The Compass DeRose Guide to How to seal a room:
Why Duct Tape Doesn't Cut It

Written by Steven J. DeRose. Last updated 2003-03-22.

Duct tape and plastic sheets aren't a lot more than a placebo when it comes to civil defense or emergency preparedness. They're good things to have around the house anyway, but not for the reasons so many people were buying them recently. This page talks about how to really seal a room.

Plan just what you want the perimiter of the room to hold up against:

• Normal air leakage
• Positive and negative wind pressure, as in a hurricane or tornado
• Air pressure from a blast wave (this is a much bigger deal, including putting blast-valves in any vents and ducts, buying special doors, etc.)
• Water
• Fire
• Bullets or shrapnel
• Sledgehammers
• Gamma rays

What you fix and how you fix it depend on what you're trying to do. Intact painted drywall is a pretty good barrier against air leakage and will hold for a little while against fire; it's useless for much else. On the other hand, small cracks or even gaps in gamma shielding don't effect the shielding value much (gamma rays travel in straight lines, and even if the crack is perfectly straight, only the few rays that chance to be perfectly lined up with it will get through).

If you're below ground level, air sealing has to be especially good, because most chemical warfare agents are designed to be heavy so they don't just float away. Thus they tend to collect in low areas.

Evaluate the construction materials themselves

Obviously if you have a suspended ceiling, it's not going to do much for you. Take the tiles all out and do the complete inspection on whatever is behind them. Take off any molding and trim that might be hiding gaps (and since that's the purpose of trim, it usually is hiding gaps).

Here's a table of what's good against what. All cases assume the material is intact. Paint, stain, or other sealants can make a big difference. These are just a general idea -- actual effectiveness depends on thickness, combination with other materials, shape and placement, etc.

Consider switching metal electrical boxes to plastic ones with fewer holes. Check your electrical code; if permitted, seal holes with silicone caulk or similar. You could also foam the area around the box in the wall. Or, just move or remove troublesome boxes.

Window glass is good against gasses (though of course you have to check the several layers of framing and mounting surrounding it). However, it is very breakable, after which its protective properties aren't so great.

Search for any leaks or gaps

Go around every inch of the room's walls, ceiling, and floor and look for any openings. For example:

• Every electrical outlet, fixture, or wire (most electrical boxes have extra holes in them, as well as a gap around them in the wall).
• Every plumbing or other pipe connection
• The joints where ceiling, floor, and walls meet
• Any actual cracks in floor, wall, or ceiling
• Heating and air-conditioning ducts
• Chimneys and stack pipes (plumbing vents to the roof)
• bathroom fan, over-stove fan, dryer, and other exhaust vents
• Where doors meet their frames
• Every inch of the perimeter of any doors and windows: inside and outside the frame, behind the molding and trim
• If there are closets or similar spaces, check them just as thoroughly
• Where possible, check the other side of the walls and ceiling too. Even if you've got the inside side sealed, it's cheap insurance (and good home maintenance) to fix any cracks on the other side too
• Over-ceiling openings to other rooms. Especially common with suspended ceilings: the walls often stop at the suspecded ceiling height, and then there's a gap of inches or even feet all along the sides. This is bad for fire-protection as well as sealing; in some buildings, however, that space has to be open because it is used as the cold-air return for the heating system

Evaluate the general strength of construction

How did the builders attach things? Consider:

• Adding hurricane ties for every joist.
• Attaching sill plates to the foundation with serious anchors.
• Cross-bracing between joists to make lots of little triangles so they can't torque out.
• Improve mounting of door and window frames.
• Reface exterior or interior with brick or reinforced concrete.
• Put thick drywall over wood for some fire protection.
• If joists are just sitting on I-beams, sills, or posts, attach them for real.

Information on how to do these kinds of changes can be found in a good home-improvement book.

Consider the shape of the structure. Basement walls typically extend above ground for a foot or two. If they're exposed and make a nice right angle with the ground, pressure concentrates at that corner and can generate tremendoues shear force. Instead, use fill dirt to make a nice curved corner -- this improves drainage and also makes the wall more aerodynamic -- a pressure wave is dissipated far more gradually, so generates a lower maximum force on the wall. If you're worried about blast, stray gunfire, and the like, which direction(s) are the most likely? Place your shelter accordingly, and consider whether and how to beef up your walls in general. Sorry to mention it, but I live about a block from where one of the Washington sniper incidents happened, and many people live within range of hunting areas -- the possibility of stray gunfire is not limited to war and drug areas.

On this last point, Kevlar wallpaper is possible. Fiberlay and FibreGlast provide Kevlar and a huge variety of other materials (epoxies, foams, fiberglass, abrasives, sealants,...). You can buy Kevlar fabric by the yard like any other fabric, $20-30 per running yard. Thus, a couple hundred dollars per layer to cover 2 walls of a moderate-sized room. I'm looking into how many layers it takes to stop typical bullets, but my DSL just went down (speaking of disasters :). Ahhh. A vendor of body armor says typical "bulletproof" vests have 22-24 layers of Kevlar(r) 129 fabric, so make that several thousand dollars total.

Some alternatives would be to add steel plating, replace the wall with brick, solid cement blocks, or concrete, or to fill the wall with bricks, blocks, concrete, sand, gravel, or similar. If the sheathing on both sides can hold it, the last few options would surely be cheapest, since you could just drill a hole in each stud space and pour in the fill. But drywall probably won't take the pressure of 8' of wet concrete, sand or gravel piled behind it. I tried testing once with sand but it got messy and I gave up.

Fix the known problems, for starters

If the materials are poor, either replace them, or cover them completely with more effective ones. Painting wood with Kilz is a good step. Gaps, thankfully, are pretty easy to fix.

If construction is not strong enough, reinforce it.

Some of the useful materials for fixing gaps, cracks, and leaks are listed below. Ones that solidify in place will make a much better seal than any kind of tape.

• Plaster spackling compound
• Caulking. Silicon is particularly chemical-resistant. Special fire-resistant caulks are readily available, and may be required by code where utility connections go through important walls.
• Latex patching cement
• Hydraulic dement This is mainly for patching below-grade
• Paint, especially Kilz™
• Polyurethane wood finish
• Epoxy
• Spray-cans of foam. These come in many styles. Some go in and expand a lot; some expand only slightly; some remain flexible; some are also good insulators; fire-resistant ones may be available too. The expanding ones are excellent for shooting into a space you can't fully inspect; for example, at some corners of my basement, the sill plates didn't meet up -- judging from the amount of light coming through, the gaps were huge even though I couldn't see them well. So I foamed the whole space, and lo! the cold wind stopped.

Do take care when sealing things, to allow for work you may need to do later. For example, if you take your thermostat off the wall and use epoxy to fill extra space where the wire goes into the wall, it will be a pain later if you have to fix anything. Likewise if you foam inside a wall with wires or pipes running through. This is not a disaster, but it may be a pain, and if you can choose other methods (say, silicon caulk instead of epoxy) you may save headaches in the future -- and the chance of headaches from having to fix wiring, is higher than the chance of having to use your shelter anyway, so worth considering.

Systematically go around and seal every crack you found. Note any other potential leaks that you discover while going around fixing the first ones.

Handle the big openings

Consider replacing window glass with something like glass block (3-4" thick), acrylic or plexiglas™.

Otherwise be sure to have something solid pre-made and ready to mount to cover the window. For example, a panel with pre-drilled holes and bolts that thread easily into pre-placed threaded anchors or holes in the wall around the window. The sheet could be 1/8-1/4" steel, or 1/2-3/4" plywood (polyurethaned). Actually test-mount the things, don't just get them "ready".

Test the quality of the seal around the edges of such covers. A layer of compressible weatherstripping may help.

For anything you'll need to put up in an emergency, mark it clearly for where it goes and which way is up. Tape bolts to the panels so they won't get lost, and include a few spares. Use the same size and type of bolts for every panel. That way you can trade them around if a few get lost, and you'll only need one size wrench.

Make covers for air ducts as well. One easy way is to make them the same size as the existing decorative duct covers, and mount them with the same screws. If there's ample space, you could keep them behind the regular covers, in the duct where they're handy but out of the way.

Make sure you have all the tools needed, stored where you'll need them. At least have a wrench and screwdrivers for putting in the bolts, and a hammer simply on principle. I think it's better not to depend on your regular tools, which could be somewhere else when you suddenly need them (like in a crawlspace where you were working on some home-improvement project).

Add weather-stripping around doors. A flexible strip can give a decent seal to the floor, though it won't hold against water or any non-trivial air pressure. Back it up with duct tape (hey! a use I can support!).

On plastic sheeting

The chances of getting a good seal of your house or room with duct tape and stock plastic sheeting seem poor. Plastics may be airtight (or not -- they vary a lot), they're prone to cuts and tears, and they are not proof against all chemicals. Try pouring a few household chemicals on a sample (one at a time please, and do it outside in very small quantities) -- I'll bet you can find a bunch of household products that eat right through. Then try the same thing with duct tape -- which also is porous, so makes a poor gas sealant.

Check your work

See if your gas or electricity supplier will come out and do an energy audit. The better ones will look at your house with an infra-red camera and point out exactly where heat is escaping. Check those places for air leakage as well.

You might try testing for air leakage by closing yourself in the room while a lot of sticks of incense are burning (in safe holders, please!) all around it. See if you smell anything. Smoke bombs would work as well, but you don't want the aftereffects. You can do the same experiment in reverse: light a lot of incense in the shelter, and walk around all sides of it to see if you smell anything.

If you're concerned about water, be in your space the next time there's a big rain storm, and look carefully for any leakage. Or generate your own rainstorm with a garden hose.

I know of no good way to test fire protection. Perhaps your local fire department has someone who will come and look things over.

For physical force protection, try (or at least think about) hitting any side of your shelter with a sledgehammer. If that makes you worry, then your shelter probably isn't strong enough.

Having done all that, realize that you still need air. Some of the very energy-efficient, tight home of recent years have been unhealthy because the air is not replaced often enough. The only practical solution is to bring in fresh outside air -- precisely the thing you were trying to avoid. See the Compass on airflow for more information.

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