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  1. #11
    Join Date
    Mar 2010
    Location
    Canada
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    86

    Default Re: Insulating ductwork?

    Quote Originally Posted by keith3267 View Post
    You misunderstood dj1. What he was talking about with the insulated ducts was flex ducts. They are a flexible duct with about 2" of fiberglass insulation around them. They are cheap and easy to install, but if mice or other critters get under your house, they will tear them up. You can see this stuff at any of the big box stores like Home depot and Lowes.


    If you have cold floors, you could start with closing your foundation vents. That will warm up the crawl space, especially if your present ducts have any leaks in them. Insulating the rim joist around the perimeter will do more for you than insulating between the joists. If you do decide to insulate between the joists, the vapor barrier goes up and you don't need a lot of insulation.

    Just like the walls and the attic, the warm air in the joist bays will rise to the top, but unlike the walls and attic, warm air rising is a good thing as it gets trapped against the floor boards. The air under the floor stratifies and insulates, you just need enough insulation to keep air movement to a minimum.
    Interesting.
    I can assure you this is not quite correct. I highly doubt the ( air leaky ) crawl space will provide warm enough air to rise and warm the bottom of the floor. Air that stratifies ( meaning it moves ) will not insulate.,only trapped ( non moving ) air would act as an insulator.
    Adding insulation to the underside of the floor is most likely required by code and it does work to prevent heat loss from the living space above and help warm the floor to increase comfort.

    If your subfloor is either plywood or OSB then you don't require a vapour barrier since those panels are considered a vapour retarder, I would recommend using unfaced batts and applying housewrap to the underside of the joists. The house wrap will be an air barrier preventing cold air drafts from the crawlspace entering the joist bays and reducing the insulation performance of the batts.

    One area that's often overlooked is along the base of the wall. Sealing along the baseboard will prevent cold air infiltration and cooling the floor.

  2. #12
    Join Date
    Mar 2010
    Location
    Canada
    Posts
    86

    Default Re: Insulating ductwork?

    Quote Originally Posted by JGKC9AYC View Post
    My wife & I purchased our first "real" house after living in a manufactured home during all of our married life.
    The house is a small, ranch-style house on a concrete block foundation. We were told it was built in the '40's, but if so it's built suprisingly well.
    My question is about the ductwork for the central heat & air. We recently put in a new gas heater & it seemed that when it first kicks on, cold air comes out of the registers. It eventually warms up, but it's not real hot. I contacted the company we purchased it from & they came right over & checked it out. He told me that the registers weren't insulated & that was the reason I was feeling the cold air initially. He stated that they need to be foil wrapped at the joints & foil "bubble wrap" around the ductwork. He stated that fiberglass insulation wouldn't last long. He gave me a quote of "about $1000" to do the job & that it would take a good days work to complete. I called to get a second opinion & he told me the same thing except that he wanted to use the fiberglass & that it holds up well. He climbed into the crawlspace & checked everything & said the ductwork was in good condition. He gave me a quote of $1290.
    The house is about 32'L x 28"W. I looked at insulation at Home Depot...the foil "bubble wrap" type. If I figured right, the insulation & foil tape would run about $400-500. Is this a job I could do myself or should I leave it to a pro?
    Also, while the crawlspace does have plastic sheathing on the ground, there is no insulation between the floor joists. Should there be some type of insulation there?
    Thanks for any help.
    Yes,the ducting should be insulated and all joints sealed. The air sealof all joints with foil tape is very important ,don't rely of the duct wrap for this.
    Which type , bubble wrap or foil/fiberglass , is your choice. I can say from personal experience the foil/fiberglass wrap works well.

    One thing one has to wonder. Is this duct work old?
    If it has been in the crawl space unsealed and uninsulated for quite some time then you might consider having the ducting cleaned , perferably with a powered brush/vacumm system. There might be a fair amount of gunk inside those ducts if they are leaky being inside a crawl space. The other consideration is mould/mildew buildup being inside an unconditioned space , leaky and uninsulated.

    Insulating the floor joists will be a wise thing to help keep your floor warmer and increase your comfort. Also, with insulating the floor joists will add more insulation to those ducts within those spaces. Attaching housewrap to the underside of the floor joists will keep colder moving air reaching the ducting and floor improving the insulation performance.
    Seal the wall / floor junction at the baseboards to prevent cold infiltration helping to keep the floor warmer also increasing comfort.

    Can you do it yourself ?
    Sure you can , just take your time and do a good job and it will pay off in comfort.
    Perhaps the money you save on doing this yourself could go toward the duct cleaning.

  3. #13
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: Insulating ductwork?

    Quote Originally Posted by LeonardHomes View Post
    Interesting.
    I can assure you this is not quite correct. I highly doubt the ( air leaky ) crawl space will provide warm enough air to rise and warm the bottom of the floor. Air that stratifies ( meaning it moves ) will not insulate.,only trapped ( non moving ) air would act as an insulator.
    Adding insulation to the underside of the floor is most likely required by code and it does work to prevent heat loss from the living space above and help warm the floor to increase comfort.

    If your subfloor is either plywood or OSB then you don't require a vapour barrier since those panels are considered a vapour retarder, I would recommend using unfaced batts and applying housewrap to the underside of the joists. The house wrap will be an air barrier preventing cold air drafts from the crawlspace entering the joist bays and reducing the insulation performance of the batts.

    One area that's often overlooked is along the base of the wall. Sealing along the baseboard will prevent cold air infiltration and cooling the floor.
    I can assure your that it is quite correct, you have misread or misunderstood my post. Stratified air does not indicate moving air, it means that the air has developed a temperature difference and has formed layers based on the temperature difference.

    If the joist bays are open to the crawl space below, the movement of air will prevent stratification from occurring. Once the bottom of the bay is sealed off, then there is trapped air and it stratifies, trapping the warmest air against the floor. The air in the bay is warmed by conduction and radiation of heat through the floor.

    BTW, one common misconception of heat flow is that heat flows mostly upward. This is completely false, heat flows to cold, uniformly in all directions. Warm air rises, but heat flows in all directions. As I stated earlier, I don't think his current installation is going to be a problem and your recommendation to use unfaced batts with housewrap at the bottom is a good one, I'm just saying that 3.5" batts will work just as good as 9.5" batts as long as the batts are installed at the bottom of the joist bays, the air trapped above will be just as effective as more fiberglass, as long as the rim joist is insulated.

    I also agree with your last sentence about sealing the baseboard, in theory at least. Sometimes the baseboard itself is not the best place for the caulk, depending on the flooring, but by removing the baseboards, you can caulk the sill plate to the floor, then put the baseboards back. I have done this when the floor is carpeted. You can also put a bead of caulk along the backside of the baseboard where it attaches to the sill plate.

    One last thing, the floor is the only place I would have a gap between the insulation and the interior surface. In walls, unlike the common practice, I put the insulation flush to the interior wall and leave any gap to the exterior of the stud bay. For faced batts, that means stapling the wings to the edge of the stud, not to the face.

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