Image Credit: Ed Welch
Image Credit: Ed Welch Bleeding heat. Taken from the exterior of Welch's house, this infrared photograph shows the high temperature of his radiating rim joist.
Image Credit: Ed Welch
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Five years ago, I completed a green remodel of my family’s home. We included green features like solar electric panels, a solar hot water system, recycled lumber, Trex decking, and many other green products. But since the remodeling work was completed, we’ve had years of consistently high propane bills, and I began to question whether my house is actually any greener or more sustainable that the average home.
For the past three years, our home has consumed between 1,300 and 1,500 gallons of propane per year. That’s “sustainable,” all right — for the oil industry! I have since learned that while using recycled, reclaimed, and renewable products is a noble green building objective, it’s more important to produce an energy-efficient, durable, healthy building that conserves water. Once that’s done, you can supplement your energy with renewables and fill in with as many green products as you like.
An energy audit
Frustrated by our high energy bills, we hired Balance Point Home Performance Contractors to test our home and help devise strategies for reducing our energy use. These energy auditors performed a blower-door test to determine shell leakage, used infrared thermography to locate leaks, calculated the required heating and cooling loads according to Manual J from the Air Conditioning Contractors of America (ACCA), tested the safety of our combustion appliances, and measured the actual efficiency of the heating and cooling systems.
When we remodeled our house, we had chosen to install a radiant floor heating system. Although it was more expensive than other heating systems, it was supposed to be more comfortable and more energy efficient. The radiant tubing was installed in the joist bays below our subfloor. The HVAC contractor chose the less-efficient method of hanging the tubing in the bays instead of stapling it up to the subfloor with heat transfer plates. The contractor also connected the tubing to our uninsulated rim joists, which does a great job of radiating heat to the outdoors. I was advised to use foil-backed fiberglass batts underneath the PEX tubing to reflect heat upward. That’s fine, if you want to keep all the critters warm and cozy in your crawl space — because the fiberglass batts leak heat like a sieve.
Furthermore, it turns out that our boiler was considerably oversized. According to the Manual J calculations, our peak heating load is 46,000 Btu/h, but somehow we ended up with a 90,000 Btu/h boiler — twice the necessary size.
Fixing these problems won’t be cheap
Now, we’re facing the need to downsize the boiler, remove the tubing connections from the rim joists, add heat transfer plates in our joist bays, and install rigid foam insulation under our PEX tubing and at the interior of our rim joists. Then I’ll need to air seal the whole assembly. What an incredible waste of more energy — my energy!
The solar hot water system with a tankless water heater as backup has never operated effectively. Solar hot water systems will likely be standard practice someday, but for now they are expensive, and most plumbers aren’t experienced in the correct installation of solar equipment.
In retrospect, our tankless water heater was a mistake. Some tankless systems are designed for solar backup, but we mistakenly chose a model that was not designed to accept preheated (solar-heated) water. In general, tankless water heaters provide minimal energy savings, cost about three times as much as a storage unit, and actually waste more water than a storage unit due to the delay in firing. They do, however, produce an endless supply of hot water. But, how is that a green selling point in water-challenged California?
Stopping air leaks
Building shell leakage, in my opinion, are responsible for some of the worst energy losses in most homes. Air leaks are caused by builders’ lack of knowledge and poor workmanship. Air leaks into attics can be easily reduced by using spray foam to seal ceiling leaks and by installing an air barrier on the back side of kneewalls. Air sealing roof vent channels and interstitial cavities is not hard work; it just needs to be done. Reducing shell leakage is probably more important, more effective, and less expensive than the majority of green products on the market.
With the 50% California rebate we received in 2004, our photovoltaic (PV) system might appear as a grand success. But it will take approximately 15 years for the PV system to repay itself. It might be a fine investment for those with a big budget, but PV should be way down the list when money is limited. We cannot simply buy sustainability; we must achieve sustainability through careful workmanship throughout the building process.
On the positive side, we did make some smart green choices. We used structural insulated panels (SIPs) for our addition, and they have a high R-value and perform well. The Loewen casement windows also function well with minimal air leakage. And we love our interior cabinets, trim, and doors, all made from recycled Douglas fir and cherry certified by the Forest Stewardship Council (FSC).
Indoor air quality must be carefully considered when building a new home or remodeling. Tighter homes require a good mechanical ventilation system. Get fresh, filtered air from clean places, not stale air leakage from crawl spaces and attics. Test your home for radon. Test all gas appliances for combustion safety. Use carbon monoxide detectors, and be careful if your garage is attached to the house. Car exhaust and other toxins can leak into your home when it is under a negative pressure.
All new equipment needs to be commissioned
Finally, don’t assume that any particular building system or mechanical system will automatically be energy efficient. Regardless of the type of equipment you choose, any system can either perform beautifully or perform pathetically. Be sure that any work you do is tested by a home-performance contractor to be sure it performs to energy efficient, healthy standards. I wish I had known that five years ago.
— E. Conrad Welch owns Dovetail Construction and works as a green building consultant.
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30 Comments
Fixing the underfloor insulation...
Just curious....I've been advised by a number of people that the best fix is to remove the fiberglass insulation, add heat transfer plates to attach the tubing up to the subfloor, insert rigid foam blocks behind the PEX at all rim joist locations, then cut strips of rigid foam (probably 2-3" polyisocyanurate) to be inserted within each joist bay (leaving a small space for the PEX, maybe 2"), air seal around the entire assembly, then reinstall the existing fiberglass because it is already there. It will be grubby, hard work since half the house has only about 12" of crawlspace! Yikes, I am not looking forward to it.
But, someone else suggested just sealing the existing vents, insulating the foundation walls with 4" foam on the inside, add about 2 feet of rigid foam on the ground around the interior perimeter.....10 mil plastic over the ground, etc. Create conditioned space in the crawlspace area, eliminating the need for the more difficult work within each joist bay. Balance Point Performance Contractors don't think it work very effectively because of the Delta T between the conditioned crawlspace and the 50 degree earth....essentially we would waste a lot of energy trying to heat the earth. Any other opinions?
Still another option
Ed,
Here's a third option: insulate the rim joists, but don't disturb the fiberglass under the PEX. Once the rim joists are insulated, you could install continuous rigid foam under the fiberglass batts, attached to the bottom of the joists, with the seams sealed with contractors' tape. That's a lot easier than putting the foam above the fiberglass.
After that work is done, you can take a break. You still might want to insulate the crawl space walls at a later date... but that work would be less pressing if you first improve the insulation under the PEX.
Continuous Rigid Foam idea on the bottom of the joists
Yes, Martin, I had thought about that option. Not sure exactly why I discounted it. Maybe I like harder work! I would probably attach the foam with fender washers and screws so it would not fall down in the future. What about the heat transfer plates? Just forget about them?
I've often wondered about crawlspaces where fiberglass batts are used in a vented foundation. Seems like the insulation would be 'wind washed' like crazy, making it virtually useless. In this case, the insulation is at the top of the assembly, while in a vented attic, the exposed fiberglass is at the bottom of the assembly...hopefully making it more effective.
Recycled lumber=Energy Efficiency??
Recycled lumber and Trex decking has nothing to do with energy efficiency. That is like asking your solar electric system to fold your laundry... what did you expect?
This article is a good example of what not to do. Don't use uneducated contractors, research the products you choose for your home, don't install renewables before making your home as efficient as possible, and don't expect your solar electric system to fold your laundry!
And we wonder why people think green building is expensive, when you don't go about it right you'll be wasting a lot of green!
Thanks for the self righteous input, Anonymous
You're right, Anonymous, recycled lumber and Trex has nothing to do with energy efficiency. But it does drastically impact the budget.....exactly my point....learning to use green dollars more effectively. I'm willingly admitting my mistakes or I never would have written the article. You seem to know it all, so why waste your time on such forums.
Hard Lessons
Ed,
Sounds like you learned a lot of hard lessons about "green" building. Ideally, "green" should be simple and straightforward, starting with a healthy and efficient envelope. All the rest - solar thermal and electric systems - are optional extras that may unnecessarly complicate the house system and add to maintenance and replacement costs.
You don't indicate what part of CA or what kind of climate you're in or how big the house is, but 46,000 btu/hr peak load is not a particularly efficient house. The last house I built, here in 8500 DD Vermont, has a peak heating load of 16,000 btu/hr for its nearly 2,000 sf and an annual heat load of less than 300 gallons of propane. And this was a low-tech home that cost well below the area average.
As for your radiant floor system, I would agree with Martin about using underjoist rigid foam board to isolate the floor system from the ground under the crawl space and from air movement. The suspended radiant tubing is a fine system and heat distribution plates are not necessary, but the tubing shouldn't be contacting any framing (it should be suspended from hangers between the joists and there has to be a radiant barrier no more than 2" under the tubing. A 4" air space in a floor with low-emissivity foil on one side provides an R-9 insulation equivalent. I use bubble-foil for under the tubing, which is a more effective radiant barrier than foil-faced fiberglass. But additional insulation is necessary to separate the radiant space from the cold crawl space and the joists need to be radiantly isolated from the ground (especially since they are closer than the code-required 18").
Great post. Thanks for the openness.
Great article, Ed. I appreciate your willingness to share so we can all learn from your experience.
Incidentally, you may not need to buy a different boiler. If I understand correctly, many low-mass boilers don't lose that much efficiency when they are sized incorrectly. (Though of course if you were starting over, buying the right size would make more sense.)
Here's a wacky thought: depending on the water temperature, size, and some other factors, your instantaneous hot water heater might be repurposed as a decent floor heating boiler, though there are a number of factors involved in that setup which may make it a bad idea. Hopefully you've found someone who knows what they're talking about with the radiant heat and they can help you figure out some of these things!
Lastly, I wouldn't be to hard on yourself. Five years ago many of us in temperate climates were not up on the real home performance issues in existing houses. The word is getting around, and your post will help people see the light.
Tahoe region in CA
Robert,
Thanks for your input. We live in the foothills of the Sierra Nevada Mountains, about 2500', near Lake Tahoe.....cold, but not severe....not sure about the heating degree days....I need to find out that number. The house is 2700 square ft. 46000 BTU....not so efficient? I'm not sure how to assess that. We have good windows, R-19 fiberglass in the walls, cathedral ceilings with R-30, decent southern exposure, etc. My Title 24 engineers automatically added 30% to the 46,000 (for extreme weather fluctuation days)...he originally calculated 52,000 BTUs + 30% bringing it to 68,000...and the HVAC contractor covered that with a 90,000 BTU furnace. Go figure...we got an oversized boiler, no matter what.
So you think it would be just as efficient to install rigid foam on the underside of the joists as opposed to installing strips within the joist bays. That would certainly be easier. And you don't think the heat transfer plates would be necessary.....also, easier.
Anyway, all still a pain in the neck.....yes, I have learned a lot about building an energy efficient home.
heat transfer plate alternative
another option that could relly improve the performance woud be to use 1/2" pipe straps to press the PEX tightly against the underside of the sub floor. We've had good success with this. They go in with 1/2" hex head sheet metal screws very quickly (great application for one of those tiny little Bosch drills) and you can easily carry a pile of them in a bag that you lay on your belly as you squirm around in that 12" space. After that putting the foam on the underside of the joists makes a lot of sense to me and I agree with Robert (again! that's twice in a week!) that there is little penalty in the over-sizing of that boiler. Maybe you can put in an indirect water heater off the excess boiler capacity and sell the demand water heater to a friend. No problem with using a solar water heater as a pre-heater to an indirect heater.
Thanks for sharing your hard lesson. Many of us have learned the hard way about unintended consequences of well intentioned actions, in energy efficiency especially. I was under a house today servicing a project we did about five years ago with one of my employees and we were just shaking our heads at how far we've come and how much we continue to learn and to improve every year. It's just part of that continual improvement and re-education we are all here for.
Ouch!
Ed,
Sorry to hear that your "Green" experience has been so expensive and frustrating. Good luck with renovating the renovation.
Green lessons
Thanks everyone. I admit being the victim of good marketing programs. I did not know better and fell for the 'buy sustainability' movement. I think we all need to help direct green building priorities....towards better, smarter building practices.....all working towards healthier, more durable, more energy efficient structures. Right now, I'm stressing energy efficiency because of obvious reasons....my failures! And because of climate change, etc.....annual energy bills that keep coming......until you fix/reduce the problems.
Michael.....so you think heat transfer plates are necessary to make the system much more efficient....hmmm....that was my original thought as well....but just screwing rigid foam to the bottom of the joists would be much easier rather than removing the existing fiberglass, adding heat transfer plates, reinstalling the fiberglass, then installing the rigid foam. Funny, when we did the infrared thermography, the floor shots actually showed the PEX patterns under the floor....like they were attached to the subfloor as opposed to hanging in the bay. I think the heat transfer plates would make the system respond faster certainly....but would it save me more energy for all the extra effort...that is the question.
Robert...can you explain why the PEX tubing should not touch the floor joists? Certainly it would heat them up, but if the rigid foam was on the bottom, the heat would at least be held in the bays. I suspect, the worry is drying the lumber too much, perhaps causing checks, cracks, etc. It will be a challenge to keep the tubing off the joists when we pull it back to insert foam blocks (2-3") on the rim joists.
Lessons learned the hard way
Ed: Green building has never meant to be the cheapest way out. There are many more parts to "Green" then just energy savings. Recycled lumber and Trex decking are green, but don't provide any energy savings to your home. Energy is just one of many categories in the ratings. You have to include water conservation, site selection, home orientation, materials, disaster mitigation and other features into your "green" designation. Next time, consult with a "Certified Green Professional" before committing money to an upgrade.
There may be no monetary value to building Green, but the impact on the environment is certainly worth it.
Great Article
Ed - great article - you did a good job of summarizing a lot of problems we run into daily. I am often amused (and frusterated) as a remodeling contractor with potential customers that want to install a solar water heater in their house even though they have no insulation in their walls. I actually just wrote an op-ed on this very same topic and sent to my local paper (and will probably never be published - solar is more sexy than air sealing so they will not write about air sealing) hoping to educate the public. I would love to print this article and give to all potential customers - do we need permission to reprint articles from the website?
Pipe straps, not heat transfer plates
Ed, The Pipe straps I'm referring to are those little 2" x 3/4" copper stirrups that you can buy by the bag at a plumbing supply store, not the expensive aluminum heat transfer strips.They just snug the pipe up tight to the sub floor when installed every two feet or so and they hold themselves to the PEX while you shoot in the hex head screws. Even with having to pull and re-set the fiberglass it would be a pretty quick job to do it, so long as you pull an area, screw it tight and immediately re-set the fiberglass. I think it would be worthwhile esp if you could hire a couple of youngsters to work with you and knock it out in a couple of days. Definitely throw some rigid foam on those rim joists while you're down there!
fix your crawl space
Ed
Our North Country perspective would be to treat the crawl space as conditioned space with air sealing and insulating, then address any other airsealing issues revealed by the blower door test. after that is done mess with the tubing, adding rigid foilfaced iso is a pretty typical installation.
Green Priorities....
Walt--
I understand that green includes more than just energy efficiency. I value Trex, recycled lumber, etc. But, if climate change is a major reason for our movement, energy efficiency has to be at the top of the list. Because, if I do not fix my 'green' home, I will be burning 1500 gallons of propane every year for the next 50+ years.....that is about 75,000 gallons of fuel! That, in my opinion, has a much greater impact than most, if not all, of the other issues. By using Trex and recycled lumber, I did save a few trees...and I am happy about that....but, I think green should first prioritize energy efficiency, durability, and good indoor air quality. First, spend your money on the issues that have the most impact. LEED and other green organizations have been hammered for green certifying commercial buildings that are only marginally better in energy efficiency than the average building....and sometimes worse! I hope in their reevaluation of the LEED for homes ratings that they strongly emphasize energy efficiency....so homes cannot receive any ratings without proven performance results.
Pipe straps
Michael,
Pipe straps, huh...I will look into them. Send me a link if you have time. Of course, if I am already taking down the fiberglass, I'm not sure that I will save much unless the straps are actually more efficient than the heat transfer plates....or considerably less money. I'll compare them. My savings will be installing the rigid foam on the bottom of the joists as opposed to insetting them in each bay.
Thanks for the help.
Print it, hand it out, Danny
Danny,
I'm happy for you to take the article, print it, hand it out, etc......I don't think GBA will mind. We do need to try and spread the word....green building is so much more than just buying products...we have to learn to build smarter, with a very keen eye towards energy efficiency.
If you use our content ...
If anyone wants to print or share our content, we ask that you include attribution along the lines of, "Content provided by GreenBuildingAdvisor.com."
Thanks! We're glad that people find our articles and blogs useful.
Using Content
Understood - will do. Will be more credible coming from a source like GBA - if I pretend to have written it may not hold as much water. Thanks.
Radiant heat / transfer plates
I built a new house in '02 for my parents, the structure is a large ranch (36 x 80) and made of ICF's (Insulated Concrete Forms), the walls are 11 inches thick with an R-value of 52, from footer to to eave, the gable ends of the house are conventional wood frame and batt insulation. I put the radiant heat in the concrete basement floor, as well as, the "staple up" method to the floor joists. I had not done it before, and the system was engineered by a local HVAC supply corp.... so I did as they told me..... later learning of the heat transfer plates. 1/2 of the basement (there is a wall across the middle @ 40 ft) has reflectix's stapled uner the pex, and then 2 layers of 5/8" sheet rock, per code, as there are 2 9x7 garage doors into the end of the basement with bedrooms above. The other half of the basement is still completely exposed from below, no insulation or ceiling. The heat system is 2 in 1, there is a fuel oil boiler as back-up, installed in a utility room in the basement (never comes on), the actual heating system is an Aqua Therm outdoor wood boiler which is installed in a 3rd bay of the main garage (which also has the radiant heat in the floors of all 3 bays, which is irrelevant really here but there is about 8000 sq ft of heated space altogether) The house didn't seem to warm up as we had presummed it was going to, the floors did not really feel warm, (they were not cold, but they didn't "feel warm" ) so, in the 1/2 of the basement where the piping is still exposed, I installed the heat transfer plates with the pex dirrectly to the underside of the floor. It made notable difference upstairs. The floors are actually "warm" now, in that 1/2 of the house. In retro, to me, it makes clear sense as metal is a much better conductor for the heat than wood, thus, taking more of the heat from the tubing and putting it into the floors where you ant it.
While simply putting the tubing on the bottom of your floors with the pipe straps would most likely help you out, personally, I think the transfer plates are worth the money and effort if you want to actually realise a true difference and get what you were looking for from the radiant heat.
As for the insulation: as I had previously mentioned 1/2 the house has the reflectix as well as 1 1/4" of sheetrock, its really a perfect test situation as the basement is divided exactly and sealed with fire doors between the 2 halves and the wall is the same R52....... the upstairs is about 75 degrees while both sides of the basement stay about 83 degrees all winter, and the heat in the concrete floor of the basement, in 7 years, has never once come on, the heat is completely from the pipes in the flooring above.. (It's elders in their 70's they like it warm)
THe absolute best, is the piping in the concrete floor, there is a 10 x 10 breezeway that connects the house to the garage which is concrete floor with ceramic tile, there is nothing like that, the floor feels so good...... ya wouldn't think of laying on a tile floor as being comfortable, but it is with the radiant heat in it.
Good luck and Happy Holidays to all!
Heat Transfer plates...
Thanks Rick.
Yes I will compare the heat transfer plates with the pipe straps. I don't think any price difference will make that much difference.....I just want efficiency. Yes, our bathroom floors don't seem warm either with the radiant....but then someone reminded me that thermostat setting is about 68 degrees...so while it does not seem cold, it certainly does not feel warm.
Insulation before heat
Readers following these comments may be interested in my take: while Rick provides details about the temperature of his parents' basement (83 degrees) he doesn't provide enough information on insulation. Really, whether or not you have heat-transfer plates for an in-floor radiant distribution system should be way down your list of concerns. First off: are the basement walls insulated? Is the basement slab insulated? Is the floor under the breezeway insulated? If so, with what materials? What's the R-value?
Proper insulation can help make any space comfortable, regardless of the heat source.
For readers unfamiliar with the product: Reflectix is a 5/16-inch thick product consisting of bubble-wrap plastic sandwiched between layers of reflecting material. According to the manufacturer's own ASTM C518 testing, the product has a dismal R-value of only R-1.04. Anyone interested in slowing heat flow from joist bays should be using rigid foam, not Reflectix.
foil faced bubble wrap HRVs ERVs
The other thing I understand about this foil faced bubble wrap is it becomes even more inefficient after it gets dusty which happens quite quickly in most situations. Here in Canada we are going through this house tightening thing as well but you I am seeing even older homes get so tight you need to poke holes in it to keep mold from forming and getting choked out by dryers, range hoods, central vacs pushing air out thus allowing combustion air etc. to be sucked in. An hour of planning can save days of work so be sure to always include an HRV or better yet an ERV in any long term plans you make and as well make up air for combustion appliances as required. Lastly but probably most importantly a carbon monoxide detector. I do not want to over whelm but if you start tearing out stuff to put more insulation you may want to get some duct work in there while you are at it. Ian Ruitenberg AirTech Equipment
Insulating/tightening older homes
More a questiontouched on by Ian R. I have read that simply increasing the insulation in older homes can lead to moisture retention problems? Could someone address this.
Insulation and moisture
Lou,
This is a big topic. But, to briefly answer your question: yes, simply adding insulation to an older home can, in some cases, worsen moisture problems.
Buildings with little or no insulation tend to have dry, well ventilated walls and ceilings. They are kept dry and warm because heat is always leaving the house during the winter. Adding insulation can reduce air flow as well as reducing heat flow. The outer layers of the wall or ceiling assembly therefore become cooler and (because of the laws of physics) damper.
That said, an experienced building performance contractor or insulation contractor can usually insulate an older building without causing harm or damage to the building.
Insulation comments
This is interesting reading. I thought I might add some comments and share my thoughts.
On the comments about the effectiveness of "Reflectix" product, I believe that the R value is not what is important in that situation. The heat plate and reflective material products are not working on the principle of mass thermal resistance, but rather they are working on the principle of infrared reflectance. If we can believe the information provided by the bubble foil manufacturers,and the like, the effective R value is much greater than the actual R value because these products reflect the energy at the infrared spectrum and do not need to resist thermal conductance through convection. My guess is that in a light weight radiant system, like in this situation, the IR reflective component will be far more effective, and possibly the most critical component to system performance and efficiency, than thermal mass resistant insulation. My thought being that you want to reflect and return the radiation to the backside of the subfloor rather than just relying on creating a warm void in the joist space. However, in Ed's case, where he appears to have an uninsulated crawl space, the thermal layer between the floor system and the ground would be needed as well.
When it comes to insulating at the rim and header joists, it would seem to be the perfect application for spray foam. Cutting and sealing rigid foam blocks is not going to give as good results and will probably take much more time. This is part of why I am a big fan of exterior insulation, especially for foundations.
cold floors
I live on the coast of northern california where winter nights temperatures drop to the low 40's. Our townhouse has a ventilated crawl space with 6 inches of fiberglas insulation between the floor joists. The crawl space ground is not covered, is pretty dry and is sloped two feet to 7 feetfrom front to back. Our lower floor is carpeted but is always much colder than the upper level. We have a forced air gas furnace What would be the best way to insulate the crawl space? Would reflectix bubble foil be of any help?
Response to Harry Corey
Harry Corey,
1. Reflectix bubble-wrap is nearly worthless. It has an R-value of only R-1. Read more here:
Is Bubble Wrap Duct Insulation a Good Idea?
2. You asked, "What would be the best way to insulate the crawl space?" The answer is:
- Seal up the wall vents
- Install a polyethylene ground cover
- Install rigid foam insulation or closed-cell spray polyurethane foam on the walls and rim-joist area.
Read more here:
Crawl Spaces
floor fix
I can sympathize with you about the floor. Fixing it well is difficul. radiant barriers in general don't work well unless they are airtight, so putting foil under the floor joists is not great unless the rim joist is airtight, even if there is rigid foam below the foil.
Martin, I have to ask you not to suggest poly in a crawl space ever again, use EPDM roofing instead. Foam will not bond to poly but you cannot get it off EPDM, and EPDM is at least as good as a VDR. it will last longer too. the price difference is not that great either. if you're lucky you can find a commercial roofer in your town who throws out scraps and use those.
As fro this fellows floor, I spray a lot of closed cell foam, so I may be biased, but here is a a fact of failure, and a solution that works.
I was looking to make staple up radiant workable in crawl spaces and above garages. I knew the theories all point to reflective surfaces under the tubing (most say with an air space between the tubing and the foil), and 'insulation' at an R value of at least 4x the R value of the flooring under the foil. My pea brain figures closed cell foam is R-7 (mfr's#) per inch, so it will work in restriced spaces. I put up the foil and sprayed R-21 foam under it and thought 'GENIUS'!! ....oops.
I quickly found out that if the tubing is run to the perimeter like the pic shows, and the foil is too, and we used foam to insulate. a few things happen. the foamed house has low load, so the heat doesn't run as constantly. the tubing between the foil and the floor is not in a sealed bay if the rim joist in that 3/4 inch space is not insulated. When (notice When, not If) the tubing freezes, you lose at least 300 sf of the floor heat, and someone notices that right away. Then it takes several hours to find the problem and fix it.
Better to have the tubing guys run the tubing so it ends about 8 inches shy of the rim joist. Now think before you yell, most exterior walls are 2x6, and the Rim joist is 1 1/2 inch thick, so the bottom of the inner surface of the exterior wall is at least 4 inches away from the inner surface of the rim joist before you add sheetrock and trim. your client is not going to get a foot in against the wall far enough to feel you missed that edge.
After the tubing is run, run foil faced bubble pack under the tubing and extend it past the ends of the tubing about 3 inches. that leaves about 4-5 inches of the sub floor exposed to the space below the floor. now spray foam in the rim joist , seal it to the sub floor, and cover the foil all across the space with 3 inches of foam. the beauty is it doesn't matter if the pipes are in contact with the foil, it works. dust will have a hard time getting in to screw stuff up too.
One problem is you cannot allow any tubing in the cavity that is parallel with the exterior wall on the floor edge becasue you can't get the foil in there and keep it away from the rim joist. i have been skipping that for about 10 years now in very high end homes with no complaints.
A corollary:
When you try to convert a deck into a room and the deck is closer then 2 ft off the ground, you can remove the existing flooring and attach housewrap to the underside of the floor joists. then spray 3 inches of closed cell foam on the house wrap from above, run it up the rim joist and seal to the floor (we suggest leaving enough floor so you can do this, one guy just attached his bottom plates to the floor frame and put the floor inside the wall.) Now drop foil faced bubble pack in the joist bays on top of the foam. Then run radiant tubing (and any other plumbing) in the floor joist bays and floor it. if you have drains or other apparatus that hangs below the floor you want to keep warm, put the housewrap below it and hold the house wrap away from the object with a piece of rigid foam.
simple and practical.
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