foam board with foil and air gap or just more insulation
I have a house just south of Portland OR USA. It was built in 1968 with an integrated electric radiant heat system in the ceiling. I have btw. 3.5 and 5 in. of blown in cellulose insulation over it. I can feel the heat radiating up into the unfinished attic. Because it is radiant heat, I am reading that I can reflect that energy back into the house. I am planning on installing a 0.75 in., 2 sided foil faced foam board first before I install R-30/38 insulation. Insulation type not determined yet. I am reading conflicting ideas of weather to have an air gap or not. And if an air gap, how much, 05 to 1.5in. If foam board is installed, I plan on a loose board fit so moisture is not trapped. Any ideas on foam board or not. And if foam board, how much gap? Also, just add more of the same insulation, add something else on top of the old, or remove the old and put in something else.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Is the electric radiant all original circa 1968?
If so, are you married to this system? At 56 years old, it's an aged (read: sketchy) and grossly inefficient system.
Your time and money is better invested on a safer, more efficient, modern system. Electric resistance heat has an extremely high cost of operation.
No, I am not married to it. I have gotten estimates to install a mini-split system, $19.5K to $27.2K. Right now I am investigating having solar installed on the roof to pay for the electric the heat system uses and even make some money. (I have more them enough southern facing roof). I can not afford to do both at the same time even with the Gov. incentives. That's why I'm trying to figure out the best way to DIY fix the insulation. I figure, if it is good enough for the radiant system, it will be great for when we have the mini-split.
A PV sized to operate your current heat system will be much larger (and more $$$) than one sized to operate mini splits.
Phase 1: splits and insulation. Phase 2: PV.
Do nothing with current- to repeat- it’s unsafe and expensive to operate.
Some of those systems were recalled and have caused fires. Further, rodents love to chew romex. If your attic is wired with romex, it’d be mighty accessible to little critters. If it’s BX, well, that’s a fire hazard unto itself.
Electric resistance baseboard heat is wildly expensive to operate, I’d think this system is worse- because it operates contrary to basic physics. Heat rises.
To answer your question, in short, replace your heating system. Yes, you need more insulation. You also need to air seal. Neither should be done in consideration of an archaic heating system, rather moving toward an end goal of making your house safer, more comfortable, and less expensive to operate.
To analyze this, provide the following info:
1. How much energy does the current system use to heat / cool? There is a post around here that you can use to calculate that by estimating your usage in the heating season based on the difference in usage in the shoulder season when you use no energy heating/cooling.
2. How much money do you spend on heating/cooling with the existing system?
3. How much will it cost to remove all the insulation from the attic, AIRSEAL the attic, then blow in new insulation (cellulose preferred).
4. What are the variances in the mini split costs you have gotten?
Without data we don't know but I will guess that airsealing and putting ALOT of insulation into the attic is not very expensive. The airsealing and insulation will lower your heat load so you will use less energy after that. Thus you can go with the smaller HVAC system due to less load. Then maybe there is enough left over to put in the solar panels, which will be a smaller system due to reduced load. If all goes well, you can then dramatically reduce your heating costs.
I forgot to say to say that the system is original. 1- of the ceiling heat systems has failed and a forced air wall heater was installed. 1- window AC unit built into a bedroom wall. Wiring is a combination of braided cloth sleeve and plastic sleeve. I installed a powered louvered gable vent last summer. It could use some more soffit vents. The house is 900 sq. ft., on 1- level, 2- bed, 1- bath, kitchen-dining room and family/living room one big space, integrated garage, laundry room and workshop. Active 55+ retirement community, I'm 70. I'm still planning on DIYing replacing the house galvanized water pipes with PEX and the galv. waste water pipes with PVC, leaks waiting to happen. I moved into the house in July so I do not have a years worth of electric costs. The PV system is being sized to zero out the electric cost over the year. I do not have the specs. for the mini-split available right now. No one has mentioned removing the old insulation, just adding more on top of what I have. So I do not have a line item cost for that. Thermal imaging shows walls have insulation but not great. When I pressed them about the type of heat I have they blew it off because they had never dealt with this type of system before and did not know how to handle it. No one has mentioned airsealing before, what is it?
The quotes you received for a mini split system are very high considering the size and volume of your house.
Air sealing and improving insulation, as suggested above, will reduce the size of the mini split system your home will require and thus reduce the cost of your mini split system.
The cloth wiring you have also needs to be replaced. The cloth sheathing dries and crumbles with time and becomes a fire hazard. It is extremely difficult to work with.
Did you have a home inspection?
I would suggest connecting with a different contractor. One that has attempted to sell you a $27k mini split system for 900 sq ft house is bogus. Insulation upgrades, air sealing, and an appropriately sized mini split system is a better use of funds and will cost much less in the long term.
Further, there could be a state weatherization program that’s worth looking into.
Don't waste any money on using foil-faced rigid foam like that. You have several issues here:
1- I kinda doubt you're feeling "radiant" heat here. I think it's more likely you're feeling convective air flow, which means you probably have an air leak somewhere, maybe a lot of leaks. Air can move through fibrous insulation, including cellulose (although cellolose is better than fiberglass in that regard). While an air sealed layer of rigid foam could help with that, it's an expensive way to do it, and could potentially introduce moisture issues. You'd be much better off removing the existing cellulose, air sealing with caulk and canned foam, then applying a new layer of cellulose thick enough to reach your final R value target (probably R49+).
2- You would need an air gap between the foil facer and the side of the stackup that you're trying to "protect" with your radiant barrier. This means you'd have to prop the rigid foam up above the cellulose layer somehow, while also keeping that interstitial air space sealed from the outside world to avoid convective airflow which would otherwise cancel out any gain you might get from the radiant barrier.
3- You would be adding a vapor barrier that would potentially cause moisture problems due to condensation. If you have air leaks, which I suspect you do, this is even more of an issue.
My preference here would be to remove the old celullose, air seal, then put in new cellulose, or just blow additional loose fill cellulose over the existing layer to get more R value. Cellulose does help to limit airflow somewhat (not as well as a proper air sealing job though), so the thicker layer of cellulose will help to reduce airflow from leaks somewhat.
Bill
Bill-
Ceiling radiant was a short-lived form of electric resistance heat. That’s what is going on here, not failed insulation and lacking air barrier. Although, that needs upgrading too.
Ah, gotcha. I had misunderstood, and thought you were having a less-than-ideally-insulated attic, which is a common problem.
You could try using some foil faced polyiso spaced up off the radiant heating panels on the attic side, but I'd put that UNDER the cellulose, not OVER it, for best results. You may have some improvement doing that. I'm not sure how that would compare with just adding more cellulose though -- radiant barriers and air gaps are usually said to add about R1 to the assembly, but you have something of an unusual situation here, so you might see a bit more benefit than you would if you were just insulating normally.
Bill
Thank you all for your information and help. It looks like I have more investigative work to do related to mini-split installers.
Bill, I do have a lack of insulation, R15-19. It sounds like I need to move or remove the existing insulation to look for air gaps and fill them with caulk or/and foam, then add (insulation board) additional insulation btw. R42-48 (cellulose). If I do install foil faced foam board, what size air gap should I archive?
I did not know that the fiber sheathed wire was a fire hazard. I looks like I should remove or abandon the fiber sheathed wire and replace it with Romex wire. It will be a hassle, but I can do that (while doing the air gap seal work). I am already adding and/or reworking convince and/or safety electrical wiring systems.
Normally you try for a 1" air gap with a radiant barrier, but 1/2" should be OK too. It's not as critical as it is for things like roof vent channels, which are mandated by code to be at least 1".
Current code in most areas for attic insulation is R49. That should be your target, although anything more than what you have is a plus if you're limited.
What is this "fiber sheathed wire" you're referring to? If it's knob and tube wire, then yes, you have to remove it -- you can't bury that stuff in insulation. Knob and tube wiring is SINGLE CONDUCTOR wires, normally with a braided exterior, that run through ceramic tubes in studs and joists, and across joists and rafter using ceramic stand off insulators (the "knobs"). Connec tions are made by soldered splices that are normally insulated with tape.
There is also an old style of NM wire that has thermoplastic insulated wires inside of a braided outer sheath. This is a cable that CAN be buried in insulation. It's similar in physical size to modern NM-B cable, just with a braided outer sheath instead of the more modern plastic outer sheath. There is also an old R type cable that has a braided outer sheath, and wires inside that also have braided outer coverings over rubber insulation. I've never actually thought about R wire in insulation before, so I'm not certain as to code requirements for that and insulation, but I personally would replace that stuff too and would NOT bury it in insulation.
Bill
It is the older NM wire with the silvered braided sheth.
What are your thoughts for adding more cellulose insulation an top of the old or remove the old and install all new?
Also, is it worth the time and money to install the radiant barrier if I'm going to install a mini-split system in the next 2 to 4 years?
My own preference would be to replace the old braided-style NM cable with more modern NM-B cable. The big difference between the two is that the insulation on the conductors on the old stuff were rated for 60*C, the new stuff has insulation on the conductors that is rated for 90*C. The reasoning was that the wires would be safe running at higher temperatures, and they could expect to operate that way *if buried in insulation*. You still have to size the cables from the 60*C column on the ampacity chart either way. Basically, the newer NM-B cable is safer when run at heavy loads with the cable in insulation. If you have access to the cable now, this is the time to replace it.
My guess is you won't see much difference with the radiant barrier, so I would either insulate ONE radiant panel as a test prior to insulating them all, or decide if you're going to insulate them or not based on how difficult/expensive the project will be to complete. Once you replace the system with minisplits, all the effort and expense that went into your radiant insulating project will be lost.
Bill