Tuesday, November 6, 2012

More than you ever wanted to know about attic insulation

Our attic, devoid of even a scrap of insulation!

When we bought our house, the attic had no insulation whatsoever.

Proper insulation is one of the major challenges of an older house. One thing many people don't realize is that proper insulation is as much about saving your roof as it is about saving money. You can't simply fill your attic full of insulation and expect to save hundreds of dollars on your heating bill. You also have to make sure its properly ventilated to avoid the many dangers of humidity and moisture. Here's a brief explanation before we describe what we did in our attic:

Issue #1. Humidity

Have you ever noticed on a cold October morning that the inside of your car windows are frosty, but the outside isn't? This has to do with moisture in the air: the air inside your car is humid, while the air outside is dry.

This can also happen to your roof. The warm, moist air in your house rises to the attic, where it meets the cold surface of the roof. Guess what happens then? Just like inside your car, the moisture condenses on the inside surface of the roof deck. When it's heated during the day, the condensation runs off the roof deck into your insulation, causing mold, rot, and a whole host of problems. You can avoid this by properly ventilating the space between your insulation and roof deck.

Issue #2. Ice Damming (The big problem)

If you were to take a drive through your neighborhood on a cold winter night after a snow and look at the roofs of the houses, you might see some interesting things. On some roofs, you would see a nice, even blanket of snow. This is a good thing. On other roofs, you may notice that only the eaves are covered, while the peak of the roof is bare.This is a bad thing.

Why? On a warmer part of the roof, snow melts into water, flows toward a colder section of the roof, and then refreezes, forming an ice dam. Ice dams can force open the seams of shingles, develop into leaks, and ruin your roof.

A properly insulated attic goes a long way to protecting your roof from ice dams. So remember: when you insulate an attic, you are not simply trying to keep heat from escaping, you are also trying to keep the interior surface of the roof deck the same temperature as the exterior surface (that is, cold). But how? Enter the "air gap".

Solution: Mind the Gap

Plastic baffles maintain an air gap between the roof deck and the insulation.  
Ever wonder why old houses have empty walls? It's because air is a great insulator. Even modern insulation (and things like down jackets and knit sweaters) use this principle. If you install insulation directly to the interior surface of the roof deck, it will transfer heat, warming the roof (which, as previously explained, is a bad idea). On the other hand, installing baffles between the roof and your insulation provides an air gap, which allows for better air flow and ventilation, therefore minimizing humidity. 

Our Insulation Challenges

Loading up on insulation.

Insulating old houses is tricky any way you slice it, but it's especially tricky when you want to retain the attic as usable space, which we do. We are not planning to use (or heat) the attic this winter, but we do plan to finish it eventually, something we need to keep in mind as we prepare our insulation now.

Our house is essentially an  "American Foursquare," except that it has a gabled roof instead of the typical hipped roof. It lacks vented eaves, which poses a challenge. There are options for older house without standard eaves, such as drip edge vents. But our roof poses problems for this option, too.

We are planning to redo the siding next summer, and during this project, we will likely have to build new "ladders" for the soffits and fascias. When we do that, we can also add soffit venting, which will be a big help for our roof and insulation.

Insulation plan for 2012.   

Until then, the challenge will be ventilating without the soffit vents. To do this, we are going to add a couple of fanned gable vents to each knee wall.  This is less than ideal, but should work for at least a year. We are most concerned about the front of the house, since all of the existing static vents are on the back of the house.

Initially,  Justin had hoped to be able to use the gable fans to move cool air from the side of the house that didn't receive direct sunlight to the side that could (see the fan layout in the the image below).  He figured out that this wouldn't work with the existing roof vents. So instead, all the fans will pull air in. This should help move hot air from the front of the house up and over the peak to the rear static vents.

Top view of one possible plan for venting (note: the fan directions are wrong).

It's Never Just One Project

not only did we have to insulate...we also had some construction to do.
As we've mentioned in earlier posts, the previous owners did not always make wise decisions when "fixing" things. Unfortunately, they didn't take ventilation into account when they re-shingled the roof. Also, while re-shingling, they neglected to address areas of rot around the chimney flue, caused by (can you guess?) poorly installed insulation. They had also removed all the old paper insulation, and it's safe to assume they did so because it, in turn, had been improperly installed back in the 50's, and therefore developed mold and rot.

The sad remains of a "cripple" that was holding up the roof around the chimney.
Justin had to address this issue shortly after starting the insulation project.

2X6s poorly installed and unsupported by the previous owner.
The rotting led to a slight sag in the roof around the flue. The previous owners "sistered" 2x6s to the existing rotted out cripples, and didn't even bother cutting them at the correct angles. 

2X6s unsupported, and not properly mitered.
Justin added some additional cripples and properly mitered "sisters", and carefully removed and replaced the rotted out material. For added support of this section of the roof, he also added a knee wall (pictures taken while in progress).
Justin builds reinforcement for the roof

There is a lot left to be done. We need to complete the knee wall the previous owners started (with spacing of 24" on center). We've insulated behind the knee wall with R38 over the floor for now, and plan to add more insulation after this season.

R38 fiberglass bats behind the knee wall

The knee walls and sloped roof sections have been insulated with R13. Ultimately, when we finish the attic, the R13 will also get a layer of 2" R10 rigid foam, followed by a layer of 1/4" plywood, and then 1/2" drywall.

R13 fiberglass insulation on sloped roof sections

All of the rafter baffles have been connected at the peak, and duct taped together. Our roof is framed with out a "king beam", which obstructs air flow; otherwise, this wouldn't be possible.  

So lots more work to do this week. Good thing the kitchen is on hold for another 2 weeks while we wait for our granite...though we should be back on that soon, too!

No comments:

Post a Comment