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Unique situation for a hydronic floor system

tmtrux | Posted in Mechanicals on

I am about half way through a whole house renovation of a 1888 2 story Victorian over a ventilated crawl space in New Orleans. We have been upgrading the insulation as we go with R36 closed cell insulation along the roofline in the conditioned attic and R3 zip sheathing and R15 mineral wool batts on the walls. I am ordering Indow-windows for the old single pane windows.

The last line item is the floors. The first floor of the house is 3″ tounge and grove heart of pine wood flooring 1.25″ thick over 2×12 joists. Nothing else, zero insulation. You can see the crawlspace in spots. Due to flood zones, creating a conditioned crawl space is out of the question.

The first floor is not comfortable in either season, cooling or heating, but much more so during the winter. This likely due to the lack of any air barrier and insulation on the floor and the fact that house was designed for passive cooling prior to HVAC.

We have 2 separate variable speed forced air heat pump systems. One sized for each floor.

The question is, would installing a hydronic floor system over this crawl space increase our comort level over just spraying 2″ of closed cell foam under the floor and joists in the crawl space.

I feel like now with the forced air system and 12 foot ceilings we can not get comfortable on the first floor because all the forced air heat rises and the little bit that is left escapes to the cold crawl space though the sub-optimal floor.

I am looking at this strickly from a comfort perspecitve. My only experince with heated floors was in a small bathroom and they were nice. The system would likely only be useful for about 3 months out of the year.

My thoughts on potentially putting it in would be:

1. I could set it at a set temperature and I wouln’t need to run the first floor heat pump as much and lose as much hot air to the ceiling. We turn off the first floor heat at night b/c all the bedrooms are on the second floor.

2. I would be closer to the source of the heat.

3. All the other selling points of “radient heat”. Less forced air/dust/noise/better overall comfort.

4. I could zone it to be just the rooms we spend most of our time in. Living room/kitchen. I wouldn’t have to include all the formal rooms b/c they are mostly unused unless a party when people are overheated anyway.

I know our comfort will improve simply with insulating and air sealing the floor. Would the Hyronic Floor system really add to the comfort during our three damp cold months?

any thoughts ?

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  1. Expert Member
    Dana Dorsett | | #1

    Radiant floors would indeed raise the comfort level of the first floor more than just insulating it with 2" of closed cell foam, but it's expensive, and not the only option.

    Installing 2" of foil faced polyiso on the bottom side of the joists with the seams taped would meet code-min, and would be cheaper & greener than 2" of closed cell foam sprayed on the underside of the t & g flooring, and it would protect the joists from moisture accumulation during the cooling season. Air seal & insulate the band joists and foundation sills with 2" closed cell foam, and seal the polyiso at the edges with can-foam. If you want better than code-min installing 3" of foil-faced polyiso isn't necessarily insane.

  2. GBA Editor
    Martin Holladay | | #2

    Like Dana, I vote for air sealing and insulation rather than a new heating system. His suggestions are good ones.

    The climate in New Orleans is quite mild compared to the climate we face in northern New England. I understand that you are uncomfortable, but if you perform the required air sealing and insulation work, you'll find that it won't take much heat to tip the balance from uncomfortable to comfortable.

  3. DIYJester | | #3

    Being in a similar situation, I also say go with the insulation.

    I have a slab on grade hydronic system that the original builder/owner decided that he would "insulate" with dirt. I say insulated because the house was sold to me with a fully insulated floor. I proved there was no insulation with my air hammer and drill.

    I decided to put R-8 Roxul Comfortboard on the exterior of the foundation walls. At the SE corner of my kitchen, which is insulated now vertically 4 ft deep, the floor is about 63 F (bare concrete) with 72F inside air. The NW corner of my home, which I haven't completed yet, the floor was 41 F, which is only about 10 F above the outside are temp. These temperatures are with me only running my wood stove for heat.

    I think the information Dana and Martin gave are solid, you will probably have a nice warm floor, but there are better solutions. If you used an insulation like Roxul water/flooding may not be as big of a deal since it is not very hygroscopic of a material. With the 11" of rain we just had, the exposed areas of the Roxul seem to be holding up just fine.

  4. tmtrux | | #4

    thank you all for your answers.

    I did not originally include the foil faced polyiso as an option because I have a couple of acquaintences here who have tried that approach with marginal results. One who strongly recommened against trying it. Mostly due to the fact that is it very very hard to get a true air barrier with all the penetrations from the plumbing, gas, HVAC, electrical, etc.. found in an old crawlspace. .

    The green building contracting community here isn't that developed and its hard to find someone to do that job as well as it needs to be done, but I guess as the saying goes, If you want it done right, you will have to do it yourself.

    I was thinking that I might gain a bit more advange with the closed cell by encapsulating my HVAC ducts to help prevent leakage to the crawlspace. With the polyiso, the ducts will be below the polyiso unless I box them in.

    Dana, I would assume that you would also put a HVAC duct between the joists to keep the space between my floor and the polyiso as a conditioned space? Or would you just rely up the tremendous "natural" ventilation that is already there and assume that it will equlibrate easily.

    Mike M.
    I am surprised to see that much of a difference, with the exterior roxul. Do you run the hydronic system at all? I understand that without the subslab insulation, a larger percentage of the heat just makes the worms warm.

    thanks again for the answers. surprised no-one came out in support of the hydronic

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I would assume that you would also put a HVAC duct between the joists to keep the space between my floor and the polyiso as a conditioned space? Or would you just rely up the tremendous 'natural' ventilation that is already there and assume that it will equilibrate easily?"

    A. The polyiso on the underside of your joists would become your thermal envelope. Everything on the interior side of this polyiso is indoors -- it is warm in winter, and the polyiso keeps it warm. Everything on the exterior side of the polyiso is outdoors.

    You don't need to have a duct between your joists for the insulation to work. If the room above the joists is warm, the joist bays will be warm too -- as long as the polyiso seams are taped to reduce air leakage.

  6. DIYJester | | #6

    I was a bit surprised by the addition, but not too much. Bare concrete has an R value of something like 0.07 per inch at 150 lbs. So an 8" foundation wall provides ~ R-0.56. If I'm truly getting the full R-16 out of the 4" of Roxul, that is something like 25x the insulation that was there previously. I should also not that the top 3-6" of my foundation wall are completely exposed.

    I have not run the hydronic system since the first winter I lived here. I ran it realized my house was getting colder at night with the electric boiler humming for 12-16 hours a day. My electric bill went from $40 to over $400 by the time January and February hit. This was set at 68F, by morning sometimes it was down to 65 in the house.

    I bought the house because it was supposed to be energy efficient. The guy who sold me the house, Jeff/John Weiss (two names should have clued me in, he goes by both), left out 6 years worth of electric bills for people to see when showing his house. There was not one bill in the winter for over $30.

    After the sale and I started lawyering up. he claimed the primary heat source for the home was the wood stove. Unfortunately the cost to sue him became nearly the cost, or more to fix the floor. The lawyer said we'd easily take the case but since John/Jeff retired and moved to Arizona, I would have to sue again to try to get the money after the first suit.

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