Staple-Up PEX in an unconditioned cellar
Background: 1700s farmhouse with cellar\crawl. Cellar\crawl is just slightly above freezing December-March. Staple-up radiant PEX (to the underside of the subfloor) with aluminum plates, 8” o.c. tube spacing.
R-21 unfaced batts will go between the joists, in direct contact with the plates for the 1800s addition. Same approach for the cellar portion of the original 1700s structure.
The flooring in the original structure is rough cut, so not all floor boards come in direct contact with the aluminum plates beneath the underside of the floor.
There is a crawl area under the original structure. Do you guys think it is worthwhile to install 1/2” polyiso (2” away from the underside of the subfloor, and about 1” from the tubing and plates) in the crawl area, in an attempt to lessen the heat flow into the crawl? The idea of only having batt insulation under the plates seems insufficient given the situation.
Comments
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How much batt insulation (R value) can you fit in ?
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.0 -
I would recommend this article on how to insulate crawlspaces:
It may turn out that the best way is to insulate the walls of the cellar and crawl space with spray foam. This article shows how:
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If that is the rim joist at the end of the logd, get at leas 3- 4" of insulation there. Cut some foamboard to fit as close as possible then use cans of spray foam to glue them in place. That can be a huge heat loss area.
I would just put fiberglass batts against the plates, 6" or more if the space is cold. You will need to make some custom length wires to hold he insulation in place between those logs?
How thick is the floor build up, the planks plus the finish flooring?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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hot_rod,
The finished floor height is 1-3/8” in the original structure, and 7/8” in the addition.
It is the “rim joist” at the end of the log. The timbers are 7x7s. So, maybe 2” rigid foam (versus 3-4”).
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Ironman,
I can fit a little over 7” of batt insulation. I have been using 5.5” R-21batts so far. I thought about adding a layer of rigid foam beneath the batts, but I wanted to see if I could get away with just the batts.
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try the batts and see how warm the space becomes. That will give you some idea
Heat travels to cold, driven by the temperature difference
That thick floor may take some high temperature . So more incentive for heat to go down
Was there a heatload calc and design done?
With those log joists you will not get 8” spacing across the floor, so hopefully you get enough btus delivered to the space
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
hot_rod,
Ran load calcs (and did a blower door test). Conservatively speaking, building heat load is roughly 70K Btu. 1st story heat load is roughly 30K Btu.
The rooms in the original structure (with the log joists) are mainly entertaining spaces, so I can live with the output being less than ideal.
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Foam board insulation underneath the batts (and round joists!) would be ideal—with taped seams too. Definitely put some kind of foam (board or spray) on the rim-joist beam like hot_rod said. There is significant unheated floor sq./ft. area with those big beams. The present 8" o.c. plate-spacing" is kind of OPEN to interpretation… shall we say.
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psb75,
So, fill the entire joist cavities with batt insulation? No air space between the batts and the rigid foam, correct?0 -
no air space required with transfer plates
Your heat transfer is by conduction, the tighter the plates the better. With plank floors that is a challenge
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
The foam board attached to the joists are essentially and insulated floor beneath your heating plates and batt insulation. The space below the foam boards should almost be considered as "outdoors" as it is an unconditioned crawlspace.
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How about adding more fasteners to the aluminum plates with large washers to increase the contact area?
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That is only adding more metal mass to the plates. Not increasing contact area.
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I would have installed 3 loops per joist bay instead of 2. This project may require a supplemental wall panel radiator to offset floor load. The floor will require some serious temps to reach 85 degrees. I'd be interested to know what that is….?
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Unless the new fasteners are able to apply enough force to bend the metal into contacting the wood in more places.
I was thinking there must be some flexible heat-transfer material out there that could be put between the plates and the wood, like what is done with heat transfer compound and/or pads on circuit boards.
NJ Steam Homeowner.
Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
See my sight glass boiler videos: https://bit.ly/3sZW1el0 -
some graphite paper of various thickness. But is it worth it?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Interesting structure .. being an old house guy I have run into a few "logs" … never that large !
IMO — More should be done to better insulate the crawl space. How big is it? .. height? What is on the floor?
Most of my projects have been in PA … Those stone walls are heat sinks (and they leak). I can see you have foamed to minimize the leaks — did you get a price to spray foam the walls ? You can drop a lot of $$ today on those foam panels and it's going to use cases ($$) of foam cans to seal around the logs. Foam the walls and you are almost done. Is there any other heat in the structure ?
Did you do a heat load on the building / room above? You may have 8" between the plates but not across the floor. Here is a picture of an 1870's stone church conversion I did … this is the main living room floor up in the timber frame rafters of the building. The floor was already there (it was no longer an active church) when I bought the structure so the cost to do anything other than adding retrofit plates was too expensive. You can see how many plates I used — they were the heavy plates and I used the 3/8 tubing on advice from "hot rod" here. I have used plates before but never the 3/8 tubing. Anyway — in this case I was trying to make sure I got as much out of the those plates as possible because I'm only making one temp water from the boiler and the rest of the structure is Warmboard. There is a 6"/8" traditional 3/4" board beach floor above. The structure already had some foam as you can see in the picture. I had the foam guys come back and skim the old foam and foam the crawlspace that had not been done. In my case the house also has ducted AC — so I put in a furnace as well and made provisions for some heat in the crawl but it's never been needed. Foam done correctly is amazing. All the walls and floors were filled with traditional fiberglass batts. Batts up tight against the plates — that is the way I have always done the plates. Years ago there was a theory to use the AL faced batts and place the VB up against the plates .. I used that on my first project. I don't think you can even buy that today and I don't think I would if available.
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I was thinking more like a heat-transfer caulk or some kind or squeezable rubber-like material, but yeah I have no idea if it's worth it. I just thought there must be some kind of substance like this for less than perfectly flat underfloor material
NJ Steam Homeowner.
Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
See my sight glass boiler videos: https://bit.ly/3sZW1el0 -
In think Watts Radiant still offers graphite plates. They are flexible and do work better on rough, uneven surfaces.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
I was taught to seal the band joist or cap the bay's. Spray foam is best. Staple up a 1/4" bubble wrap 3" below the tubing to create a hot box and then install a fiberglass insulation or spray foam under bubble wrap to fill up the joist cavity. Black Onyx was great in the day you could staple up onto sheet alumium or by itself directly to the sub floor.
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That's a first … it's surprising the stuff out there that even with lots of sleuthing … gets missed.
Have used the AL undermount … I like working with the plates that snap hold the tubing. It also seems like they would transfer heat better from the pex
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those graphite plates have the aluminum component to grip the tube tightly.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0
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