We recently finished building a custom house for a client who had requested that the house be especially airtight for the sake of conserving energy and providing increased comfort. This is a typical goal of ours anyway, but because the client had made a special request, and because the house was a very simple, efficient shape, we had an opportunity to run with it. Long story short, the house achieved an airtightness rating of 0.9 ACH50. If you’re not well versed in nerdy building science lingo, that is about 6 times more airtight than the average new home, and only .3 ACH off the airtightness requirements of the most energy efficient building standard in the world. So, a pretty decent result for our client.
There are hundreds of things to detail properly to achieve an airtight home, but one of the most commonly overlooked in a typical new house is the attic access hatch. This is generally about a 22″ by 30″ hole in the ceiling of your house into unconditioned space, so it certainly deserves some consideration. (*Special note: make sure to check your local building code for attic access size minimums to ensure that your finished opening will still meet these requirements). Having searched online for a simple design in the past, I knew that there was literally not a single good article or resource on constructing a truly airtight attic hatch. What follows, then, is a simple 10-Step Guide to building your own airtight attic access hatch for very little cost, a few hours of your time, easily available materials and methods accessible to anyone comfortable with power tools. Relevant pictures will precede each step and its accompanying text.
– Two, 7′ long pieces of interior door jamb stock
– Two, 7′ long pieces of kerf-in weatherstripping
– One piece of MDF (Medite) or Melamine at least as large as your attic access opening
– Four draw catches
– One 4′ x 8′ sheet of 2″ XPS (extruded polystyrene rigid foam)
– One tube of construction adhesive compatible with foam (ex. PL Premium or PL 300)
– Paintable caulking
– 3″ Trim-head screws
– Caulking Gun
– Utility knife
Step 1: Purchase some 2×4 interior door jamb stock from your local building supply centre (or any shop that supplies doors for homes). This generally comes in 7′ lengths, and you will need three pieces of it for a standard-sized attic access hatch.
Step 2: You’ll notice in the photo below that interior door jamb stock does not have any kind of weather stripping in it, for obvious reasons. However, your building store will sell “kerf-in weatherstripping” that they install in exterior door jambs to keep out rain and wind. You will also need three 7′ pieces of this kerf-in weatherstripping. You will use a table saw to cut a kerf into the jamb rabbet on your door stock to accept this weatherstripping, as can be seen in the photos in step 3 below. You could also just buy exterior door jamb stock with the kerf in it already, but here in BC that stock is now moving towards a composite product to better resist moisture damage, so it is much more expensive and totally unnecessary in this application.
Step 3: You’ll notice in the pictures below that now you will cut the saw kerf into the jamb stock, and in the photo on the right you can see the kerf-in weatherstripping inserted into it. This is a flexible vinyl covered foam that squashes nicely to form a reasonably air-tight seal, but only when pressure is applied on it. More on that later.
Step 4: The next three pictures show the stock kerfed and assembled into the shape of our attic access hatch (make sure to square your frame at this point!). Rather than cutting it to fit dead tight in your framed opening, leave a little room for shimming it even around the edges so that it appears centred in your opening. The second image above shows a close-up of how you will have to cut back the jamb on one piece to form a kind of extended rabbet or lap joint to make the corners fit together properly. The third image shows how it will look once the weatherstripping is cut and installed all the way around the inside of the hatch. Notice that we are viewing the “top” or attic side of the hatch.
Step 5: Next you will want to cut a piece of MDF, melamine or other smooth-faced panel good to fit inside of your attic hatch. It will sit right on top of your weather stripping. Something paintable is best, and heavier is better as well, as it will help to keep pressure on the weatherstripping. Lay it inside of your attic hatch, press down slightly to squash the weatherstripping by placing a heavy weight in the centre of the panel, then trace the top edge all the way around the inside of your jamb stock with a pencil.
Step 6: Next, set a table saw fence so that when the top edge of the access hatch is placed against the fence, the outside of the blade will cut just into the pencil line, representing where the top of your panel will sit once it is pressed down tight to the weatherstripping. You may want to have someone help you steady the frame as you cut it, and be mindful of where the blade is as you push the piece through. Once finished, the panel should rest in the hatch just proud of the cut you made in the frame, as pictured in the second picture below.
Step 7: This simple cam hardware shown in the pictures below is what really makes this entire thing effective. In exterior doors, the strike and latch hold the door pressing tightly against the weather stripping to form an airtight seal. In most typical attic hatches, there is only a thin piece of stick-on weatherstripping placed on the top edge of the drywall protruding into the attic opening. Not only is this stick-on weatherstripping a poor air sealer, it also is notorious for falling off in dusty environments (say, for example, like an attic where there are mounds of fluffy blown-in cellulose insulation!). But beyond that, if there is not constant pressure applied on an air sealing weatherstrip, it is far less effective. So, this above hardware does the trick to pull that panel lid down, keeping constant pressure between its the panel and the weatherstripping. By flipping the attic hatch over at this point, you can now see the underside of the lid (which you will see when you look up at it after it is installed), and it will be straight-forward to locate the hardware, pre-drill, and install it. Be a good craftsperson and lay our your hardware so that they are all equidistant from the panel corners!
*It is worth nothing that not all cam hardware will function at 90 degrees like this, so it will be important to look at the parts before buying them to make sure that the hook will still engage on the catch at a 90 degree angle.
Step 8: Before installing the attic access hatch, you will want to insulate the backside of it before putting everything in place. Here in Canada, our minimum attic insulation requirement is R-40 (that’s a metric u-value of 0.142 if you’re in Europe or actually using the units we’re supposed to be in Canada!). So, in our case, this means about 8″ of XPS rigid foam (which has an r-value of roughly R-5 per inch). This foam should be cut to the outside dimension of your attic access hatch box that you have pre-assembled. When you have four pieces cut, glue them together with a construction adhesive designed to work with foam, and then glue that entire foam block to the backside of hatch panel, centred (as it is here). When the lid is finally installed, the foam will lap over the attic hatch frame helping to reduce the thermal bridging of the door jamb stock. Every little bit helps! You may want to pre-paint and re-mount the hardware on the panel lid as I have done here (same with the hatch frame), so that there is less work to do over your head once it is all installed. (Make sure to paint all six sides of your panel, so that it doesn’t warp over time.)
Step 9: Below is a shot of the access hatch frame installed into the attic opening. As you can see, the drywallers have done their regular thing, whereby the ceiling drywall protrudes into to the framed opening of the attic access hole. You will simply set your access hatch on top of that drywall, centre it in the space by shimming between its backside and the framing wherever your screws will be, and then fastening it into place. Please note: It is very important that you detail your attic access hatch properly here with caulking. As can be seen in the picture, the hatch frame’s bottom edge is caulked to the drywall edge. If this is not done diligently, then your attic hatch can be as snug as it wants to the weatherstripping with no net result; the air will simply leak between the drywall and your hatch frame, completely negating the effort you’ve put in. I actually put a bead of caulking along the top edge of the drywall before setting my hatch in place as well, just so there are 2 levels of protection to keep our assembly airtight. If you really want to go the extra mile, you should spray foam any small gap between the framing and the outside of the attic access hatch frame as well (but anyone going to those lengths should probably have their attic access in the side of an exterior gable wall and not even have a hole in their ceiling air barrier anyway!).
Step 10: Next, simply place your panel with the foam attached to its backside into place, and snap shut your draw catches. They will snugly pull the panel down against the weatherstripping and keep your heated or cooled air where it belongs: in your house, not your attic!
Hopefully you’ve found this guide clear and practical. It’s a nice example of what a knowledgeable custom builder can do to help make homes more energy efficient and comfortable, and it’s a great project for any homeowner to apply them on their own existing home. View more of our blog articles, and see other examples of our work by visiting our main page here: www.buildbetterhomes.ca