insulating concrete slab radiant heat
madmax
| Posted in Energy, Heating & Insulation on
We’re building a house and plan to have radiant heat in a finished walk-out basement. I’ve been reading about slab insulation and I’m confused whether 2″ “pink board” is the only route. I’ve heard some contractors are now using rolls of foil/double bubble/foil. One 3rd party website ( not a seller of products) says use the board in basement slab but it’s okay to use f/db/f in crawl spaces which get radinat tube heat to warm the floor above.
Anyone have some experience either way?
Thanks
Mik
Replies
foil products under slabs are a waste of money. better than nothing, but not even in the same league as rigid foam.
In joists, never rely on reflectivity to do the insulating job. You can use it, but always use products with real R-value in addition and consider the reflectivity gravy. Personally, I wouldn't bother. I would focus on sealing the joists against infiltration and getting an appropriate R-value in.
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
R-value makes sense. I gravel is in place so I'll have to scrape away 2" for the board, but I supose there is more than 2" of gravel. I'll use a vapor barrier underneath the board.
Thanks,
Miki
Miki,
I would advise that at least 3/4'' of foam board be installed at the edges of the slab to break the thermal syphon of the foundation wall. I set mine to the screed line b/4 the pour.A great place for Information, Comraderie, and a sucker punch.
Remodeling Contractor just outside the Glass City.
Quittin' Time
When I installed the insulation for my shop slab, I used the same thickness as under the slab, but cut an angle at the top of the insulation used on the side to end up at 1/2 inch thick at the top. I then put 1/2 inch up to the screed line (about one inch) and after the pour peeled that 1/2 inch out and inserted the closed cell backer rod and then put in the NP1 sealant....worked great.....a fully insulated and isolated slab. --Ken
Thermal siphon (aka thermal bridge?)
I am having a house built in which I planned to heat exclusively with RFH, both the walk-out basement and main floor. I discussed the 'requirement' for a thermal break at the perimeter of the slab as well as including one to create a wine cellar. I learned after the fact that neither were installed. There was some rigid insulation added to the exterior of the foundation at the 'walk-out' side.
I believe that the absence of the thermal barrier renders the radiant heated slab a waste of time and money, that my boiler will be always be running to heat the ground surrounding my house. That is, unless corrective steps can be implemented.
The trouble is I don't know which of the following solutions addresses this problem the best (and least expensive) way.
1. Dig out the backfill and intall rigid insulation on the rest of the foundation's exterior walls.
2. Cut the perimeter of the slab and install the thermal break.
3. Re-install the PEX in a gysum base over the existing slab? (This could cause problems with thresholds and plumbing.)
4. Abandon the RFH in the lower level and install mini-splits.
Am I missng something?
Richard
Not a good outcome in the beginning.
What do you consider the chance for a successful outcome in cutting, removing and insulating the perimeter? Who'd pay? How close to the edge of slab is your hose run?
What is your idea for perimeter finish on these foundation walls?
I'll direct your post to a radiant specialist-maybe he can come up with a fix.
what
did the plans show for slab insulation......
did the contractor follow them.. or did he just substitute his method for yours ?
all of our basement slabs are on a minimum of 2" ( 2 lb/cf ) EPS Performguard under the entire slab
the edge perimeter is 4"
i would think a RFH on an uninsulated slab would be a huge waste of heat
Mike -
I agree that the RFH in the slab, as it currently stands, is useless. I'm trying to find a way to make it work.
The plans do not show any type of heat. However, the GC and I discussed the need for a thermal break at the perimeter as well as one to isolate a wine cellar. I'm not sure if he dropped the ball or the sub cut corners or worse, didn't/doesn't have a clue. (see attached pictures of the PEX layout)
Ultimately, the GC is responsible, but I don't feel confident that he knows how to fix it.
Thanks!
If you "discussed it" then
If you "discussed it" then you're responsible. Get it in writing.
Insulating concrete slab radiant heat
DanH -
Thanks for the response. There are a couple of things to consider -
1. Acceptable building practices, basic heat loss knowledge (heat goes to cold) and customer satisfaction.
2. Since there was nothiing in writing about putting radiant heat in the slab, I guess I don't have to pay for the materials and labor that were associated with it.
That said, I'm looking for a solution not to place blame.
Richard
I'm just saying that all the GC needs to say is 'That's how we always do it" and he's off the hook, unless you can prove a code violation.
And your pictures appear to show foam under the slab. So at most you're missing the perimeter insulation. I don't see how this renders your in-floor heating inoperative.
A boat with a hole?
Ok, maybe inoperative isn't the right wording. It very well work at heating. But I'm fairly certain there will be an unnecesary drain of the heat that I will be paying for to the outside world.
How would you feel if you bought a beautiful (and expensive) boat only to find it had a leak that can't be easily fixed. Yes, it works but...
Again, I've posted to find a reasonable solution!
What you've got is a situation where roughly the outer foot of the floor will be a few degrees colder than it would have been otherwise. The rest of the floor should be fine.
I'm doubting that replacing the system with "splits" would be more cost-efficient.
Here's your real world info
Just visited with a client about a HUGE real detailed deck (and deck is way too small of a word here).
Walked into the house which I've done extensive work on over the years. Shoes come off as usual.........
Whoa, cold tile at the entry-I know there's hose under there but no insulation. Only a 4x4 area just in the door. All slab, unheated garage below, no insulation on that slab/ceiling of the garage. He's losing heat.
Step in further, very comfortable floor heat. There is hose under there as well, but wood frame under w/insulation.
That you have foam under is going to be a big difference.
To the perimeter, and this is my house now. I insulated with inch and a half against the block foundation and between the slab. Even with that foam at the perimeter and 2" underslab, the area at the wall is much cooler. In the kitchen, around the island and in front of the perimeter base cabs, the hose is not run under the cabs. You can feel the difference in floor temp at the toe kick.
In both these floor areas, the hose is run maybe 6" away from the walls and cabs. I don't seem to get much lateral warmth except between the hose runs. At the outside walls I ran the hose on 6" centers for maybe 2- from the walls, then graduated to 12" centers. This was detailed by the hose supplier-closer together at the outside walls AND there FIRST coming back into the room with the "cooler" hose returns. Made sense to me-it (hose) starts losing temp (heating the floor) as it goes through the slab.
This is why I mentioned not being able to follow your hose runs from the pictures-I couldn't tell if your supplies went to the outside walls first . I don't remember from the pics if the centers were closer together at those exterior walls.
Take from this whatever you can, I'm no heating expert by any means, but am enjoying warm floors for the past 25 yrs.
Losing Heat!
Calvin -
Thanks for the response.Yes, the foam underneath is definitely a plus. If that hadn't been done I would have told the GC to jackhamer it out and start over. The perimeter is another matter. The side of the house that is the walk out (framed walls) has what looks like 2" of rigid blue insuation outside the foundation. My concern is the foundation walls and footing that are below grade. I think there will be continious thermal siphoning there. I liken it to buying a nice (expensive) fuel efficient car that has a leak in the gas tank. Every mile driven costs a little more than it should...
As for the pex in my slab, I suppose I could get theic pics enlarged and try to follow the circuits with a colored marker but I shouldn't have to. I just wanted efficently heated floors!
What to do to achieve that...
I think it would be fairer to compare it to that fuel-efficient car with an extra mirror, or maybe with a window open at highway speed. The heat loss into the earth below through the 2" foam will be several times the loss through the sidewalls. And the loss through your walls and windows and roof will be greater still.
you can insulate the foundation walls on the inside, and that's fine. was the slab insulated underneath? If so, then good. perimeter insulation COULD be installed around the inside of the foundation walls as well if you can cut the slab at the wall face for a 2" gap. that would be my preferred solution, if you can imagine your foundation all rigid foam going right to the bottom of the slab, to its underside insulation then that's the ideal solution. no path from floor slab to wall. of course, you would want to be sure you don't have any pipes within 2" of the foundation walls before doing this! and there may be structural concerns based on the wall construction method.
I can't think of anything cheaper or easier than that, especially if your GC will at least do the labor for free to rectify the oversight. you want to insulate foundation walls anyway.
If you had to build up over the floor, you'd want a quote on those minisplits as a comparison.
Interior insulation
I agree that cutting the slab is risky, especially seeing the pictures of the PEX layout. While with the inside insulation is also an option, it doesn't completly stop the thermal bridging.
Instead, I'm thinking the most thorough solution is exterior insulation, rigid foam against the foundation walls. However,it will be a pain to get to the foundation, some of which is 8+ feet below grade.
Remember that I will be paying for that lost heat, not the GC nor any of the subs...
Interior insulation
I agree that cutting the slab is risky, especially seeing the pictures of the PEX layout. While with the inside insulation is also an option, it doesn't completly stop the thermal bridging.
Instead, I'm thinking the most thorough solution is exterior insulation, rigid foam against the foundation walls. However,it will be a pain to get to the foundation, some of which is 8+ feet below grade.
Remember that I will be paying for that lost heat, not the GC nor any of the subs...
I'm still confused -- you have foam under the slab, which is where the vast majority of any heat loss would occur. The lack of foam around the edge, while not an ideal situation, is not the end of the world either. It's certainly not sufficient to render the system inoperative. My rough calculation is that it will add about five cents per hour to your heating costs if you heat with oil, less if you heat with gas.
I just did some research on this topic... not the insulation per se, but the buildup under a slab. Bottom line, for the slab I'm now doing I built up under the insulation with fill (sand), installed a VB (poly), laid down the insulation (2" Foamular) and then added a 2" 'blotter' layer of sand over the insulation. The reasoning is that this sand will absorb the bleed water from the concrete as it's curing, and help prevent 'slab curl', which can occur when the slab can dry from the top but not from the bottom because it's poured over a non-absorbtive material like foam or plastic. The water in the blotter layer will eventually have to exit thru the top, but the underside of the concrete will not be sitting in its own water.
You should check into this and possibly put something like sand over your foam. Also, make sure you are compacting the sh!t out of the fill you are adding. I've been running a walk-behind Bomag vibratory plate compactor for the past two days straight.
<<I've been running a walk-behind Bomag vibratory plate compactor for the past two days straight.>>GOOD!!! <VBG>Power always thinks it has a great soul and vast views beyond the
comprehension of the weak; and that it is doing God's service when it is
violating all his laws. -John Adams, 2nd US president (1735-1826)
y y y eaaa a hhh hhhh !
Great info, thanks, Miki
Foil bubble wrap would be totally useless under a slab. The R-value is much less than 1. Foam board is much, much better.
I would not even hire a contractor that proposed bubble wrap. That one thing alone tells me he has little professional knowledge.
RFH
Probably the best way to proceed, is to lay 4 inches of Styrofoam over the floor and then lay pex followed by a layer of OSB.
The Syrofoam will save most of the heat moving downwards by conduction.
This format will enable quicker heating (less mass) and quicker cooling - making the home cheaper to heat.