I have a new Carrier high-efficiency furnace in my attic. This came about when I took the baseboard heat out of the upper level of my split-entry so as to get central air and a humidifier.
I insulated all the duct and built a small “room” around the furnace to keep the drain pipes and water pipes for the humidifier from freezing. I used 1/2″ foil-lined foam board for the “wall.” It got so hot in there that the thermal overload kicked out and I had to vent it into the attic.
But now the snow on my roof melts, and is filling up my eavestrough with ice. I recently saw a reference on converting the attic to a heated area in FH mag. If I do this, do I have to seal up my ridge vent, gable vents and soffit vents? what about next summer when I want to use my whole-house fan, or my attic vent fan?
Do they make fiberglass insulation to fit the 24″ center of my roof trusses? Or should I use the foamboard again?
Replies
Flatthumb,
1. If you insulate your sloped roof deck, you may (and probably should) leave your existing soffit and ridge vents intact. Install vent channels between the roof sheathing and the new insulation. Plug up your gable vents and insulate your gable walls.
2. If you insulate your roof deck you will no longer be able to use your whole-house fan.
3. You will no longer need an attic vent fan, which is just as well, since there is no reason to use electricity to vent an attic.
4. Yes, they make 24-inch-wide insulation. Fiberglass batts provide more R value per dollar invested than rigid foam.
I recently relocated a furnace and hot water heater to attic in cold climate, your post is of interest to me. I did not build room around either, but used 2" foam boarding to fabricate ductwork and was careful about insulating metal ductwork, mainly concerned about sweating in hot humid summers. This is new install so don't have track record to tell you how it goes.
Where is all that heat coming from? I figure your furnace is electric ignition, not standing pilot for high efficiency. It should be drawing cold air in from attic space for combusion air, keeping the area cool. Did you use b-vent piping or single wall vent piping? That could be a good source of heat, and less safe, if your furnace is gas not electric.
The hot water heater I am using is instant-on and installed to masonry backer board. It has a standing pilot so the coil should not freeze up. The supply piping extends prox 1' from attic floor, to keep from freezing I am using a little low-temp plug that kicks on around 40 and off at 45 or so. Then resisitance wire designed for keeping pipes from freezing, which runs across galvanized plate providing some delta-t to air rising into coils. Any opinions on this intall?
To your post, I think I would remove your box and let it open to the attic. I can't see a good reason to keep the furnace isolated and go through the business of trying to make a cathedral ceiling in your attic. Sealing the eaves will be a bitch because it's such a tight corner, and you would need to do that so air flows from your soffit vent up to your ridge vent and provides air flow across the roof (avoiding ice dams, etc. that destroy roofs). They make plastic trays that fit in between the roof rafters and provide an air channel, and then the insulation is installed over this. The plastic tray runs eave to ridge and is installed against the roof decking.
My last thought is - why have a humidifier, and why not use localized humidifiers? I would never have a central humidifer. They can cause a lot of structural damage to a house in a few seasons - warm, moist air flowing through electrical sockets and around windows, only to condense on cool studs and siding, etc. A portable steam-type humidifier in the bedroom, etc. doesn't dump nearly as much moisture in the air yet accomplishes your purposes. Those central ones dump gallon on gallon in the air of water.
remodeler
The furnace on this installation is a high-efficiency one, with a separate PVC run to the exterior for cumbustion air and exhaust. Thus the interior is not pressurized, either positive or negative, but the air simply circulates in a closed loop.
It is absolutely essential to have the furnace in a heated area, as moisture is removed from the air by heating it. Quite a bit of moisture, really. The manufacturer suggested installing a clear hose from the internal drain to the drain piping so as to be able to see if any plugging occurs, and water is continually running through this clear hose.
Carrier also insists on heated pipe-wraps, like are used the mobile homes here in Michigan, to ensure the drains don't freeze. The first one I hooked up immediately shorted out and my pipes froze, necessitating complete replacement. Incidently, I hope your local code calls for a drip pan under the attic furnace with a separate drain. If I hadn't had this in place on mine, I would still be replacing drywall! So with the heat tape fiasco, I decided to build a small "room" around the furnace and drain lines. It turns out the furnce radiates enough heat to keep the room at 80 degrees, and the drains running. I think this should answer your question about what I think about the heat tape on that water heater.
I think the heat that is melting the snow on my roof comes from the ductwork ( I love the idea of using the foam board!) which I have insulated, in some places twice, with foil-backed fiberglass batt, which is standard duct insulation here.
Some heat is probably being lost from the existing insulation in the attic, which is old and has been repeatedly disturbed by me with the furnace installation and all the recessed lighting I have been installing. It consists of loose pink wads of fiberglass insulation with a hard yellow fiberglass "board" laying on top. This stuff is nasty to work with and almost impossible to pet back in place once disturbed. Maybe once all the improvements are done, I will be about replacing it with something modern.
Insulating the roof joists would be MUCH easier to do, give me a ton of isulation, and a heated attic. But we use our whole-house fan quite a bit in the spring and summer, and would like to keep this feature.
My wife and I tried room humidifers. What a nightmare! On the cold water models if you don't keep them cleaned bacteria grows in them. So we switched to a heated water type. If we kept the humidity at a comfortable setting, condensation ran down the inside of our thermopane windows and actually rotted some of the wood trim. The floors also expanded because of the moisture (but only in the bedroom), causing some of the aok to warp. No, a whole house humidifier was the way to go, and we love it.
i have a hydro air system and was thinkin of putin in a humidifier in the second floor zone,which is in a attic.i didnt put it in because of the fear of pipes freezing.i used glcol so the heat pipes would not freeze .i am in ny and it is 0 today. i also thought of doing the insulation bit. l has anyone done it yet. it want to know the pros and cons before i do it
My furnace is a new install - 1 month - and we have had some heavy snow of late. I see what you mean, it's keeping the roof clear of snow in a wide circle. I'm unfamiliar with the high-efficiency condensing furnaces. Mine is 80%, necessitated by what was in stock that day in a horizontal model. It has a powered vent. I don't understand the heavy need for moisture in the combustion air on yours - I would like to understand how the condensing principle works. Anyway messing with insulation is one of my least favorite chores. If I had to do what you're doing, I might consider using the plastic liners to provide air flow across the bottom of the roof decking, using 1/4" hardboard due to light weight nailed as ceiling material to bottoms of roof rafters, removing soffits and closing off the space boxed in by the hardboard, roof decking, and rafters, and blowing cellulose insulation in from the top. I got the idea your attic is stick framed, if trussed it would be much less pleasant I would think. I would build a jig of some sort to press the ends of the hardboard up to the rafters in the eaves so that I could wood-glue and not have such a bad time in this tight space.
Lastly, I've planned to use this idea a long time - positive air flow across the roof decking. Small solar panel driving 12v-style computer fans in the ridge vent off a thermostat. Sounds cobbed, but if you have outside air pulled consistently you should not have warming of roof in winter / hot attic in summer due to thermal gain. Otherwise the system depends on hot air rising with slow air movement.
remodeler
Do you have both the inflow and outflow ductwork insulated? I ended up buying too much unfaced r19, and used the excess to build an A-frame around the longer duct run, so now I have r8 ducting inside an r19 shell. Seems to have made a difference as the air coming from that duct is way hotter than it used to be. When I reinsulated this Fall, I used a can of that "great stuff" foam and filled in the area around where the ducts pentrated the ceiling. Those bends create turbulence and you get more heat loss there, hence needing more insulation.