cold slab in cold massachusetts

[thatgarngirl]

Planning a substantial reno to our 1971 2200 sq ft ranch on an uninsulated slab. 99% set on getting a Garn boiler that will be used to heat the house and a detached 1800 sq ft garage. Currently burn wood as sole source of heat so we are no stranger to the idea. No plans on skimping on any part of the project but of course the project has to make economical sense.

We are planning on tile throughout (low maintenance for our dogs and found some gorgeous wood look alike tile!) and in floor radiant would be ideal! Would likely be using something akin to warmboard, thermal board, etc. Our concern however would be placing this over the uninsulated slab. Dont want to be wasting wood to heat the earth if we dont get that heat back. Will be doing perimeter insulation however under slab insulation is obviously not an option at this point. Thoughts on this? Do we need to insulate over the slab and build a new subfloor? Trying to keep ceiling height loss to a minimum...

Other option of course would be panel rads but would prefer to explore infloor heat first.

Hoping some fellow boiler/gasification users can share their experiences. Has anyone here had experience with this or done a similar project? How did it work out? We are looking to running low water temps through the system and keep garn firings as spread out as possible.

Thanks in advance!

 

[BoilerMan]

Welcome to Hearth!

It may be cheaper to do a pour-over. 2" of blue board with pex directly on the BB, wire mesh (may not be necessary) and a 1.5-2" new screed of concrete over that. You'd loose 4" of headroom though.

TS

 

[Boil&Toil]

If you don't have major headroom issues, @BoilerMan's approach is probably the most effective (and probably also most affordable). Gypcrete (which is often what's used on the top layer) makes a fine tile base. If you have BOTH good drainage and good, deep/wide perimeter insulation you can "get some of it back" in the form of having a vary large thermal mass, but if the insulation is not up to snuff or there's water moving through under there, the heat all leaves. Also, a very large thermal mass (as your radiant source) can be less than ideal - moving from "steady heat" to "unresponsive bleepity-bleep" which overheats you when the weather warms up, and leaves you firing like crazy to try and get warm when it turns cold suddenly. So having the radiant and a little mass isolated from the uninsulated slab can be a good thing. If you are really tight for vertical space even an inch of blueboard would make a difference; but be sure to get the perimeter insulated very well in that case.

 

[AK13]

There is absolutely no way I would ever put radiant down in/over an uninsulated slab. I'd want insulation under and at the perimeter for sure.

My personal opinion is that radiant floors are overrated. And I don't care much for pex in general. So I'd go with panel rads. But if you are set on radiant absolutely insulate the slab.

 

[ewdudley]

But if you are set on radiant absolutely insulate the slab.

Even if not going with radiant, seems like a high priority should be to insulate the floors.

 

[STIHLY DAN]

This is 1970 in mass, you probably have no perimeter drains either. Concrete floor is probably 4 inches. You could dig out the floor and foot of dirt. That would give you basement drainage, ability to load lots of insulation under the floor and have nice head room.

 

[thatgarngirl]

Thank you everyone for your input and time, I truly appreciate it! Such a balancing act between cost, efficieny, comfort, and aesthetics. I would love to insulate the floor regardless, but the husband FREAKS out at the thought of rehanging the doors and raising the kitchen cabinets and whatnot. He thinks I watch too much HGTV and underestimate how much these projects entail (probably true). Since we are redoing so much already I'd like to put in the extra effort and make it "perfect" as we won't be going back and changing anything again...

Stihly Dan, no basement here. Just slab on grade, sorry I should have said that in the first place. Don't know the actual thickness of the slab, but I can tell you that it rises 8'" above grade and that the perimeter sinks down AT LEAST 30" below grade (arms weren't long enough to dig further down, will know more when the ground isnt so frozen). Don't know if it sits on footings, is monolithic, etc.

Thanks again!

 

[Boil&Toil]

You probably will have a "frostwall and footing" arrangement. I don't think monolithics were all that popular yet at that point in time. Odds are excellent you'll find the footing about 18" below where you've already dug (ie, 48") and if the DH is freaking about doors and cabinets, you'll be wanting to find the footing and install a boatload of insulation from sill to footing, and a drain tile to daylight (unless you find one in good condition down there.) No minor job. If he's freaking enough that you're not going to put radiant in the floors, I would strongly suggest considering (and this is a whacky one by most people's lights) putting a couple of loops around the outside of the house, under the insulation, and do make it serious insulation. Point being, your tiled uninsulated slab is going to be cold as heck if you're heating with panel radiators unless you can get some heat into it. Perimeter loops will offset the cold edge.

I'm still half-kicking myself for not throwing a radiant loop in (inside, below) before I backfilled my frostwall before pouring the slab (which has 3, and sits on 2" XPS, has insulation between the edge and the frostwall, and the frostwall is insulated outside with 2" XPS down to and out from the footer) - or into the frostwall pour itself. Might or might not have proved useful, but can't be done over.

 

[maple1]

Don't think you mentioned before - what's on the floor now?

And do you find the floor to be cold now?

I think if it were my place, I would do my best to insulate & ensure good drainage on the outside of the footing/wall, put my tile over a (thin-ish) layer of insulation on the slab, and use panel rads. Sounds like the challenges you face in trying to get in-floor heat (that works right) might be too big to overcome - or impose severe marital stress.

 

[Fred61]

I wouldn't do it. Avoiding water or moisture is one prerequisite for efficiency of a slab in addition to the insulation. Chances are there is no vapor barrier and if provisions were made to eliminate moisture when it was built, the method used probably falls far below the requirements recommended for radiant in-floor. Any moisture in direct contact with the concrete will draw heat away at warp speed. There are folks who have tried to cheap out on slabs by laying a sand bed beneath the slab and eliminating the insulation and vapor barrier and they also failed. They're walking on a useless pile of dollars. I suggest you put your time and money on alternatives.

 

[boilERIK]

Where in MA do you live? I am a builder contractor in western MA. My suggestion as others have stated would be to insulate the frost wall from the footing to grade with at least 4" of blue board. at grade install 2"x24" blue board perpendicular to the foundation. make sure this foam is pitched slightly to move water away from the foundation. Put 3/4-1 1/2" drainage stone on top of the on grade foam this minimizes splash back onto the siding and keeps the foam in place. Above grade install as much blue board foam as you can without creating a shelf which sticks out past your siding. you can finish the above grade foam with a roll on stucco finish. This insulation assembly really allows you to load up on insulation below grade . The horizontal insulation on grade further insulates by keeping the frost off of the frost wall foundation and it provides a nice drain plane to keep water away from the foundation. Be extra vigilant to seal all of the joints. There are a variety of tapes and spray foam products available.

How involved will your interior renovation be? I wouldn't touch the slab unless you have a large budget and feel very strongly about installing radiant heat. It seems to me that the most bang for your buck will be installing panel radiators. My wife and I live in a old colonial built in 1747 we have done major improvements to the house. I have in-slab radiant in one half of the house and panel radiators in the other. The radiant slab is nice in full blown winter but I don't feel that it is that much superior to the half with the panel radiators. the floors are cold on that half of the house but they aren't that much warmer on the radiant side either. The panels are more responsive during the shoulder season as well which is nice. good luck sounds like you have your work cut out for you!

 

[ewdudley]

Above grade install as much blue board foam as you can without creating a shelf which sticks out past your siding.

Where siding goes down nearly to grade I've used rubber roofing as flashing to get out over the vertical blue board that does stick out past siding, then slope away under a layer of cobble.

Stones moved away:

Stones in place:

 

[boilERIK]

That is a good way to flash the foam insulation but you want to avoid bringing grade that close to the siding, make sure that the rubber roofing is sealed to the foam so that any moisture under neath the rubber roofing won't migrate up into the wall assembly when it heats up. You will have major rot issues with wood siding and if you have vinyl or aluminum siding you are trapping a ton of moisture behind the siding. This can lead to mold and mildew in the stud bays and rot in the framing members close to grade. Keep at least 8" between siding and grade, that is at a minimum.

 

[thatgarngirl]

Wow, thanks for all the replies and suggestions! This will all definitely help when it comes time to make decisions.

The house right now has a combination of tile and carpet throughout, except for a few places where the carpet was removed leaving the bare concrete slab (bathrooms originally had shag carpet, that can't be sanitary). I can stand the cold floor, but husband says the simplest remedy is to invest in good slippers. We want to go with tile due to his allergies and OCD and our large dogs.

Our house does have wood siding that currently sits 8" above grade.

We are located in Southampton. I'm pretty sure we will go with panel rads but I hate to rule out infloor until I know more about the costs/work involved. We are planning on turning our attached garbage into a bedroom and adding a walk in closet and bathroom, so we were also thinking of infloor in the "new" house and panels in the "old." Our budget should be pretty healthy but we hope to offset costs by doing "dumb" work (digging trenches, etc) ourselves. Husband is a firefighter, so we have a whole station guys willing to come over and help out.

 

[STIHLY DAN]

Turning a garage into house is just a sin.

 

[boilERIK]

I agree Where are going to put the garn?

 

[thatgarngirl]

Little attached garage that is being converted is being replaced with mega detached but close 1800 sq foot insulated/heated steel building with a loft. So don't worry, the Garn will have a very spacious place to live! The garage is slated to go up before we tackle any of our major house projects so we we have somewhere to be when there's too much commotion going on in the house. NOT cheap however for this structure, which is why I think we are going to have to compromise and get panels for the house instead of splurging on infloor, while ironically the garaged WILL have infloor....

 

[boilERIK]

Why a steel garage? Your insulation dollars will go further with a wood structure. Metal dramatically effects the r value of a wall assembly.

 

[thatgarngirl]

We obtained many quotes for both. Came down to $$$. Wood was about $30-50k more than steel counterparts Do you think a steel building with above average insulation is a bad idea? Hate to spend so much on something that won't perform... Steel building manufacturers assured us that we would be fine but of course they want us to accept their bid!

 

[eauzonedan]

Similar situation with a walkout lower level in northern wi.....including dogs. My project is still in process bit I proceeded as follows: heavy plastic sheeting on the slab followed by flat framed treated 2x sleepers with Warmboard. This keeps the limits of the aluminum skin away from the exterior walls a bit (a good thing). Followed up with 1 1/2 of rigid in the gaps. First area was framed 18" on center but shifted to 24's as it was plenty beefy and got me a bit more foam under the floor. Raising doors was Pain. It if you want radiant floor and there is no insulation in the slab raising the floor height is the only option. Looked at gyp but in smallish areas the cost was as bad as the Warmboard (crazy cost but again limited options). I also liked a low thermal mass concept for spring and fall temp swings. Plan on spending a lot of time shimming the sleepers to get a level finished surface. Find the highest point in the slab and bring everything up to it. Looked long and hard for exterior doors in non standard heights and found one that could be cut down (a royal PIA but again minimal options). Warmboard shipping took some work as my tractor won't lift over 1k# and they like 3k# pallets. Also the shipper used enclosed trailers which complicate fork access. Overall Warmboard was the best choice of a lot of less than ideal ones. Dan

 

[boilERIK]

I am sure you would be happy with the steel structure unless you had an identical wood building right next to it. I am not a big fan of steel buildings from an aesthetic stand point and I would argue that the wood building performs better than a similar steel building but I do sell/erect wood structures for a living so I am a bit partial as well. pay particularly close attention to the air sealing details involved with the steel structure. Neither wood or steel will perform well if you have large amounts of air leakage. will the steel building manufacturer provide provide you with heat loss calcs or estimated air movement figures?