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A bit lost on Cape attic insulation (rigid foam board in between rafters)

Have a very old cape with an attic. In that attic there is currently some old batt on the floor that is seemingly doing nothing. In order to improve energy efficiency and to prepare for HVAC equipment to go up there, looking to make the attic a conditioned space.

My idea is to place rigid foam board up on the attic ceiling, in between the rafters, tape them together to prevent air flow, and then to spray foam (out of a can) the edges to fill in any remaining cracks.

1) The attic appears to have no ventilation (soffit, ridge vent, etc). Do I need to worry about this?
2) Do I need to put some foil backing up against the roof sheathing before the rigid foam board
3) How do I attach the rigid foam board to the roof sheathing
4) How do I attach one rigid foam board to another if I am stacking the rigid foam board to achieve my desired R-value

Any other considerations I should make?

I would be super grateful for any advice or direction on where to find answers.

Re: A bit lost on Cape attic insulation (rigid foam board in between rafters)

A Cape Cod house is a "story and a half" house with the second story built into the rafters, so it is typically only about half as wide as the first floor. It has a steeply pitched roof, normally 10/12 to 12/12. The second floor rooms typically have a small ceiling, low walls known as knee walls and a slanted section between the ceiling and the knee walls. They have almost no roof overhang (eves) so that lifting forces from storm strength winds are minimal.

When these houses were first built, they were not insulated so vents would not be installed behind the knee walls or the attic and since the houses were not that tight, it was not a problem. The heater would be either a central chimney or a large coal burning furnace in the basement.

My first question, based on your current plans, do you really have a cape or is it a one story house with a cape style roof? Second, what kind of HVAC equipment are considering?

Based on your answers, you could be going about this all wrong.

Re: A bit lost on Cape attic insulation (rigid foam board in between rafters)

The house has 2 stories AND an attic. It has a center chimney. The roof is steeply pitched and the second floor has bedrooms but a great deal of space is lost due to the roof pitch (space goes into the eves).

As far as HVAC. Right now, the house is oil fired with baseboards. Conversion to gas with forced hot air is the plan (perhaps hydro-air).

Thank you for your response. If I haven't answered your question, please let me know. I am grateful for the help.

Re: A bit lost on Cape attic insulation (rigid foam board in between rafters)

You house should look like one of these.


Yes there is an attic above the second floor, but it doesn't go the full width of the house. My house is almost a cape style but I have eaves and a wrap around porch. My attic has about 7 ft of head room but there are capes that have as little as 1.5', and the attic is only about 3' wide in those cases.

Here is the problem with your plan, a gas fired heater is going to need combustion air. You do not want to be using conditioned air for this for two reasons. The conditioned air will be completely lost and will have to be made up with outside air that has to be heated first. The warmer conditioned air will make the flame burn cooler. Strange as it might seem, the cooler the combustion air, the hotter the flame, go figure.

My suggestion is that you insulate the attic floor to a point above the joists. Put a platform up there to mount the HVAC on that is above the insulation. Install a gable end vent at the peak of of each gable, a small one will do unless your attic is as large as mine.

Tear out the sheet rock or lath and plaster from the slanted walls. Put in the vent spacers against the underside of the roof sheathing, then fill the remaining void with the proper thickness of fiberglass batt insulation. If your rafters are 2x6 or smaller (I hope not smaller), you may want to furr them with 2x2's or 2x4's to get a good cavity depth.

Now put 4x8 sheets of 1/2" or 3/4" thick foam boards glued to the rafters (or furring strips). If you use foil faced boards, one side faced only, foil side down and unfaced batts, you can use a foil backed tape to seal the edges and make a vapor barrier.

If you use unfaced foam boards, then use faced fiberglass batts and staple the wings to the ends of the rafters, preferably with a little overlap of the paper. You probably won't be able to glue the foam boards on to this so just use a few screws. The sheet rock will hold the foam in place. Use 1.75 or 2" long sheet rock screws on this, not nails, to hold the sheetrock in place.

Insulate behind the knee walls and the floor behind the knee walls. Adding vents to the kneewall cavity should not be necessary, especially in an old house, the natural infiltration should be enough. The kneewall and floor needs as much insulation as you can fit so that the cavity stays as cold as possible so that ice dams do not form.

BTW, why are you putting the HVAC in the attic, is it just for the second floor? If you are heating the first floor, the ducts will need to go down an interior cavity space about one or two feet square. If you run it down under the roof of the slated section, you will cause ice dams to form and that will damage your house.

The older houses had the furnace in the basement and in the coal fired days, there were no ducts, just the hot air rising through vents in the floors and ceiling of the first floor. Personally, I would keep the HVAC in the basement but then if you are planning to convert the basement into living or recreational space, it would get in the way.

There is a third option and that is what we call a "gas pack" here in the south. The HVAC is a single unit mounted on a pad outdoors. The duct work goes through the foundation walls and into the crawl space (water table is too high here for a basement). That is what I use. It doesn't take up any interior room in the house and you never have to worry about carbon monoxide in your house.

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