Thread: Attic framing clerance

This isn't about a specific house, but in general. I know we all see the framing tight up against the block in the attic space. I can probably count on my two hands the number of times I've actually seen it spaced away the 2" that it is supposed to be.

I have two questions.

1: Is this something that came about later in the building codes, or has it been around since the 60's? It's just every house in my area has the same thing. None of them are spaced away from the block.

2: Is the purpose to keep the heat from extending through the block and igniting the framing?

I've never seen the framing charred or signs of overheating, but I guess it could happen in a chimney fire.

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Jim,

I would guess yes to both, but I could not say for sure. I also see wood framing touching the masonry chimney. I have touched masonry chimneys in the attic while a fire is burning and it is noticeably warm, but not to the point of ignition.

The 1" clearance to combustibles has been traced back to 1927 with the Uniform Building Code, which also required chimney liners.

The air space performs several critical functions. In the event of a "chimney fire", meaning burning creosote in the flue, there can be around 2,000F generated. As the masonry absorbs this massive heat, it is conducted through the masonry walls until it reaches a thermal break such as an airspace. If it is in contact with another material, heat will be conducted to that material. If it is a combustible such as wood, pyrolysis will begin, which is a chemical breakdown of the wood thereby lowering its ignition temperature.

When masonry gets hot, it expands. Circumferentially, this is referred to as 'hoop stress', which tends to push out. If there is framing tight to the chimney, this resists this outward expansion greatly increasing the stresses in the chimney. The result can be catastrophic interior damage. Even unchecked, hoop stress often results in flue tiles ripping themselves apart. That's on reason for the required airspace around flue tiles. Room for expansion.

If there is an airspace, there is some cooling of the outer surface of the chimney as opposed to insulating it with direct contact. We want to dissapate heat.

The gap supposedly allows for inspection of the masonry, too. You do not want to see offsets at framing as heat tends to concentrate at offsets.

The required firestop must be of low mass materials. Otherwise, it will conduct heat to the surrounding combustibles. Sheetmetal or cementitious backer board are the intended materials but even these have to be caulked to form an 'effective barrier' and are not bulletproof. No insulation up against the chimney, esp. urethane foams. The problem with insulation is, it works. Instead of cooling the chimney surface, it keeps it warmer along with the surrounding combustibles.

Energy raters with blower doors will often point to air bypasses around chimneys and try to seal and insulate them as hugh thermal holes in the envelope. The best you can do is to caulk the firestopping and construct an attic insulation shield around the chimney where it approximates the framing but attic shields must breathe.

This is a critical area of inspection and it is one of the areas most often where things were done wrong and a common point of ignition.