DIY Radiant heat in slab questions

amo675

Hi,

So long story, I am converting a 1 story brooklyn warehouse into rentable artist studios, one of the renovations is a tear up and repour of the concrete slab. Right now we are in the process of designing a radiant system to handle the heating requirements of the space. I feel fairly confident with the current plan but there are a few areas i am not so sure of and cannot seem to find concrete (no pun intended answers.


Here are my main concerns:

Supply line size
Boiler/pump location
Oxygen barrier pex or not
Pex spacing & design pattern
Concrete pour thickness: 6"
Expansion joint pex treatment


Well i might as well detail the plan, here goes:


At present: The slab has been removed, plumbing work completed and grade is being compacted

We plan on installing
10 mil vapor barrier
1 in rigid foam insulation + 1 in “crete heat” insulation + radiant nubs to speed and ease insulation
2 in foam thermal break between slab and wall, 1 ft above and below slab on ext walls

we were planning on using a 3/4" pump and running 3/4” pex supply through the walls to each studio to be heated as a supply, at which point the manifold would step down to 1/2” in the floors spaced at 9” apart and then a 1/2” return to the boiler room

the boiler room would be located in a small basement that can be seen in the lower right of the drawing, the basement is uninsulated but has a gas line and is directly below the chimney as it seems to have served as the boiler room for a previous itiration of the building.

each room would have it’s own zone and thermostat, and we were considering having 2 zones in the largest space on the right (in the drawing) we would love to make this as tech saavy as possible, and have been looking at thermostats that can be controlled online

the dashed lines in the drawing are 6x6 acrylic skylights to be put in

the builing is a 1 story warehouse, that is about 2’6” above grade on one side and meets the grade as we get to the other side (where the boiler room would be located)

the ceilings are 13’

currently the walls and ceiling have r-19 insulation but we are considering changing the ceiling insulation to r-35 or higher, the only thing holding us back is the old wives tale in brooklyn that you want some heat loss through the roof to prevent hydrostatic shock on the few days that might occur, to me that seems like a huge price to pay for the potentially 2-3 days a year that might occur, i was wondering if it would be an option to throw a zone in the roof to be turned on ONLY on those days as a snow melt.

we were also wondering if it would be better to have the radiant boiler to supply the building hot water, or use a regular hot water heater. we have roughly a dozen sinks and one shower, so the hot water demands would not be extremely high if that’s something to consider

thanks so much in advance for any help, if there is someone that would like to dicuss this project in more detail i would be open to hiring a consultant for more detailed assistance.

I Am anxious to get the pipes in the floor and the slab poured, but the boiler and pump setup can be done later as we currently have 2 operating modine blowers in the space.

Snowmelt

all nice but no heat loss done on structure so i don't know what you need as far as pipe size or how many loops

Gordy

The heat loss is the first step. This dictates flow rates, supply temps, tube spacing, and tube size, circ size etc, etc.

We can not recommend that 3/4" piping is sufficient to supply the loops at the manifold with out the fore mentioned info. I can tell you that 3/4" pex will carry 42000 btus. I can also tell you 1/2" return to the boiler is not sufficient. Especially if think your supply is 3/4".

You are off on the right foot with the slab detail.

amo675

Could you reccommend someone that would be able to do that? We are quick learners but have been having trouble with the heat loss calc. Part of the problem is the fact that we are building in exhaust fans to be used on demand, if there are fumes in the rooms that need ventilation. Also we have never done heat loss calc's before and it is making my brain hurt

Gordy

Use the slant fin program it's free. Just input numbers. Be very diligent with the numbers. Or go to find a pro on this site may be one in your area.

amo675

Ok thankyou! i will check out slant fin.

Rich_49

You will need to account for the CFM those fans move when exhausting . Crete heat and 1" is not your best assembly , the tubing will be at the bottom of the pour and the entire install will suffer . Tubing is best in the middle of the pour . Look into BarrierX5 , it is R10.3 when there is a six " stone detail below , it is also vapor barrier with the adhesive right on the product . There will be less seems and the assembly will be superior .

Rich_49

Look at Barrier X5 for your insulation . Less first cost and the assembly you are thinking of will have the tubing at the bottom of the pour as opposed to in the middle where it will work best . You will need to account for the ACH when those fans are running also . ACH is mostly overlooked and is a killer , Have seen lots of jobs go very far South when this was overlooked .

You only usually get one chance to do this right, do not waste it . If you'd like you can contact me , phone call is free .

Gordy

Rich,
I'm finding the barriers r value charts a little hocus pocus. While the cost, user friendly roll of vapor barrier/insulation is a time saver. When insulation products,start using sub grade details as part of their r value.......hmmmm.

That would mean 1" exps is really an r 15 with 6" stone sub grade.......

To many variables to affect r value in a sub grade detail. Stone compaction. Moisture content, fines in stone type etc. no mention of a pea gravel r value which is used quite often in my area.

Rich_49

Well Gordy , Many folks ignore ASSEMBLY details . What can I say , they readily state a material only value of R 5 . People cannot ignore assemblies any longer . Too many folks think they have a wall R value of 19 when in actuality the assembly value is much less due to thermal bridging and other factors . Maybe that's why we find so much fudge in calcs and everything else , overcompensation . I assure you that no Hocus Pocus is going on with NOFPs testing and reporting . Would you think that 1" EPS has an R value of 5 as stated while NOFP claims 5 at a 1/4" thicker product . Maybe others are promoting Hocus Pocus .

Why would 1" EPS be 15 when 1 1/4 is R 10.3 ? Don't understand the logic there . Remember also that this product has a permeance rating that is factored in . The vapor barrier is for real also . Last year after we installed we experienced a rain storm which effectively put 4" of water in our basement PAN . 2 sunny days waiting only took it down about a 1/2 inch . We finally had to pump it .

Zman

You are getting some good feedback here on the radiant heating part.
It appears that you are considering the ventilation side of this as an afterthought. Keep in mind that any air that you remove from the building ends up being replaced by outdoor air. You need to provide for this and figure out how you are going to heat or cool that air. Will you need AC in the summer?
To me a job like this is more of a forced air job with the addition of radiant floors for comfort.
I would think that a project like this will be considered a commercial space and be subject to the requirements of the mechanical code.
Finding a local professional to provide consultation would help answer some of the "big picture"questions.
Carl

Gordy

Rich said:

Why would 1" EPS be 15 when 1 1/4 is R 10.3 ? Don't understand the logic there . Remember also that this product has a permeance rating that is factored in . The vapor barrier is for real also . Last year after we installed we experienced a rain storm which effectively put 4" of water in our basement PAN . 2 sunny days waiting only took it down about a 1/2 inch . We finally had to pump it .

Sorry rich meant Xps being r 10 with subgrade assembly not r 15. Miss typed.

Paul Pollets

R10 ridgid foam is recommended, rather than R5. Perimeter insulation is also used to create a thermal break at the foundation wall. Zoning a slab stops the flow and performance is somewhat compromised. Constant circulation and variable temperatures work best. Most pros know how to do this. Software from the radiant manufacturers (Uponor; Viega) takes the guesswork out of the design. The Slant Fin app does not calculate radiant design, only baseboard or radiators. There is often a 30-40% difference in loads when using the SF calc, rather than a radiant load software program.

Zman

What kind of work is being done in the studios? Kilns? Airbrushing? Brush painting?
The air change side of this is huge.
As much as I love radiant, you are focusing on the wrong part.
A high mass micro zoned radiant system may not be appropriate for the application.

Rich_49

You may also want to entertain the thought of something other than exhaust fans . Those fans if they move alot of air have to bring new air into the building from outside . Maybe a balanced ventilation system would be more well suited to the energy usage and the artisans well being while plying their craft and healthier for them , the energy bills and the building .

amo675

Hi, ok so on the exhaust issue, i have been considering dropping the fans as they are rarely used but nice to have for the one or two times a year you need them i guess my thinking was just that since the skylights are not vented, theres no way to get spray/glue/airbrush fumes out, since the ac will be handled by mini splits.

I am confused as to the issue with the depth of the pex in the slab, everything i have seen says that its not impereative to have it in the middle of the slab it if the slab to grade is well insulated the only downside being it will take longer to heat. Is this mistaken

Gordy

Depth has to do with response yes. It will take longer to heat, and fly wheel after its satisfied. If it's in the middle the heat will reach the top of the slab faster, and be farther from the insulation. But then this becomes a mute point if the slab is on line maintaining set point verses a cold start up, or set back conditions. Obviously the thicker the slab the more it becomes an issue.

Zman

Are there any permits required for this and if so what are the applicable codes? I would hate to have you find out after the fact that ventilation is required.

SWEI

The general recommendation to prevent cracking is to keep the top of the tubes at least 1-1/2" below the top of the pour. Once you factor in the 1/2" - 5/8" OD of typical in-floor tubing, you end up about two inches down -- which just happens to be the middle of a typical 4" residential slab. If the slab gets thicker, you don't want the tubing further away from the heated space. Keep it 1-1/2" to 2" down and things work a lot better. Responsiveness of course, but it also helps reduce back losses.

amo675

Ok so i know everyone's gonna yell at me for this but part of the thinking for putting it at the base of a 6" pour was that if anyone wanted to attach something to the floor there would be a ~4" safe zone

SWEI

Smart contractors use thermal imaging cameras for this.