that's why this is the typical detail. So maybe I'll just have to live with the fact that I won't feel comfortable if I try to sleep on the floor next to the exterior wall when it's cold outside. Not the biggest compromise I've ever made in my life.Even though we're talking about concrete and sand, I'm not very comfortable with my own suggestion that it's o.k. to trap moisture there...
*yrp.... like i said before.. there is no reason you should have a greater edge loss ... there are several ways to detail this...you're in charge.. work it out...
*I think you meant "put sleepers and plywood OVER a finished concrete floor".Personally? No. Assuming that a VB was installed under the slab.....I would simply install the higher density (1")rigid insulation over the concrete floor, nice and snug then cover with a layer of 5/8 T&G plywood. Glue the edges and stagger the sheets. Then reverse the pattern and cover with 3/8" underlay, glued and screwed to the 5/8".In effect create a wall to wall waffer.No thermal breaks.Gabe
*I meant sleepers and plywood under the finished concrete floor. Slab, rigid insulation, sleepers, plywood, finished concrete floor. Or skip the sleepers and plywood for this approach?How about painting a water barrier on the slab? I don't know what product, specifically, would work for this. But if you can paint the inside of your basement to stop it from leaking, surely that kind of stuff would work as a vapor barrier. (?)Would you expect to get any grief from building inspectors for this type of construction, it being rather atypical and all?
*This is a related question. I am planning to modify a screened porch, replacing the screens with tall casement windows, and covering the existing slab on grade with wooden flooring laid on cdx over treated sleepers. The sleepers will be shimmed to eliminate the 1/4 inch to the foot slope of the slab. Two walls of the porch are exterior walls of the house. I plan to leave a 3/8 inch gap between the edge of the floor and the house walls for breathing. The space will not be heated, and will still be essentially an outdoor porch that can be closed off during inclement weather.The problem with the slab is that when the weather has been cool but turns warm and humid, moisture condenses on the slab to the point of running off(The location is western Pennsylvania). It would appear that the slab is installed over well drained gravel, because it does not move at all during the change of seasons, but there is probably no moisture barrier beneath, because moisture does migrate up through the slab, showing itself where the concrete is tightly covered.My question is: should I install a vapor barrier over the slab and insulate the tapered space (3 inches to 1 inch) or perhaps re-think the whole flooring approach and lay decking over the sleepers and eliminate the insulation?
*Jay,You are 'hi-jacking' another's post :o) But I'll answer you anyway.The moisture you experience sounds like it is due to condensation, as you described it, like on the side of a cool drink on an warm, humid day.Your problem is not the construction of your slab (adding insulation or vapor barriers over it or under it will not avoid the temperature/humidity difference in your case, will they? unless you add heat), your problem is the presence of warm, moist air contacting the surface of your cool mass.To avoid this you want to do one of two things to eliminate the big differential that causes your condensation: - eliminate the source of the warm/moist air (remove the 3/8 gap venting, which is working against you in this case). - eliminate the cool mass (heat the slab).The former is probably easier than the later. Of course, you must consider why you provided the vents in the first place. If they were provided to prevent moisture problems then you may have done exactly the opposite of what you should have done (bringing in the moisture rather than taking it out!).The consensation you describe occurs everywhere in nature whenever there is a sudden swing in air temp (stand on the shaded side of some large rocks on a suddenly warm day and feel the moisture condensing!). The problem with our enclosed spaces (porches, garages) is choosing between heating that space or preventing that moisture laden air from getting in there all at once (at least until the slab mass temp has more closely reached the air temp).Or add a floor drain and get a squeegee / mop.
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Tell me what you think about the following construction technique. Pour slab on grade - float smooth. Glue rigid insulation on top of that, tape seams. Then vapor barrier, then 3/4 plywood subfloor. Bearing walls would be brought down to concrete.
I'm sure this can work. In fact, I don't even think it's an original idea. My biggest question is: how would you fasten the subfloor? Also what would you spec for the insulation? It not only has to insulate, but also support the floor load.