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Community and Q&A

Help with green addition and hydronic heat demo

Mike M | Posted in Green Building Techniques on

I am in the process of designing a pretty substantial addition onto my home in central Missouri. I understand that our relatively low electricity costs will not be around in the next 5-10 years in the US and am planning for this.

The addition is going to be 2×6 (24″ O.C) construction with engineered 2×4 roof trusses and only a single top plate over a new full basement. The majority of the original construction is 2×4 (16″ O.C) with engineered trusses and double top plates, slab on grade. One portion of the original home is 2×6 which is on a wall that is 85% windows southern facing (passive solar design).

The current home has slab on grade with hydronic floor heat. The addition that I plan on adding will be a full basement next to one side of the home. I’m not a concrete guy, and the contractors I have spoken with have talked of installing a “box” between the slab side and the basement. My guess is to prevent cave in of the original footer and wall when the hole is dug.

The unfortunate part about the floor heat in the main section is that the seller/builder forgot to insulate the slab, yet advertised the home as such. During the addition I intend on demoing out the original slab (it’s a floating slab) and installing the required insulation prior to repouring so that the heating system is even worth using. The option of raising the floors 2″ to install foam above it and use a new heat source is not really acceptable to me. I would also really like to design the subfloor to handle extra load to allow me to pour concrete (possibly lightweight) on top of the sub floor (with reliefs) to allow for expansion of the floor heat to the addition.

During the addition I am also planning on having foam board added to the exterior of these walls. I would really like to add up to 4″ of XPS foam to the OSB after the vapor barrier is laid (please correct me if this will cause moisture issues) on the addition. The original portion of the home with the 2×4 construction I would like to get ~5-6″ of XPS to try and equal the insulation of the addition, I believe the vapor barrier is already installed over the OSB (under the siding).

Another possibility I would like to entertain is somehow getting my AC ducting within the conditioned envelope of the home. The ducting is currently ran in the unconditioned attic using pre-insulated duct work which I understand to be terribly inefficient. Also, due to the truss configuration, the majority of the long runs are perpendicular to the trusses making it hard to create its own envelope. The majority of the original home also has 12 ft cathedral ceilings.

Do any of you Green builders/designers have experience with such projects? Any help with these questions below or other problems I may create is greatly appreciated.

-Is the insulation idea between the 2×4 and 2×6 wall interfaces going to cause any headaches as long as I use furring and window boxes?
-Are there other options for AC ducting routes that I might research?
-If I put venting in the roof rafters to allow my soffits to breathe, can I just have another 12″ of cellulose blown in to increase the R-value of the cathedral ceilings?
-Will covering my soffits with that much foam on the exterior cause air flow issues in the attic, and moisture? (there is a ridge vent the entire length of the home.

Thanks again!

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  1. Richard McGrath | | #1

    Michael ,
    Slab radiant really is not your best option . Have you thought about radiant walls or ceilings ? Also quite possibly don't require a boiler for any of these as you mentioned you have in another discussion I was viewing . I also believe it was you that mentioned john Siegenthaler's text , MHH , what edition do you have or have you read ? there is really much more to designing these than you may be aware of .

  2. Mike M | | #2

    Richard, without demoing the slab I will have to raise the floors in my home 2"+ floor coverings and abandon the in-floor heat when I do the addition as it does not meet code without insulation. I am in the process of reading the 3rd edition and have had a few talks with John on this subject in the past. Other than demoing, he recommended plate style systems which still don't bring me up to code. Also without a boiler for this type of system I would need some other heat pump and heat exchanger system which I don't already have.

    Adding insulation above will also make the passive solar approach to this home useless as the thermal mass will be insulated from the home. Radiators could be used, but again, the 2100 sq ft of thermal mass that I have is useless and I would still be left with running a lot of wiring or tubing. There are also no load bearing interior walls according to the truss specifications so gutting the interior is not a problem.

    To install radiant wall or ceiling heat, my attic would have to be re-insulated completely. Wall heat would be pretty ineffective in my main living space as it is nearly a 50x20-30' room with 12' cathedral ceilings, so I think without heating the middle of the room, there will be large differences in room temperature. Another thing I should note is that the guy who built the place also didn't insulate the waterlines in the attic per code, and these must all be redone. Due to the current truss layout and cathedral ceilings, I would rather not do any work in the attic as most of the piping will run perpendicular to the trusses and need a lot of extra insulation since they are nowhere near the conditioned space.

    I would rather spend an extra $10,000 to demo the slab and correct the problem rather than cover it up if possible.

    Since electric heat is my only option where I live I think it will be the most efficient to continue to purse radiant in floor heat. Using a resistive coil in an air handler would be probably just as ineffective as my current floor heat given the minimal insulation on the installed ductwork in the attic. A heatpump would work for 8-9 months out of the year.

    My background is in nuclear power and as a boiler inspector so I have no issues with water and wiring. I much prefer to work with this type of system over a conventional air handlers. The zoning portion is also not a concern as I have the option to use my own microprocessors based on Arduino to control if the cost of commercial systems becomes prohibitive.

  3. Richard McGrath | | #3

    What type of product will you then be using for your slab before re pour ? Have you performed a room by room heat loss for the home as of yet including the new addition ? You may want to entertain using the slab tubing as cooling too , this will enable you to decrease duct sizing by a substantial amount as it will only be needed to remove the latent heat from the home . I'd rather not discuss too much about passive stuff on the site since it inevitably turns into a rather unpleasant discussion about first cost and antiquated thinking about the various technologies available that when looked at several years ago have now evolved into truly usable and highly efficient systems . feel free to Pm me if interested in some ideas for your bigger more efficient home , No gimmicks , no sales pitches , just a pretty good opinion on some oft overlooked technologies . [email protected]

  4. Mike M | | #4

    I'll shoot you an email with the details I have at the moment. Most are pretty vague as I'm still in the planning and design phase.

  5. GBA Editor
    Martin Holladay | | #5

    Let me get this straight: your existing house is on a floating slab, and you want to demo the slab?

    OK, this is possible -- but it's not easy. How will the wall loads and roof loads be supported while the slab is being demo'd?

    If I were you, and if there are no structural problems with the existing foundation, I would leave the slab the way it is. I would retrofit vertical rigid foam insulation around the perimeter of the slab, properly protected on the exterior and equipped with flashing to manage rain.

    In-floor radiant heating systems are expensive, and your dollars are better invested elsewhere. For more information, see All About Radiant Floors.

    There is nothing wrong with abandoning the tubing that you have in your slab and coming up with a different heating system. That's one option.

    Your discussion of "vapor barriers" is confusing. In case you don't realize it:
    (a) Plastic housewraps like Tyvek are not vapor barriers -- they are designed to be vapor-permeable. (b) In your climate, you don't want interior polyethylene.

    For more information on installing rigid foam on the exterior side of your wall sheathing, see How to Install Rigid Foam Sheathing.

    Q. "Is the insulation idea between the 2x4 and 2x6 wall interfaces going to cause any headaches as long as I use furring and window boxes?"

    A. That depends on whether you get the details right. There are lots of ways to mess up any construction project. Study up on GBA if you want to stay out of trouble.

    Q. "Are there other options for AC ducting routes that I might research?"

    A. Here are two suggestions: (a) Read this article: Keeping Ducts Indoors, and (b) Consider heating and cooling your home with ductless minisplits.

    Q. "If I put venting in the roof rafters to allow my soffits to breathe, can I just have another 12" of cellulose blown in to increase the R-value of the cathedral ceilings?"

    A. Maybe -- as long as your rafters have enough room to accommodate that much cellulose, and as long as you specify ventilation baffles that are stiff enough to resist the pressure of dense-packing cellulose. For more information, see:

    How to Build an Insulated Cathedral Ceiling and

    How to Install Cellulose Insulation

    Q. "Will covering my soffits with that much foam on the exterior cause air flow issues in the attic, and moisture?"

    A. Your question is confusing. If you have soffit vents, that means that your soffits are perforated to allow air to enter at your soffits. Clearly, you wouldn't want to cover your soffits with foam if you have soffit vents.

  6. Aj Builder, Upstate NY Zone 6a | | #6

    Totally being honest.

    I would sell the home you don't want and buy or build the home you do want.

    There will be NO absolutely no payback period ever. You and I will be long dead prior to such; nuclear engineer and or your basic multi decade builder experiences and knowledge or not so much...

  7. Mike M | | #7

    Thank you for all of your help. I have read all of the articles you posted prior to this and was just trying to clarify some of my questions.

    First off, there are no load bearing interior walls according to the truss details or schedule. I have included a picture of the original plans I received from the county. GE02 is the only truss that has load bearing points along the wall. I intend on getting with a structural engineer to verify that I can support this load using a few trusses similar to T01 which only have load bearing points on the external walls. I plan on removing anything below the top plate (interior walls only) and opening up a 6' rear door to allow a small Bobacat in for the demo. I understand hyrdonic may not be my friend, but to my understanding, the home is not up to code and when I do the addition, I'm required to bring it up to code or I won't get my occupancy permit back. I have absolutely no intention on installing foam board on the interior walls.

    Thank you for the information on vapor barriers. I was under the impression that Tyvek sheeting was a barrier. The drawings show sheetroock, studs and cellulose, sheathing, and vinyl. No details on any vapor barriers or Tyvek.

    1. I will study finished wall thicknesses more to ensure this will not be a hiccup. It seems that given nominal stud thickness and finish thicknesses though that this will vary up to 1/2" depending on the materials.

    2. If you look at the floor plan, the concerns are the AC ducting in the east side of the home (left) and the cathedral areas. I would like to make a box, but I'm not sure how to do this without making it an eyesore. I think the minisplits are also an eyesore as this was an option when the AC was installed. There would have been nearly a dozen of them hanging around the house.

    3. The trusses are engineered 2x4. I would be blowing insulation over top of this to almost fill the attic. I would extend any soffit vents using channels or foam venting to ensure they weren't covered and can flow to the ridge vents.

    4. The question about the soffits was based on adding 6" of foam to the exterior walls and covering a portion of the vents, not actually placing foam board over them. If the soffit is 10" and I cover 6" of it with foam, I was concerned it would limit the airflow. I can calculate what the IRC requires easily, but they are not perfect as many articles here I've read state.

    AJ Builder,
    Would you suggest I lie to the next buyer and tell them the floor is insulated to code and the domestic waterlines don't freeze? I am morally and legally obligated to disclose these facts if I were to sell the home. I would not be able to sleep ant night knowing a family with children was living here with 50 degree bedrooms at night.

    The cost to repair the floor without new tubing or insulation was quoted at $25k including re-pour. I will be removing all the walls, fixtures and plumbing, so this price should go down drastically. I also can't sell a house for nearly $75k less than I paid due to market conditions and 2 foreclosures in my small neighborhood of 9 homes. We all have nice 5-10 acre lots in a good location close enough to the city.

    The original seller (GC) for the home claimed the floor was insulated and that he averaged $20 a month in heating costs. I found out from the utility the house was vacant the majority of the 7 years he owned it and the costs below $20 were basic fees and not usage. He claims he was not aware of any of these conditions when he sold the home. My lawyer told me to take him to court I'm easily looking at $25k, plus more to try to collect. He married a woman with a special needs child, retired, and moved to Arizona. The lawyer said Arizona is a state that makes it nearly impossible to collect this debt from a moron like him. He took his Porsches and left me high and dry (he had a pair of them in the garage when I first saw the house).

    I have framed three homes in my life, and done more wiring and plumbing that I care to admit. I used to install and start up boilers, piping, electrical, controls, tuning etc. I am not worried about doing a bit of hands on labor to reduce my costs. There are also no licenses required here except to install my septic.

  8. GBA Editor
    Martin Holladay | | #8

    Is this slab poured on the interior of a stemwall foundation that extends below the frost line? Or is it a true floating (thickened-edge) slab? If it is a floating slab, and you are demolishing all of it, there will be no foundation to support the exterior walls while the demolition happens.

  9. GBA Editor
    Martin Holladay | | #9

    You wrote, "The drawings show sheetroock, studs and cellulose, sheathing, and vinyl. No details on any vapor barriers or Tyvek."

    Well, your walls don't need Tyvek -- but if they don't have Tyvek, they have to have some type of water-resistive barrier. For more information on this topic, see All About Water-Resistive Barriers.

    You wrote, "If the soffit is 10 inches and I cover 6 inches of it with foam, I was concerned it would limit the airflow." Some soffits are solid lumber soffits with slots; others are solid lumber soffits with round vents; and still others are vinyl soffits with perforations.

    So to answer your question, you have to do the calculations. This article tells you what you need to know to do the calculations: All About Attic Venting.

  10. Mike M | | #10

    I've spoken to the guy who poured the original home. There are foundations walls and footers below the frost line and a separate pour ( I thought the term was floating slab) inside of this. I have verified this after removing floor coverings and seeing the gap between the slab and foundation wall. The gap is just under the trim so all of the 2x's are supported by the foundation wall to my knowledge.

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