Winter HVAC Hack





Introduction: Winter HVAC Hack

Block winter drafts from home air conditioning vents: fast and low-cost.

For years I've been trying different schemes to block the cold drafts from my home's  Air Conditioning vents. They are essentially holes right into the unheated attic immediately above since this is a 1950s ranch-style house.

Because the vents are aluminum I can not use a standard magnetic cover.

The vent screws just go into drywall, so loosening or removing them every Winter will not work. The drywall ceiling just gets messed up.

This year I made covers from white two-ply corrugated plastic. I used Coroplast from old election signs and commercial road spam advertising signs.

Careful now, Mr. Potato Head - don't trespass, or take down signs before an election. In my area, commercial advertising signs on state roads are illegal and so are fair game.

You can also visit a sign shop and ask for rejected signs. Believe me, they have them and will give them away free of charge.

Rubber foam weatherstripping or Mortite caulking cord and a velcro strap save the day. My only innovation is making the cover by plastic welding two-ply Coroplast.

Step 1: Finished Draft Stopper

Here's the snug and warm finished vent cover.

Totally good to go with the whole Wife Acceptance Factor thing too, amazing, isn't it?

Held on with a 4 inches velcro strap and insulated with low-cost foam weatherstripping.

Read on to see how easy it is to do this...

Step 2: Install Velcro Strap

Loop your double sided velcro strap through the vent louver.

Pass the free end through the hole in the end of the strap.

Tighten it up and turn the strap so it is close to the center of the vent.

Now you are ready to prepare your cover.

Step 3: Measure and Cut

Measure the vent to the nearest half inch (centimeter).

In my house, the vents are 13 inches by 13 inches, and 9 inches by 9 inches.

Cut two corrugated plastic squares for each vent. Use a craft knife, tin snips, shears, or even scissors.

If your vents are rectangular, be sure that one piece of corroplast has its ridges running at right angles to the first. This gives extra strength, like plywood.

Step 4: Weld Cover Pieces

Using a hot soldering iron or soldering gun, poke through the top three layers to weld the two pieces of corrugated plastic together. Make sure the plastic ridges run at right angles to each other for strength.

Do this outside, or in a well-ventilated area. i use a fan and an open basement window because the smoke is pretty foul and is likely to contain all kinds of nasty stuff.

Wear appropriate safety gear - no fooling, a gob of hot plastic in the eye is just not a good thing. When working with a hot iron, i usually have a small tub of ice handy to instantly cool any finger burns.

Step 5: Punch Center Hole

I like to use my cheap hollow punch set to make a nice round half inch ( 1 cm )  near the center of the plastic cover.

You can also melt a hole with the soldering iron, or cut a hole with a craft knife.

I usually put the hole about half an inch off center towards the middle of one side to allow for adjustments.

This is because the velcro strap is never exactly centered on the vent.

Step 6: Weatherstrip the Cover

I like soft squishy rubber weatherstripping foam.

You might also be able to use Mortite caulking cord, or recycled foam rubber.

Attach the adhesive-backed weatherstrip around the edge of the cover.

Step 7: Pull the Strap Through

Now you see why the center cover hole is so large.

I use needle nose pliers to pull the strap through the cover hole.

You might be able to use wire or string and have a smaller hole, but this is easiest for me.

Step 8: Attach Velcro Squares

Attach self-adhesive Velcro fuzzy loop squares to the surface of the vent cover.

This will be where the Velcro Strap attaches to the cover.

Step 9: Plug the Cover Hole

Use a wadded up piece of foam to plug the hole in the cover.

Be sure it will be under the Velcro strap when it is pulled tight and stuck to the cover.

Step 10: Cozy and Warm

You have stopped the draft.

You might be tempted to use white vinyl tape or duct tape to seal off the corrugated openings in the sides of the covers. But in my experience this really does not make any difference

Not only can I set my thermostat lower, but the furnace seems to run a lot less now that these major draft sources are sealed.



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    This was very helpful. Thank you.

    I modified the process by using foil backed foam board. The gasketing and tyvek holders worked well.

    Thanks again for posting.

    The most effective way to cover any vent - central air conditioning, whole-house fan, louver, etc., is with VentSTOP. The product is an insulated, non-magnetic, flexible vent cover that is applied with Velcro and works extremely well - it's very economical and reusable year after year. The website is


    Blocking a vent is ok EXCEPT if a ventfan is located directly above the vent. My brothers trailer burned down from doing this. It is a bit of a funny story because his smoke detector went off and he went into his bathroom that had smoke coming out. He couldn't believe he was seeing his TOILET was on fire. "How does a toilet catch fire?" he thought. But then he saw something dripping from the ceiling. He looked up and saw small flames and molten plastic dripping from his bathroom vent-fan! What had happened is a few days prior he placed duct tape across the vent because cold air was coming in. Later another family member turned ON the bathroom vent switch and because he had bad hearing (almost deaf), he couldn't hear the fan running and the fan was left on for days in a row. It finally overheated and caught fire. Nobody was hurt but his whole trailer burned down!! I think the VENTS you are discussing are not motorized, so it is not a concern. But I wanted to place this note so others reading your instructable do not have a tragedy.

    I have the same issue and bought some of that thin white magnetic sheets (they make fridge magnets out of it)
    Because the vents are square I just use white 3M tape and tape them up each winter around the edges so no air leaks out
    Then in spring I just pull off the tape and store the squares downstairs til next winter

    I'm confused but nothing new there. It seems you have a/c vents that are not connected to your furnace system?? Then do you have two different sets of everything?

    And the problem is that you have drafts that are coming from the a/c vents?  Is that right?  Can you put a ribbon or something at the vent and see it moving?  Do you have a ceiling fan?  Can you feel your feet being colder than your face when you stand up or walk through the room?  I'm not suggesting anything.  I'm just trying to understand on your situation.  I might suggest something later but not in my present confusion over what you have. 

    7 replies

    Good questions, in old houses like mine, we have hot water baseboard heat run from the furnace.

    Central Air Conditioning is installed by putting a big compressor outside the house on a concrete slab. Insulated pipes run up to the attic to a heat exchanger.

    The heat exchanger is connected to a long duct running down the center of the attic from end to end.

    Insulated flexible pipes about 7 inches inside diameter connect from the central duct to ceiling vents in each room.

    In the winter the attic is very cold because the roof and gables are not insulated. The attic has no floor, just wooden joists with fiberglass insulation sitting on the drywall sheets which make up the ceiling of the rooms below.

    The air conditioning duct and pipes become very cold from the freezing attic.

    Cold air from the ducts flows down into the room. Also the cold metal ducts absorb heat from the room.

    If you sit or stand under a duct, you can feel a cold draft on your skin. Heat in the room is lost through these 15 inch by 15 inch square holes in the ceiling into the frigid attic space.

    By covering the vents with corrugated plastic I am able to stop the drafts and the conductive loss. Hope this helps.

    Good explanation. Thanks.

    In my old house, instead of installing ceiling fans, I chose to use really good, personal-sized fans to move the air around.  What I found works for me is to put them on the floor and aim them toward the ceiling.  In the winter I put one in the corner of the room and leave it running at low speed 24/7.  The fan sends the cold air from the floor up to mix with the warm air at the ceiling and evens out the temp throughout the room.  I have five little fans running throughout the house. 

    I am 6 feet tall but used to know a lady who was 4' 11".  Her thermostat was mounted above her head in the house, so it clicked off when the room air at the 'stat reached the set temp.  In her house my face was burning hot but my feet were freezing cold.  She kept cranking up the heat because lived in the cold air below the thermostat.  I bought her a fan and within 15 seconds she was warmer than she had ever been in the house. 

    I'm not guaranteeing this would solve the cold air from your vent, but I would suggest that not all is as it seems in your situation.  Cold air cannot just drop from a vent.  That same amount of air has to enter the vent somewhere.  You might try a fan on the floor, or turn on a ceiling fan to the summer direction, and see if the drafts stop. 

    well actually, following on the end of your comment, cold air is heavier than hot air, so it falls to the lower part of the room. thats why your feet are cold when your head is not. the cold air from the cold pipes/vents in the roof is falling down, and filling the room with more cold air than hot air. the hot air is pushed into the pipes/vents and that pushes out more cold air and all the way during this continuous cycly the furnace must run to keep the room at the most consistent temperature it can get.

    zack I'm not sure the laws of physics work as you suggest (cold air falling out of a vent with warm air flowing in through the same hole). Nevertheless, air is apparently getting into the pipe, getting cold, and falling into the room.  I still suggest trying a fan on the floor aimed at the ceiling.  If you don't feel a huge improvement immediately, then go ahead and cover the vents.  Ceiling fan works, too.

    Why would the fan stop the drafts?  Because it stirs the air and prevents the hottest air from getting stuck at the top of the room and the coldest stuck at your feet.  When the air temp at the ceiling is 90, that hot air is going to leak upward more easily than if the entire room is 70.  Also when you have two layers of air (hot over cold), you can get convection currents running inside each layer.  Those are drafts when they are cold.  Stir the air and they stop. 

    Cold air = dense
    Hot air = light
    Cold air falls.

    Heat does not rise, hot AIR rises.

    but we were talking about cold air and hot air, so yes, hot air rises and cold air falls, thus covering the vent will prevent the cold air from going in the room and the hot air going into the attic, where it is not needed.


    It limits contact with the cold surface. There is not "cold air" coming in (unless the duct has leaks), it's just hot air rising, contacting a cold surface, becoming denser, and falling. If he would insulate that duct in the attic it would achieve the same thing without having to cover the vent.

    I forgot to add... a 1/2" x4x8" sheet of insulated sheathing at lowes runs around $9, a 3/4"x4x8' runs $10 and a 2"x4x8' runs $27

    1 reply

    Thanks L8nite for the update on the insulated sheathing. Especially how low cost a 4x8 sheet is. They probably have a much higher insulation value than my recycled sign boards. And a single 4x8 might be enough to cover all the vents in my house.

    I don't know why but I keep getting email updates on this thread, thats not a problem but it has kept me thinking. The sign boards do a good job of blocking drafts but the insulation value has to be very low. For a small expenditure you can get a 4x8'sheet of rigid foam insulation, cut slightly oversize pieces and either router out a place for the vent or use another thinner sheet to make a frame, attach in the same manner, for esthetics cover all 5 sides that show with contact or wall paper and/or paint to blend with ceiling. Not applicable in your situation but for those with wall vents a photo or some kind of picture could be glued to the front

    great idea! a little more work and maybe a 10 on the mrs scale would be to unscrew the covers, insert the panels and replace the cover

    4 replies

    Excellent comment, thanks -- I have added this to my introduction because I forgot to mention that I have tried it. The screws go into drywall, not metal or wood. Doing this every Winter messes up the ceiling. I had to repair the drywall and reinforce the screw hole with soft wood matchsticks and glue. Not only that, but plugging the vent from the inside does not stop the secondary draft from the metal conducting the extreme cold from the attic.

    One year I clambered into the attic, pulled back the insulation, removed the hose clamps and stuffed and sealed the vents. What a hassle. And it did not work all that well. You could still feel the cold dropping down from the icy aluminum vents.

    My current plan is about a quick, acceptable, cheap way to stop the drafts. So far, it works well. Thanks again for the reminder.

    Im all for the quick and dirty solution (to my wifes chagrin) I hope you understand that seeing how one person accomplised something can get the mind working on similar alternatives such as cutting the plastic sheet slightly over size and scoring around the edge to fold up "walls" more or less hiding the edges and the vent view from the side or adding a layer of foam insulation. Anyway, I appreciate the mental stimulation and your "ible" was very well done with nice pics to describe the steps

    You have improved my method. I did not occur to me to make a box by folding the edges up.

    Then I could use foam weather stripping to seal the box against the ceiling.

    Thanks for the good idea. Especially since if I did it neatly it would look better than my cut and stick method.

    instead of folding the edges, couldn't you just cut a square and then cut out the middle that is the size of your vent?