Retrofitting old home with radiant heat: suspended slab or panels

Zad

Hello,

We are a couple months away from starting a retrofit on our 110 year old home to radiant in floor heat and I am hoping someone here can help me. The decision I am wrestling with is whether to do a suspended slab or go with one of the many available panel systems. Let me start by saying that the floor is not level. When the home was built in 1904 they didn't support the floors properly (this has since been corrected) and as a consequence there are some visible elevation changes on the floor. I haven't put a transit on it yet but if I were to guess I would say we are out about 1 to 2 inches from high to low. It is for this reason that I am leaning toward pouring a suspended slab. Engineering tells me we are good to go with floor loading. The house has clear old growth fir joists 9.75" by 2.75" on 16" centres with a max span of 11' translating into 160 lb per square foot load capacity. Pouring a slab provides an easy solution to the non-level floor. I am not so happy about the mess and added moisture of concrete, but it might be worth it just to easily deal with the elevation changes and to give me the thermal mass. The panels are appealing for the same reasons cement is less so. However, if we go with one of the panel systems then I will have to level the floor first which would likely mean stripping off the 1"x8" fir subfloor, scabbing onto the existing joists to level, re-sheeting and then putting the panels on. By using Warmboard I could just re-sheet with that product and be done. None-the-less, at the end of the day I still think concrete will be less costly and less work but I still have some hesitation about going that route. Losing a couple inches ceiling height is not an issue as we are at 9'6". I am hoping by posting here that someone will be able to point out something I am missing, suggest an alternative that I haven't thought of, or point me in the best direction given my situation.

Gordy

Gypcrete

If it were me pouring gypcrete solves your leveling issues everything is done in one step. So long as engineering says the floors can support the additional load.



What's the finish floor choice going to be ?



Just member your low spots will have additional load due to the additional thickness of gypcrete. So if you have a 2" elevation difference then that floor thickness will be 2" plus the 1 1/4 min gypcrete thickness. Does who ever did the engineering realize this?

Zad

engineered hw

I am thinking glue down engineered hardwood for everywhere but the bathrooms and tile there. Unfortunately no gypcrete supplier where I live so either I go regular concrete or hire guys from 4 hours away for gypcrete (about $10K). Ya, there will be areas thicker and we calculated for 3" concrete. If I go this route, I plan to find the high point in the floor just cover the tubing at that point to prevent excess loading as you describe. Thanks for your response.

jumper

you didn't ask but

Have you considered ceiling radiant? For one thing you can vary temperature of heating water more widely.

Zad

No, I haven't

considered ceiling radiant and I don't know the first thing about it. Is it a tubing system in

the ceiling joist spaces? Or are you talking about electric infrared panels?

Zman

Options

I think your concrete option is pretty good. The thermal mass is usually easily managed using an accurate outdoor reset curve. Unless you have a lot of solar gain, you should be fine.

I also think radiant ceilings are worth a look. They eliminate the potential for many problems. Overheating and cracking of wood floor, insufficient heating due to carpet and furniture placement and solar overshoot, to name just a few.

This manual provides a great resource. http://www.uponor-usa.com/~/media/Extranet/Files/CDAM/CDAM_Manual_7thEd_0711_Cover.aspx?sc_lang=en&version=072920110236

Carl

jumper

ceiling is safer

If floor radiant is inadequate it's too bad. Period. With ceiling radiant you can increase temperature of water. I bet many with floor radiant are buying supplementary heaters this year with its extra cold weather. Ceiling is probably cheaper if you can find a strong young plasterer.

Hydro

Old House Radiant

I agree with Gordy, Gypcrete would be my #1 choice because of the way it holds the heat, and levels the floor, but I have seen it pose problems with interior elevations, like doors, windows, outlets, etc. But where I am, most of my installs here are suspended tubing, and many just because of the labor and materials savings. My own house was 2 3/4" out across my kitchen, jacked it up, added supports, got it to a 1/2", added my suspended tubing, going through 3/4 rough sown oak, with 3/4 maple on top, (my house is 144 years old) works like a champ. It has been in for 9 years, no supplemental heat, and it gets cold here in MA. ( it was seven degrees here one day last week.) Don't be afraid of suspended systems.1

Gordy

Radiant ceilings

Ya know I keep refraining from throwing that out there, and I have radiant ceilings as well as floors. The boys are right that might be your best bet if your going that far into your recon that doing the ceilings in radiant is an option. I mostly don't mention it when some one has a plan on what they want to do for radiant.



On the other hand if you want to level the floors in the manner you pursuing its fine.



You may contemplating both also floors in the bath areas or tiled areas.



The difference is radiant ceilings can run as high as 120* if need be to satisfy the heat loss ( seldom need that high of temp). Where as radiant floors are limited to 85* due to foot comfort! and certain flooring materials. At that temp you are limited to about 30-35 btus a sf. With radiant ceilings you don't have to worry about floor covering revalues decreasing output, or furniture.



It's all in the heat loss calculation.

SWEI

Suspended tubing?

or staple-up?  Or plates?

Hydro

tubing

I know we call it staple up, but its really not stapled directly to the wood, and in my heat loss programs, and in the schooling I have had, the installation was called suspended tubing. I only use plates when called for in my heat loss software, usually not too often. My home did not require any.

SWEI

Plates are good

Thus far, I have only seen one job where they were not worth the trouble.



Are you truly talking about suspended tube (as opposed to staple-up)?  We make a portion our income remediating those and for the life of me I can not imagine actually installing one.

ChicagoCooperator

Ceiling Height Limit

What is the limitation on ceiling height for in-ceiling radiant? 

Zad

Bekotec from Schluter

I just learned about a product that will reduce the weight of the skim coat by 50% down to 12.5 lbs/sq ft.. Bekotec from Schluter is a modular screed system that can be topped with either ditra and tile or glue down hardwood (I called tech support on the latter as it is not listed on their website). It solves a number of problems for me namely weight and sealing the floor so the SCC doesn't drain through the cracks. If at all possible I would like to stay with in floor radiant as it will be much, much easier for this particular application. My main worry has been the weight and our engineer is crunching the numbers now on it but based on my floor load calculation there is plenty enough strength in the floor. Thanks for all the input and I will let you know how the project unfolds.

Zad

Engineer finished

the floor calculation and with the joists and spacing and span of 14'4" the floor will be at 73% of capacity with 15 lbs per square foot in concrete and floor materials on the floor, taking into account live and dead loads. Good news! He is doing up the documents now.

Zad

The engineer

has now done a site visit and was very impressed with the quality of the materials used to build our house. He has given us a max concrete weight of 25 lbs/sq as it stands now and with the addition of one beam, 37 lbs/sq. If we can get away with 1.5" then I think I will not use the Bekotec however if the floor is severely out of level I might, depending how close I can get it using sheets of xps foam insulation.

Mark Eatherton

Don't forget to insulate the joist bays...

Or you will discover that heat not only rises, but also goes downward, causing the upper space to underheat and the lower space to overheat.



Your choice of WarmBoard is an excellent choice and will result in significantly lower operating fluid temperatures which equals $aving$.



ME