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Wall Insulation FAQ

September 7, 2013 | Posted by Mitt Jones
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If your house was built before 1960, chances are it was built with no wall insulation. This week we answer frequently asked questions about insulating exterior walls.

How can I determine if I have wall insulation?
How much difference will wall insulation make?
How much money will I save on my utility bills?
Is it safe to insulate walls in an old house?
Is it okay to insulate walls that contain knob-and-tube wiring?
Do walls need to breathe?
Is it okay to drill through my siding?
How do you dense-pack from the outside?
Will our walls look okay after insulation is added?
Can you insulate walls with aluminum or vinyl siding?
Can you insulate walls with asbestos siding?
Can you insulate walls with stucco siding?
What is the process for insulating from inside the house?
How long will it take to insulate my walls?

How can I determine if I have wall insulation?

Wall_insulation_FAQ_ir.jpgIt's usually easy to determine which walls are insulated simply by using an infrared camera, though this isn’t a reliable approach when there isn’t much of a temperature difference between indoors and out.

Another option is probing around electrical boxes in exterior walls. Please don't try this yourself if you don’t know how to work safely around electricity. Some pros like to use wood stir sticks or plastic zip ties to feel for insulation. 

Of course there’s also the brute-force method of drilling a hole in a closet or some other out-of-the-way surface.

How much difference will wall insulation make?

Insulating exterior walls arguably has a more transformative effect on a house than any other home energy upgrade. Insulating exterior walls can make a huge difference in the comfort and energy efficiency of a home. It often also noticeably reduces exterior noise.

How much money will I save on my utility bills?

How much money you save by insulating exterior walls depends on many factors, but the fairly conservative modeling software we use generally predicts annual savings of $150 to $250. The larger the house and the less efficient your heating source, the more you’ll save.

Is it safe to insulate walls in an old house?

In most houses, insulating exterior walls will not cause moisture-related problems, but it depends. In some houses built in the early 1900s—often before 1910—siding was nailed directly to the studs rather than over sheathing. We recommend against insulating exterior walls in this case, even if there is building paper under the siding. The process of dense-packing the walls with insulation is likely to damage the building paper, which is the only protection against moisture when water gets past the siding.

Fortunately, even with old houses, most have building paper and sheathing that do a reasonably good job of keeping water out as long as the exterior is well maintained. 

Is it okay to insulate walls that contain knob-and-tube wiring?

The National Electrical Code forbids insulating around knob-and-tube wiring, which is considered by many to create a potential fire hazard. The Oregon Electrical Specialty Code is much more permissive on the issue and seems to allow insulating around knob-and-tube wiring in exterior walls. We recommend against it, both because of the possibility of creating a fire hazard and because decommissioning the knob-and-tube wiring will be much more difficult if you or a future homeowner decides to do it after the exterior walls are insulated. As an alternative for customers who are opposed to decommissioning their knob-and-tube wiring, we can leave stud cavities that contain knob-and-tube wiring empty.

Do walls need to breathe?

No, walls do not need to breathe. Air leakage through walls not only causes big energy losses and comfort problems but can also cause condensation. That said, there is truth to the idea that insulating the walls of an old house can increase the likelihood of moisture problems.

In walls without insulation, water that gets into the cavities will either drain out or dry pretty quickly. Dense-packing the walls with insulation affects water retention in two ways: First, it reduces the flow of the heat through the wall assembly, which keeps the walls from drying out as fast. Second, the insulation can make it harder for water to drain.

In general, insulating exterior walls is safe as long as there is sheathing and building paper under the siding and as long as the house is well maintained.

Is it okay to drill through my siding?

I often advise customers that if their insulation contractor wants to drill through their siding, they should find a new contractor. Drilling through siding may save some money up front, but no matter how well the holes are patched, they’ll likely be visible within a few years. More important, these holes can let moisture in down the road.

How do you dense-pack from the outside?

006_Wall-insulation.jpgWith most houses, the best way to insulate exterior walls is to remove a course of siding on each wall surface, drill a hole through the underlying sheathing and/or siding into each stud cavity, and then blow insulation into each stud cavity until it’s packed tight. Then the installers plug and seal the hole, put building paper or adhesive flashing over the plug, and reinstall the siding.

Will our walls look okay after insulation is added?

If you hire a responsible home performance or insulation company with an experienced wall insulation crew, your walls should look just fine after insulation is added. Once the walls are painted, you’d be hard pressed to see any signs that the walls have been insulated. Some companies will offer to handle touch-up painting for you if you provide the paint. 

Can you insulate walls with aluminum or vinyl siding?

An experienced wall insulation crew should be able to handle vinyl and aluminum siding with no problem. Check out our blog post on this topic for more information.

Can you insulate walls with asbestos siding?

We recommend insulating walls from the inside when asbestos siding is in place.

Can you insulate walls with stucco siding?

We recommend insulating walls with stucco siding from the interior. With stucco, caution is in order. Water cannot easily get past stucco, but once it does, it also does not readily dry to the outside. For this reason, we only recommend insulating stucco walls when the exterior is in good condition and when the homeowners are committed to maintaining the stucco.

What is the process for insulating from inside the house?

Dense-packing walls from inside the house involves drilling at least one hole per stud cavity, blowing insulation into the wall cavity until it’s densely packed, plugging the holes, and then patching the drywall or plaster to match the texture of the surrounding area.

How long will it take to insulate my walls?

Most wall insulation can be completed in two to three days.

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