Two furnace’s supplies in attic and under the slab
New to the Forum.
Need some advice on how to insulate my attic. I currently have around R-38 or so in the attic.
House is built on a slab. One furnace supplies through the slab and the other (newer furnace) goes through the attic to the new addition.
With the ductwork running through the attic is this now a condition area. How do I insulate the attic area? Do I seal up the vents on the gables, and leave just the roof vents open. There are also baffles coming up above the top plates on the east side of the house should those be closed off? There is not a ridge vent only roof vents. Should the insulation be tuck tight to the far ends of the fasica boards or to the top plates? Should I also check the ductwork for leaks and cover them with insulation.
Sorry for all the questions but living here in Michigan I’m getting ice dams. I will be sealing off all the electrical boxes and recessed boxes that are in the ceiling with foam this weekend. I’m also not getting the warmth in the house that I should be getting.
Thanks!
Replies
no its not a conditioned space. if you were to foam the roof deck and close off the soffit venting then you would want to have a supply to the attic but not in the case you are diescribing.
sounds like you may be so drafty that your heat loss is faster than you furnace can handle. sealing everything you can will help will the heat loss. as you probablly know, heat rises and as it exists cold air is right behind it to take its place. seal up as many of those places you have air movement
in terms of checking your duct work, you want to seal any leaks and tape them, tossing insulation on it wont stop the air leak.
Attic issues
mark122-
Thanks for the help! I'll get the seams sealed and fill the voids around the electrical boxes.
Add more insulation where needed.
Air Seal
It sounds to me that your problem is more air sealing than anything else. R-38 is the minimum recommended for your area, but it does meet the current standard. More wouldn't hurt, but I doubt that it would be noticable as far as ice dams. You are vented, hopefully the venting is adequate. Given these conditions, to most likely source of heat in the attic is air leaks. Every hole in the drywall ceiling needs to be air sealed.
Every can light, electrical box, plumbing chase, every vent pipe, every bath fan, every register boot for the hvac must be sealed tight. Even more important is every seam and joint in the hvac ducting. The furnace fan pressurizes the ducts, making even very small leaks a big problem. Those need to be sealed with aluminum foil tape or duct mastic. This includes every transition with flex duct where the inner duct needs to be carefully sealed to the metal transitions. I would use duct mastic here although it may be possible to use tape if you are very careful.
After everything is sealed up tight, then I would look at insulation levels. I would start with the furnace. It makes no sense to insulate the ceiling to R-38 or more and have the duct system insulated to R-6 or less. The ductwork has about twice the temperature difference from inside the duct to outside as the ceiling does and thus should have as much or more insulation as the ceiling.
The difficulty of insulating a duct system is why it should be in conditioned space if at all possible. Retrofitting an attic to be conditioned space would not be cheap or easy, however. To make it condtioned space, all vents would need to be sealed and the shell (The roof from top plate to peak, gable ends, dormers and all) made air tight and insulated, replacing your existing insulation on the attic floor. This usually is done with spray foam which is significantly more expensive than fiberglass or cellulose that can be used on the attic floor. This is not a thin coat of foam but it would need to be at least R-38 as you have now which is in the neighborhood of 6 inches of foam. It night be possible to use some hybrid system of a thin layer of foam to form an air seal, combined with a cheaper insulation (cellulose or fiberglass) but that would much more difficult than it is on the attic floor as it would need to be held securely overhead to the underside of the roof. Gravity won't do the work for you here.
Air sealing is not fun or sexy so it is likely that buried under the insulation are a lot of air leaks and likely the the hvac ducts leak like a sieve too.
Attic issues
Thanks Kurt99-
I will get on the foam detail and ductwork seams asap. Also add more insulation where needed.
Question: Having outside air coming into the attic (by way of vents) not really an issue then. It's the amount of heat coming through the ceiling by way of the electrical outlets that causes the icedam's, along with not having enough insulation.
Should the Foam around the electrical boxes be the fire retardant type? Or is the regular ok to use?
Thanks again for the quick response I'll let you know how it turns out.