Ductwork-Insulate or Not?
I spent a few hours yesterday researching this question and the answers, but it seems because specifics vary, so do the answers. I'm a homeowner, in northeast Ohio, who plans to add a 1300sq.ft. "in-law suite" addition to an existing house. For a couple of reasons, this addition will require its own system, and since it is electric only, I've decided on a heat pump system (the main house has one, and we're happy with it). The addition (one floor) will sit on an enclosed, insulated, and unvented crawl space, with a vapor barrier, and a concrete floor. The space is approximately 48" high, and will extend approximately 36" below grade. I've gotten two bids for systems, that look good to me, but one contractor is convinced that the duct work must be insulated, and the other is equally convinced that it should not be. Both intend to put registers in the crawl space ducting. There is a money difference, of course, that I will be willing to pay, if I can determine that I will legitimately get a benefit from, but I don't want to spend if I don't need to. This addition is approx. 30'x40' and the fan coil will be pretty much centrally located in the crawl space. Which is better, in this application, and why?
If the crawlspace is both insulated and conditioned [has registers] there would be no point in insulating the duct work, any loss would be to conditioned space. If there were no registers there may be some benefit but not like if the walls were uninsulated.
Is there a particular reason you are heating/cooling this space?
Thanks for your response. Both of the contractors (plus two more, who's quotes I will probably reject - more on that at another time) recommended it. They said it will keep the floor warmer in the living space, and meet the ResCheck requirements.
Originally Posted by stvc
first, be sure the ductwork is sealed! -- mastic is the easiest.
second, insulate the ductwork
-- because this is not a 'conditioned' space.
-- -- unless you will be having it conditioned with a return down there and concitioned air supplied into the crawl & have that space included in the load calculations.
insulating under the crawl floor concrete perimeter [ 2ft from all edges ] will render good benefit in northern OH, most likely -- get the numbers. see BUILDINGSCIENCE.com
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!
Thanks for your response. Just to clarify "conditioned space" - the fact that the contractors want to install registers in the crawl space, does not make it a conditioned space?
Originally Posted by cem-bsee
Infiltration at it best. no insulation cold metal duct not very effictive.
GOOD WORK IS NOT CHEAP! AND CHEAP WORK IS NOT GOOD!
IF YOU THINK A GOOD HVAC COMPANY IS EXPENSIVE. THEN YOU SHOULD TRY A BAD ONE.
The supply air that will be put in the crawl will create a positive pressure in that area and create negative pressure in the living area of the home. Positive pressure will try to escape from that area and negative pressure will draw air into its area. While some of the positive pressure will find its way back into the living area, air will find the path of least resistance and follow that. And that could well be out of the home altogether. The negative pressure in the living area may draw some air from the crawl but the path of least resistance may well be the around windows and doors and such.
So you are likely to push the air you have paid to heat and cool to the outdoors from the crawl and will likely pull air from outside in the negative pressure area.
As suggested you could also put a return air in the crawl to balance the pressures or you could create a passive air way path back to the return air by having an opening between the crawl and the living area. It would have to be large enough that it would become the path of least resistance. If that is the route you choose to take you need to make sure the space is included in the load calc.
Alternatively, you could not put any supply or return air in the crawl. If you do that all you would need is a dehumidifier (probably a smallish one) to keep the humidity under 50%.
Finally, insulate the ducting so you get the heated and cool air to the far ends of the duct work and to the rooms farthest from the air handler. This is the best way to do it. As with all insulation, you pay for it once and get a divided back every day for years and years.
Thanks, Mchild. I got it all but the above. When you say "all you would need..." - you're still saying insulate the duct work, right?
Originally Posted by mchild
By the way, are you (any of you) familiar with Federal "REScheck" requirements?
Yes, insulate the duct work and put a dehumidifier in the crawl. Humidity is the only thing that you really need to be concerned with in that space. That is probably the reason some of the contractors suggested a supply down there - simply to help with humidity control. But when it is humid and and only 73* outside you will not have much if any run time run time from the a/c and the humidity in the cooler crawl will be excessively high. Install what the space needs - equipment to control humidity.
Originally Posted by Welshman
Not familiar with REScheck.
Sorry so long on this. REScheck is the form and/or program used to meet the Federal Building Energy Code (IECC). Diferent states may call the form a different name. I asked because the input data looks very similar to what goes into the Manual J, (HVAC-Calc) and I was going to ask some questions about that, but I see there is a good discussion going on another thread.
Thanks again for the help.
ANY duct work insulated with at least R-8 is the most efficient way if you ask me.of course sealed air tight first..
Insulate the ductwork and, as other's have suggested, put a small dehumidifier in the space. Uninsulated ductwork will "sweat" in the summer during a/c run and cause corrosion and higher moisture levels in the space, which in turn can promote mold growth.
My bet is that the whole house could use a little dehumidification during wet cool weather maintain <50%. How a about using a high eff. dehu like the Santa Fe compact which could the new crawlspace, the basement, and the rest of the home <50%RH during months that the a/c does not run very much. In the Midwest, the a/c does not run enough to everything dry enought to avoid musty odors in the lower levels. Units like the Santa Fe Compact are ductable to a remote crawlspace, the regular basement, and the a/c ducts to the rest of the home. Keep us posted.
Originally Posted by Shamoke
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"