new insulation in old house... which product?

doozmanAugust 30, 2008

Hello all,

I am new to construction/remodeling and would like to ask a few questions about insulating our home. My wife and I recently purchased a 1500sf 1850s era farmhouse in Northern VA. The interior of the home is in pretty good condition⦠the walls are lathe and plaster, and have been recently redone, so I would like to leave them alone if I can. The outside needs work. The siding is in pretty bad shape and there is no insulation in the walls. We are installing an open-loop geothermal heat pump to replace the old oil furnace, and we need to address the siding and insulation. There is also an uninsulated crawl space that needs to be addressed as well. My plan is to rip off the old siding and replace with hardi-plank (to preserve some semblance of the historic look). But I am really not sure what to do for insulation. Money is a factor, but we certainly donâÂÂt want to create problems for ourselves in the future.

1) Which insulation system should we install?

2) Can a regular fiberglass batt installation be done well and suffice?

3) Should we spend the extra money on full-foam, and if so, which product should we go with? Will foam damage the lathe and plaster on the inside?

4) Can we split the difference and do some sort of a hybrid system (flash and batt)? If so, which products should we use and where does the vapor barrier go? Keep in mind that I will have the exterior of the house exposed to do the work, not the interior. I keep reading differing arguments about this approach and the moisture problems it can create, so I need some clear guidance here.

Most of what I have read already relates to new construction with products being applied to the inside of plywood sheathing, and working in towards the drywall⦠however, our situation is reversed since we are taking all of the exterior siding off and re-insulating from there.

Thanks in advance for the help!


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Pumped in cellulose is usually what's done now, though fibreglas is also used. It's not something you can do yourself though, definitely a job for the pros. I had it done earlier this year on an old place. A vapor barrier was not used. And the exterior was not 'exposed' at all, small holes were drilled under the siding (one board of which was removed at a time) and the stuff pumped in. A vapor barrier should either be already in place at the foundation level, and if not it's something you want to discuss with a good, reputable contractor, not necessarily the insulation people. I wouldn't think a hybrid approach would be very good (why do it?).

    Bookmark   August 30, 2008 at 2:36PM
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I've always wondered about pumped in cellulose. Does it make wiring/rewiring hard in the future?

    Bookmark   August 30, 2008 at 5:36PM
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You should also give serious consideration to really high performance insulation in the floor of the attic because heat rises and is generally considered the most important source of heat loss. EnergyStar rated windows should also be put in the mix.

If/when you put in a new roof, make sure to get ridge vents, which will reduce the heat burden in the attic and lower your cooling costs in summer.

Insulation should also be installed under the floor of the first floor. There is some controversy over the need to insulate and ventilate the crawl space. Here in central North Carolina we keep the vents open year round except that we close the one closest to the boiler in winter. The walls of our crawl space are not insulated, but a heavy plastic sheet covers the crawl space unpaved dirt-- it's supposed to prevent moisture from rising from below.

    Bookmark   August 30, 2008 at 9:22PM
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You need to do wiring beforehand with cellulose, at least the big basic stuff with leads to clear spaces (or planned outlet boxes, etc.).

    Bookmark   August 30, 2008 at 9:30PM
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Are you sure there is no wall insulation? I can't imagine a recently redone lathe and plaster wall without insulation (and the necessary warm-side vapor barrier). Usually the dilemma is a house like mine with no wall insulation and original plaster and therefore no way to insulate without damage to the historic materials.

Don't get confused between the vapor barriers installed in the wall assemblies just underneath the interior wall finish, i.e. on the warm side of the insulation and the moisture and air infiltration barriers installed under the sheathing or other exterior cladding. The former is to block tramission of interior moisture into the insulation material where in cold climates it can condense and get trapped causing mold problems. The two later products are intended block exterior moisture, primarily rain, from being driven from the outside into the wall assembly.

The standard order of march in weatherizing an old house: insulate the attic cavities, first; then do a heroic job with the caulk gun to eliminate small air leaks, inside and out and install or replace weatherstripping around openings of doors and windows; then look at wall insulation. Note that under floor insulation is not high on this list even though it is an intuitive thing. Floor insulation projects are usually aimed at stopping air infiltration but most insulation materials aren't designed to combat infiltration, per se. However, tightening up other, higher, parts of the house will prevent them from drawing cold air up through the floor in the first place. This overall plan of attack is based on first doing the things with the most efficient payback in energy savings for the dollars invested. And sexy, or not, attic insulation and caulking are the top two.

For under your structure, you may need a vapor barrier laid on the dirt to stop moisture vapor from rising from the soil into your living space and the framing of the structure, and possibly ventilation of the crawl space (this is the controversial part and I think depends,greatly, on local conditions especially your outdoor drainage situation.)

BTW, I would carefully assess the choice between wood siding and Hardie-plank type products. They are not equivalent in how they look and I think the HP makes an old building look odd. Unless you siding is totally rotted, you can probably rehab most of it, then buy only a small amount to finish the job.

There is a series of National Park Service publications regarding the care of old buildings. One title is devoted to weatherizing old structures. I've attached a link below, scroll down the list of titles until you find the ones that interest you.



Here is a link that might be useful: National Park Service Preservation Brief Series - Wide range of old house topics

    Bookmark   August 31, 2008 at 4:04AM
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