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Thread: radiant verses baseboard on preexisting slab

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    DIY Junior Member Doubler's Avatar
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    Default radiant verses baseboard on preexisting slab

    We have a small office used during the summer with a slab on grade, total square footage is approx. 1200....what are some thoughts on heating this space for winter use as an office?

    thanks

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    In the trades Dana's Avatar
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    Assuming you're talking about hydronic (not electric) radiant vs. baseboards...

    And assuming you've already got a boiler that can handle the additional load...

    Is the slab insulated?

    If not figure a cost adder for 1" of XPS and a 2" quickset over slab (+ labor) over the existing with radiant. It'll be cushy, but figure on losing 3" of headroom, and you'll have some efficiency gains by being able to run very low water temps. If it's already insulated below and head room is an issue Warmboard(tm) or similar might be the ticket (but probably as expensive or greater than a thin slab.)

    If not and the rest of the structure is reasonably tight, 1/2" XPS between sleepers for a finished flooring (or just a vapor barrier under some reasonably thick carpeting undelayment) and some baseboard should suffice. If you want to regain some of the higher efficiency of radiant, doubling the length of baseboard that would have been required for 180F water and lowering the water temp to 130F is usually cost-effective.

    Whatever way you go, do a full-on manual-J type heat loss calculation, and make sure you neither under or overdo it on the radiation or heat source end.

    If you're talking electric somebody else needs to break it down for you... (it's not my cup o' tea.)

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    DIY Junior Member Doubler's Avatar
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    No insulation under the slab, was built as a summer camp cabin, walls are now insulated and structure is pretty tight. Would be used as an office. Losing three inchs of floor would creat a big problem with front doors as there is no head room to raise them

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    DIY scratch-pad engineer leejosepho's Avatar
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    Even though the slab is not insulated, I would consider electric heating wire and ceramic tile. You could get that on the floor while only giving up about 1/2" or so, and you could use edgings and leave more room open around outside-wall doorways, if necessary. You could possibly get all the heat you need by using a commercial-grade wire, or you could only warm the floor a bit to make it comfortable while heating the room in some other way. To address the matter of insulation, I would look into possibly placing foam vertically around the outside of the foundation since that is where the biggest loss of heat would take place. Underneath the slab, heat will only go a couple of feet or so then largely "hold" there in a plume.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If the soil under the slab is wet, the heat transfer goes up.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Quote Originally Posted by Doubler View Post
    No insulation under the slab, was built as a summer camp cabin, walls are now insulated and structure is pretty tight. Would be used as an office. Losing three inchs of floor would creat a big problem with front doors as there is no head room to raise them
    What are your fuel options, and what is your design-day heating temperature? (Or what are the temps at the coldest hours of the heating season that you anticipate needing to maintain temperature?)

    With no insulation under the slab radiant floor is pretty wasteful- you can do it, but it won't be very efficient. (If electric, it'll be ATROCIOUSLY inefficient- you lose 60-70% at the power generating facility, then half of the energy that actually reaches through your meter ends up heating the ground below your building!)

    That said- if its very dry & sandy soil you may be able to do OK with radiant if you insulate with 2" XPS down to couple of feet below the usual frost line depth around the perimeter of the slab, making sure that the slab-edge is fully covered. (How easy/hard it is to get right depends on a whole lot o' particulars.)

    But unless you're working the wee hours a lot, it's probably better/cheaper to go with something with a quicker response than radiant-on-slab and use nighttime setback for improving efficiency. The thermal mass of the uninsulated slab and some depth of subsoil will have to be maintained at temp pretty much 24/7 to stay comfortable thoughout the 9-5 workday. Even a propane-fired 65% AFUE side-vented wall furnace would probably be cheaper to run than radiant in this application (and a whole lot cheaper to install!) Using an insulating floor covering like cork or carpet can sufficiently take the edge off the coolth under foot, unless you're prone to working in your socks (which might become habitual, if you have radiant floor! :-) )

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jadnashua View Post
    If the soil under the slab is wet, the heat transfer goes up.
    Ah. I had not heard or thought about moisture content!

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    DIY Junior Member Doubler's Avatar
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    fuel options are oil or propane, thinking at this point baseboard hydronic or wall mounted propane....we could dig around perimiter qand put in some rigid board, which would help.

    finishing bathroom for heat is needed and adding insulation around from the polutside is most likely best as inside offices are finished rather nicely. we qare in the northeast and just went through a week of -5 ^ weather but normally its 25`s and up.

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    Baseboard hydronic is my choice, but what about cooling? Have you considered a split ductless heat pump rather than a wall furnace?

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