How to insulate a cathedral ceiling in a house

How to insulate a cathedral ceiling in a house

How to insulate a cathedral ceiling in a house

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There is a considerable amount of misinformation on the web about insulation of cathedral ceilings. There is a science to this that must be followed. There is no single, simple answer to this question because it depends on the climate zone you live in.

First, for the most complete answer, go to buildingscience.com and search. They give a full treatise on this topic.

The abbreviated answer is that the underside of the roof sheathing needs to be protected from moisture and condensation. However, many of the commonly accepted practices only answer part of the question or lead to unsatisfactory, partial solutions.

If you are venting under the sheathing :

The purpose of venting is to allow a clear air flow to every cavity. In order for this to occur, you need: a) a clear passageway under the sheathing, preferably 2. 1 is too small even though it is often recommended; b) sufficient venting at soffit and ridge. Sufficient is defined as the area of the room below divided by 150 to 300 depending on the conditions. Safest is to shoot for 150. That is: room area / 150 equals square feet of venting.

So, for example, if you have a 500 square foot room, then you need approximately 3 square feet of venting area at the soffit and the roof. Many soffit and ridge vent systems are inadequate in this regard, providing vastly less area than this. See Cor-a-vent’s website for details of venting. They have excellent documents describing proper venting.

Note that this precludes the side venting suggested by another poster unless you can figure out how to get 3 square feet of venting out the sides. Most people use those little 3 vent plugs that might have a square inch of air space, if you’re lucky.

Note that this is NOT the case of I can’t do it completely, I’ll do it partially. If you don’t have enough area, then the air flow will be inadequate.

Also, another problem is that most cathedral ceilings have skylights. These are blocked areas that cannot be vented without cleverness.

If you are NOT venting under sheathing

Contrary to popular opinion, venting is not necessary, and can do more harm than good if not implemented properly. Remember, the key is to keep the roof sheathing from getting wet. Most moisture is carried by air movement. For example, a recessed light installed in a cathedral ceiling is roof rot ready made.

The best solution is to apply closed cell polyurethane spray foam directly to the underside of the roof sheathing. 2-3 is necessary in most climates, but see the Building Science literature for details. Closed cell polyurethane is a very effective vapor retarder when applied to these thicknesses. If moisture cannot reach the roof deck, you’re in good shape (as long as the roofing above it is in good shape).

The advantage to this solution, as noted, is that internal warm humid air can’t even touch the sheathing, so it will never condense. This air seals and insulated. So, if you install recessed lights in the sheetrock below, you’re still protected. Every other solution fails when you have ceiling penetrations, especially if you place a vapor barrier like plastic sheeting in the ceiling. Since the vast majority of moisture movement is at penetrations. If you install plastic sheeting, the electrician will cut a big hole in it where the recessed light pokes through. Since all such fixtures leak, unless extremely carefully detailed (never done), the air will leak into the cavity. With normal insulation (fiberglass), this moisture then gets trapped in the cavity by the plastic, rotting out your roof unless you have ample air flow to carry the moisture away.

Another solution is to use dense packed cellulose. This is defined as

3.5 pounds per cubic foot. Studies on thousands of homes (see Applegate insulation’s website) have shown that fully filling the cathedral ceiling cavity minimizes air infiltration and provides a long lasting roof with good insulation. The key is that it has to be dense packed cellulose and there must be no venting into this space.

This just touches on a couple key considerations. To summarize:

If you’re going to ventilate, you have to do it right

If you use fiberglass, you have to ventilate

If you use spray foam, you should not ventilate, nor should you use any other vapor barrier.

If you use dense packed cellulose, do not ventilate or use vapor barriers.


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