radiant design for log cabin

lleo

Hi All, new user here, so happy to have discovered this forum.
I have a log cabin in the woods of upstate NY near Malone, built about 30 years ago. Previous owners used it as a 3-season property, but I would like to convert it to use it year-around.
It is 748 sq ft (22x34), built from milled, D shaped, 6" logs over a traditional vented cinder block crawlspace and 2x8 floor joists. The interior is all one space, except bathroom and small bedroom, with high ceilings (16 feet) and open loft.

I did multiple heat-loss calculations and getting a range of 35K to 70K, depending on the parameters I choose. As a log construction, however, there are a lot of unknowns, as detailed in many questions and posts on this forum. I spent so far about 2 weeks at the cabin in this cold season and have some first hand experience that may be relevant. Both times was able to make the place more than comfortable with a 1500 W/5000 Btu space heater in the crawlspace (vents closed), 5 x 1200 W/4000 Btu manually-controlled electric baseboards. I also used a wood stove with 1-2 fires a day, and the stove was needed to bring the heat up from freezing. The stove is rated at max 55K Btu, using dry cord wood. The outdoor temps when there, were from mid teens to high 20s. Of course, it also can get much colder there, with as low as -25F.

My plan is to add a concrete floor to the crawl space with rigid foam insulation underneath it, seal the vents and add rigid foam to the block walls, joist spaces, etc. Essentially bring the crawlspace into the envelope.

Given the relative accessibility of the crawlspace and the high ceilings (when one gets the stove really going, it can get very hot in the open loft area, measured over 80 F and had to open windows) I look to install radiant floor heat using extruded heat transfer plates below the sub-floor. Using 2 runs in each joint space, could fit about 1,100 feet of 1/2" PEX. This could deliver anywhere from 25K to 40K BTU/hr (30-50 BTU/hr/sq ft), although the subfloor is 1.5" pine board with 7.5mm foam-backed laminate flooring on top.

Not sure if my calculations and assumptions are right, but could be that this amount of heat will not be sufficient. I hope though that would be sufficient to keep the place from freezing over at anytime.
So sorry for the long post, thought to provide all the background and save you the need to ask, but here are my questions for which I could not handily found answers:

1. Is the overall heating output of radiant increased by the length of the pipe run? Such as instead of 8" OC would do 4" OC and a total of 1800 feet?

2. Given my setting above, the formula of of heat output=(floor temp-air temp) x 2.0 coefficient suggest that if the heat-loss is in reality 70K BTU/hr, once the interior temp drops to about 50 degrees, with a 85 degree floor temp, it could deliver ~70K BTU/hr? (85-50) x 2.0 = 70K

3. Lastly, read lot about insulating below heat transfer plates. My plan is to add a continuous 2" layer of foil-faced rigid foam below the joists, but no other insulation and make the joist space completely air-tight. Does this locked-in air create a heat-mass, which, once heated up, could deliver the heat as from a buffer? in case electricity is down , LP gas runs out, system malfunctions etc. to have a day before everything freezes over?

4. Any other ideas on increasing the radiant capacity? or to make up for the potential difference needed? Considering wiring the existing electric baseboards as emergency heat or replace them with hydronic radiators (could fit about 30 linear feet or so as separate loop, but this adds expense).

Thanks a lot, Lleo

hot_rod

The bugaboo with log construction is the infiltration, air leakage between the logs, around window and doors. You might have a blower door test done to confirm that number and show where those leaks are, to seal up.

Realistically 22- 25 BTU/ hr from a radiant floor, with ambient in the 68- 70° range. More from walls or ceiling radiant as the surface temperature can run hotter.

That subfloor dimension is working against you, and the foam backed laminate, probably R-2 or more of flooring? Any throw rugs will nick you also.

Skip the reflective insulation, not needed with conduction transfer, but do beef up to 6" batts at the very least, seal the ends well, spray foam.

8" on center is the best you can do in a 14" wide joist bay with transfer plates.

I've used kick space heaters to supplement radiant, but you need solid room by room load numbers to know for sure.

Tall log homes with lots of glass are one of the toughest loads.

lleo

Thank you Hot Rod,
From your response I gather that having say a 1000 feet of PEX pipe installed on extruded transfer plates or 2000 feet PEX pipe in the same surface area would not make a difference since they would be on the same temp. Correct? Technically the extruded plates are 4 inch wide or less, so 3 could be fitted in the approximately 14.5 inch space between 2 joists with room to spare.
I was aware of the downsides of my subfloor, the foam is about 1/8" thick, this is why I mentioned.
I would like to ask for clarification how the leakiness influence the heat-loss: the milled logs have a double tongue and groove on the opposing faces, and it seems some kind putty or caulk was used when the logs were placed over each other, so for a log constructions you cannot just 'peer' out. On the inside just about every wall surface, there is an extra layer of clear 3/4 inch tongue and groove pine laid up in various directions.
My heat-loss calculation variations were driven by this aspect, specifically choosing average construction or no insulation at all.

The volume of the structure is about 10,500 cubic feet what would an ACH50 of 5.0 or 10.0 would mean for me?

hot_rod

Softwood is around 1.2 per inch R value, so if you have an honest 6" with interior wood your walls are R-7.2, what value did you use?

6" fiberglass batts run R-19- 21 depending on the density, for comparison.

If you have an unencumbered 748 sq. ft X 25 BTU/ sq ft you could expect that floor to move 18,700 btu/hr or a bit more than 1/2 your small load number?
I doubt a 3rd tube in the joist spaces will get you over the hump. IR pics of ThermoFin for example show 8" on center pretty much warms the entire floor surface, especially with floor covering over the top which helps the temperature spread.

Remember any cabinets or furniture that goes to the floor like beds, kitchen cabinets, appliances, etc deducts out of that square footage. Called heat flux, divide out that square footage.

The biggest guesstimate is the leakage, you could pin that down or use a high guesstimate, but I think you are correct in assuming you will need more than the radiant floor to keep you warm and comfortable.

As much as I dislike forced convectors, you do get a lot of bang for your buck BTUwise with kickspace heaters or wall cabinets. You may only need them on the coldest design days.

Fin tube or panel rads are another option, but you need wall space for them. Maybe a small wood or pellet stove?

You need to massage that load number first, get as accurate as possible to see what kind of supplement; BTU you actually need.

Zman

The only heat losses I have seen that were over 30 btu/ft had 25' ceilings and windows to match. Even those were in the low 30's
I would be very surprised if your heat loss is over 22,500 in the house you describe. If it does work out that high, you need to work on roof insulation and air infiltration rather that more Btus.

Think about 2 runs of transfer plates per bay and leave the option for some panel radiators of you need a bit more.

The attached screenshot models the worst case scenario for underfloor. You can likely run way lower temps most of if not all of the time.

the_donut

Zman said:

The only heat losses I have seen that were over 30 btu/ft had 25' ceilings and windows to match. Even those were in the low 30's
I would be very surprised if your heat loss is over 22,500 in the house you describe. If it does work out that high, you need to work on roof insulation and air infiltration rather that more Btus.

Think about 2 runs of transfer plates per bay and leave the option for some panel radiators of you need a bit more.

The attached screenshot models the worst case scenario for underfloor. You can likely run way lower temps most of if not all of the time.

Every log cabin I have been in had infiltration on par with civil war era farm houses. Maybe your seams are tighter. Blower door test would be a good starting point.

lleo

thanks for all your comments. much appreciated.
As I think I will not be able to convince any building energy auditor to take on a 5+ mile dirt road to do me a blower door test in a frozen over building, this will have to wait until after spring...

a few things remained not completely clear. By using heat-loss calculators, as mentioned I get to about 35K loss. The calculators I have access to do not seem to adjust based on blower door results, and also have not such data. But I assume that some aspect of the infiltration is already factored in, perhaps based on the setting the construction type being tight, average or leaky. The 35K is based on this later.

Upon reading more on this, found a formula that used the building volume and the ACH number to estimate the BTU loss.
The formula is Q= Building Volume * ACH # * 0.018 * Delta T - so in my case 10500 cubic ft * ACH 1 * 0.018 * 70 (inside 70 F - outside 0 F) = 13,230 BTU [ACH 1 is ACH50 10.0] so 10 x volume exchanges]

I get high loss numbers by selecting no insulation in the floor, which is now the case, but once I bring the crawlspace into the envelope, will no longer be true. Based on my limited inspection I can see that there is fiberglass in the ceilings, but unable to check how much. Compared to when this was built, unlikely they went bare. Anecdotally I know that previous owners spent at least one winter there with just the stove and 5 electric baseboards (~20K BTU) and worked for me as well, albeit not tested at design temps.

This conversation convinced me to plan for some panel radiators, perhaps to replace the electric baseboards, it would be about the same footprint. I had hoped to not need a multi temperature setup. I simply have not enough wall space to fit more than 25-28 feet of radiator and even those are at most 15-16 inch tall, so cannot do all radiators and the radiant floor may not be enough.

the_donut

Most calculators have a cfm of air setting or tightness option. May also be called # of air changes per hour.The results from the door test could be entered here.

The Higher the air leakage, the lower the tightness and higher cfm of air or air changes per hour.

hot_rod

You certainly want to run the load calc within insulation upgrades included. As Zman mentioned, a load in excess of 30 btu/ 'ft would be rare for what you are describing. 748 X 30= 22,440 BTU/ hr at design perhaps?

There are loss programs that allow you to input actual infiltration numbers, but since you do not know, assume worse case just for a starting point.

With that thick subfloor you may be running some high SWT and panel rads could possibly work on the same SWT.

There are hydronic savy contractors in that neck of the woods, or consider paying one of the consultants that advertise here for a load calc and design. That would spell out numbers for the parts required in addition to a load calc number.

vibert_c

@lleo
Please outline further how you are going to occupy this log building. Will it be visited on weekends only; or lived in for a month at a time.
What is the present height of the crawl space?
You say you want to heat the crawl space; for how long? One weekend, all month or what? Do you have to keep plumbing from freezing all winter?

It has been my experience that radiant is the best concept to keep from overheating the loft when it is open. Once you have experienced the concept of radiant you will be astounded.

In the middle of the cold winter I got on a step ladder to go up to the high ceiling to change a bulb in the light fixture. When I reached the 3rd step on the ladder I could see my breath in the room air. Things were perfectly comfortable at floor level.

I reside about 1.5 hours north of Malone. I have a building similar to yours built in 1946.

vibert_c

lleo

@hot rod
the structure is not far from what was described by ZMan, lots of glass. about 330 sqft of glass, double pane though, but original to structure, Andersen 400 series or like; there are also 3*6'x2' skylights, 2 windowed entry doors. I can get big differences depending what I choose for ceiling. There is no accessible attic, so whether there is no insulation, 4 inch, 6 inch or more is a guess. The rafters are most likely 2x8 as about 10-12 inches can be measured inside the skylight, but those protrude above the roof surface. I see fiberglass next to the only electrical box in the ceiling, but again this is by looking into a 1/4 inch gap...

@vibert_c
good questions: for now it is a second home, with occasional weekend use, that is maybe once a month...
But could see more, or become a winter-long residence in few years. I have a few 'neighbourghs' one is getting through their first winter, will be interesting to get their accounts. I know radiant well, as I grew up on it, on the other side of the pond... even after 20 years, not fond of forced air, but please don't tell this to folks who talk-hvac, they were also helpful.
The crawlspace is just an inch over 4 feet, once everything is thawed out, I will need to see if could deepen and insulate, just to provide freeze coverage for pipes and maybe some storage. Do not want to heat it more then that.
For now as a seasonal property is drained and winterized

hot_rod

I bit the bullet and insulated my crawl space a few years back. I used the InsulTarp insulation that some use under slabs. With the white side up it really brightened up the space. My space is only 18- 24" so it was a PITA.

I wrapped the insulation up to the bottom of the joist covering the footing and foundation. I also block off the crawl space vents in the winter now that the crawl is insulated and vapor barriered.

I'd look to lower losses like that wherever you can. Plenty of insulated window coverings available for that glass. I use a hollow tube pleated type that easily roll and let a bit of light through. blinds.com will make any style and size.

noran01

@lleo I stumbled across this thread as I am looking to crunch the same numbers. I have a very similar setup (in Susquehanna, PA) and just recently finished the slab in the basement (10 mil vapor barrier and 2" XPS foam along the floor and stem - walls/sill is sprayed with ~4" closed cell foam). Encased in this are our loops as I plan to finish the basement. I also plan on adding radiant, via transfer plates, to my first floor; similar setup as its small bedroom, kitchen, and bathroom taking up half the first floor with a loft above (~18ft ceilings) while the other half is living space and cathedral ceiling.

I'm stuck with electric as being the only option (propane is way to expensive). I'm planning on a closed system (for freeze protection), but do plan on keeping it running throughout the cold months to keep the space at ~50F.

I was looking at https://www.ecomfort.com/heating/electro-industries-boilers.html but am having a hard time with ensuring my numbers are accurate.

I'd love to know how you made out!

Zman

I would be surprised if propane is more expensive than electric in PA. The attached spreadsheet will allow you to compare.
In either event, if you plan on being in the house for a long time, an air to water heat pump would save you a ton of money long term.
https://www.pmengineer.com/articles/94022-john-siegenthaler-renewable-heating-design

noran01

@Zman I'm not just including the cost of the propane itself, but also initial install and overall availability for delivery due to where I am located.
I've read a number of these heat pump articles and they do make sense. What I have not seen is the ability to procure one of these units in the US though. He references two heat pump configurations in the article but doesn't provide any detail on either (Heat pump No. 2 was a cold-climate air-to-water heat pump with a seasonal average COP of 2.5.) - any ideas?

Zman

This is the product he is using. You might try reaching out to the manufacture. http://spacepak.com/hydronic-heating-and-cooling