Insulation Guide

Insulation for Older Homes: A Guide for Pre-War Houses

An old house is not a new house with less insulation in it. It was framed differently, sheathed differently, and it dries differently. That is why the work has an order: air sealing and the attic floor first, the cellar or crawl space second, the walls last and only when the wall can take it.

Pre-war houses are a large share of what we work in — balloon-framed colonials and farmhouses across northern New Jersey, three-story two-families in the older cities, Victorians along the Metro-North lines through Westchester and the Hudson Valley, mill-town cottages all over Connecticut, and the pre-war stock on both shores of Long Island. The National Park Service, which sets the federal standard for work on historic buildings, puts it in one line: “It is best to limit insulation to attics and basements where it can be installed with minimal damage to the historic building.”

How an old house was built, and how it dries

Balloon framing. Until roughly the 1930s, studs ran in one continuous length from the sill to the top plate at the roof, with floor joists hung off the side of them. The result is a stud bay that is an open chimney from cellar to attic, with nothing at either end to stop the flow. Today the code requires fireblocking “in concealed spaces of stud walls and partitions… vertically at the ceiling and floor levels” and horizontally at intervals not exceeding 10 feet — Section R302.11 of the IRC, in the 2021 edition Connecticut uses, the 2024 edition New Jersey adopted, and the 2024 edition behind the Residential Code of New York State 2025, which applies to permit applications filed from 31 December 2025. A house built in 1910 was not built to any of it.

Plaster and lath. Wood lath nailed across the studs, three coats of plaster pushed through the gaps. The plaster squeezing behind the lath forms “keys” that hold the wall on, and breaking enough of them brings plaster off in sheets — the reason old walls are worked carefully, and often from outside.

Board sheathing and no wrap. Boards nailed to the studs, siding over them, and in many houses no building paper at all. The boards have shrunk and the gaps leak air. They also breathe: an empty board-sheathed wall can dry both ways, outward through the boards and the siding and inward through the plaster, which is why it has lasted a century with nothing in it.

Solid masonry and timber frame. Brick and stone houses of this era are usually solid multi-wythe walls, not veneer over studs, with no cavity to fill; timber-framed houses have posts and braces inside the walls, so the cavities are irregular and blocked.

The common thread is drying. Building Science Corporation puts it flatly: “A wall assembly has to dry in at least one direction.” Fill an old wall and you take one of its two directions away. Which one you take, and whether the other still works, is the whole question.

Why the attic floor and the cellar come first

Heat leaves the top of a house and cold comes in at the bottom, and in a house with open stud bays running the full height those are the same problem. The attic floor is the cheapest square footage in the building to fix and the least destructive: nothing historic is disturbed, nothing has to be patched. ENERGY STAR’s retrofit recommendations for existing wood-framed buildings in our zones call for R-60 in an uninsulated attic and R-49 where 3 to 4 inches are already there. Our R-value page has the numbers by state and county.

The cellar or crawl space is second, because it sets the temperature of the air the house pulls upward all winter. In a dry basement holding the boiler and ducts, the foundation walls and rim joist are the boundary; in a dry, unheated cellar with the equipment elsewhere, the ceiling between the joists can be. Our basement insulation and crawl space pages work through that choice. The sill is usually a rough, out-of-square timber on fieldstone, and closed cell foam or cut-and-sealed rigid board is what fits it.

Air sealing an old house

Insulation slows heat; it does not stop air, and fiber insulation does not stop it at all — the Department of Energy’s air sealing checklist for contractors says fibrous insulation is not an air barrier and is not to be used for air sealing. The National Park Service puts reducing air leakage at the head of its list of retrofits for historic buildings, and notes that leakage of air into a building “can account for 5 to 40 percent of space-conditioning costs.” In a pre-war house, three leaks matter more than all the rest.

The chimney chase

An interior masonry chimney runs from the cellar to the roof, and the framing is deliberately held off the brick. The code requires a 2-inch airspace clearance to combustibles for a masonry chimney inside the building, says that airspace “shall not be filled, except to provide fireblocking in accordance with Section R1003.19,” and requires the spaces where the chimney passes floors and ceilings to be fireblocked with noncombustible material securely fastened in place (Sections R1003.18 and R1003.19 of the IRC, in the same three editions). In most old houses that fireblocking is missing, so the chase is an open shaft. It gets sealed with sheet metal and high-temperature caulk, never canned foam — ENERGY STAR says to use high-temperature caulk at a flue and not spray foam, and the Department of Energy’s job aid calls for a fixed dam at least 2 inches higher than the final insulation level, with combustibles kept 3 inches off the flue.

Balloon-framed walls at the sill and the top plate

Every open stud bay has to be closed at both ends — capped at the top in the attic, blocked at the sill in the cellar. The Department of Energy’s checklist says openings like the tops of balloon-framed gable walls are to be closed off with a solid material such as rigid foam or OSB, sealed at the edges with caulk, sealant or mastic, not stuffed with fiberglass. That blocking is also what makes dense-packing possible later: Building America’s guidance for a balloon-framed house is that blocking has to be installed at the top and bottom of the walls, putting a barrier between the wall cavities and the basement below and the attic above, before a cavity can be packed at all.

The hatch, the knee wall doors and the rest

The attic hatch is a hole in the ceiling with a board over it: weatherstripping, stops, a latch that compresses the gasket and rigid foam on the back turn it into a door, and a pull-down stair gets an insulated cover. Then the ordinary list — top plates, plumbing chases, dropped soffits over cabinets and tubs, wire penetrations, and the open joist bays under knee walls. Air sealing pays for itself first, and in a balloon-framed house it is not optional. Because tightening a house changes how a natural-draft boiler or water heater vents, we check the draft at those appliances before the work starts and again when it is finished; if one is not drawing the way it should, that repair belongs to a heating contractor, not to us.

Insulating the walls of an old house

This is the question most people arrive with: can you insulate existing walls without tearing them open? Usually yes, through holes. We do that work — dense-pack cellulose and injection foam in closed walls — and the honest answer includes when not to.

Dense-pack cellulose

Cellulose blown at pressure into a closed cavity until it takes no more. The Department of Energy’s Building America program puts the target at 3.5 to 4.5 pounds per cubic foot for cellulose and 1.8 to 2.6 for fiberglass, and says dense-pack prevents settling, fills the cavity completely, gives consistent insulating values and slows airflow. That last part is what matters in an old house: at that density the material itself resists the air movement the wall has always had. Cellulose runs about R-3.2 to R-3.8 per inch, and pre-war framing is full-dimension lumber — a 2x4 in a 1910 house measures a true 4 inches, so a filled bay lands around R-13 to R-15. The same bay in a house framed after the war is 3.5 inches deep and lands around R-11 to R-13.

The method is a 1-inch or 2-inch hole in the center of each stud bay, roughly 6 inches below the ceiling, plus holes below any blocking, above door headers, above and below windows, and behind cabinets. Building America says to re-verify with an infrared camera that every cavity is completely filled before patching — which is also how a homeowner checks the work, since a thermal scan on a cold day shows a missed bay immediately.

Injection foam

The other through-the-hole method, and a different product from the two-part spray foam that goes into an open wall. Injection foams are mixed to expand and cure slowly, so the material flows around lath nails, wiring and diagonal bracing before it sets and does not build pressure against the back of the plaster. That is the whole point of them: a standard expanding foam pumped into a closed plaster wall can push the wall off the lath. How a given foam is mixed, and how much belongs in a bay, comes off the manufacturer’s product data sheet, and any contractor filling a plaster wall should be working to one.

From the inside or from the outside

Outside is usually better in a plaster house. The Department of Energy’s renovation guide says drill and fill is not recommended from the inside because it can damage the plaster, it can disturb lead paint, and it produces a great deal of dust; it also warns that drilling from outside can break plaster keys on the inside face, a real risk in soft century-old plaster. From outside, a course of siding is pulled, the sheathing drilled, the cavity filled and plugged, and the siding replaced with nothing showing. Interior drilling is the fallback for a party wall or a wall behind a porch roof.

The best moment is a re-siding. Building America calls that the ideal time for an exterior installation, and ENERGY STAR says that whenever siding comes off an uninsulated wood-frame wall, drill and blow the cavity before the new siding goes on and, in our zones, add R-5 to R-10 of insulating sheathing beneath it. That sheathing is the one move that fixes thermal bridging through the studs, which the Department of Energy notes can cut a wall assembly’s R-value by 25% to 40%.

Patching and paint

Interior holes come back as patched, sanded compound or plaster over a plug, ready for paint. We do not repaint rooms, and a patch in a wall not painted in twenty years will read as a patch until the wall is. Holes in wood siding are plugged and left ready for finish; holes through sheathing behind removed siding are invisible once the siding is back, and vinyl or aluminum comes off and goes back in whole courses. Stucco is the exception twice over. The patch shows, and Building America calls stucco “a high-risk cladding” for filling a cavity with spray foam, “as it is likely an airtight cladding system with low vapor permeance, and thus low drying to the exterior”; it does not recommend cavity spray foam in a stucco-clad wood-frame wall “because of the risk prolonged dampness should any water enter the walls.” That finding is about spray foam rather than dense-pack cellulose, but the reason behind it — a wall that cannot dry outward — is reason to look hard at a stucco house before filling anything.

When to stay vapor-open, and when to leave the wall alone

Fill a board-sheathed wall and the inward drying path closes down to whatever the fill lets through. Cellulose and open cell foam are vapor-open, so the wall can still dry inward through the plaster. Closed cell foam sprayed against the back of old board sheathing is not: it shuts that path, and the sheathing is left depending entirely on drying outward through the siding — the direction nobody can vouch for on a century-old house, with original clapboard under twenty coats of paint, or vinyl or aluminum laid over it, or stucco, and no housewrap and no drainage plane behind any of it. Building America’s finding on cavity spray foam runs the same way: fill the cavity and “less drying of the wall assembly will occur,” and how much risk that carries depends on what the cladding will let out. That is why we do not fill old board-sheathed walls with closed cell foam. Building America adds that if water has been getting into a wall, insulating before fixing the leak increases the risk of moisture damage.

Sometimes the answer is no wall insulation at all. The Department of Energy says of lath-and-plaster houses that if you want to keep the plaster, the best approach is rigid insulation on the exterior, and otherwise “you should probably leave the walls uninsulated and just do airsealing.” The National Park Service is blunter: blown-in insulation in historic wall assemblies “may trap moisture within the wall and lead to accelerated and often hidden deterioration of the structure.” We will tell you when your house is one of those. Our wall insulation page covers open walls and code R-values.

Knob-and-tube wiring, vermiculite and lead paint

Knob-and-tube has to be dealt with first

Knob-and-tube is the original wiring in most pre-1940 houses: single conductors through porcelain tubes, supported on porcelain knobs, no ground. It sheds heat into open air, and burying it changes that. The Department of Energy’s Building America guide points at the scope section of the National Electrical Code, Article 394.1, and states that the NEC does not allow insulation to be applied to or surround knob-and-tube wiring; it tells homeowners to replace active knob-and-tube before a contractor begins the insulation project, lists live knob-and-tube in the attic as a stop-work condition, and says drill and fill should not be used where knob-and-tube is in the wall, because insulation can push the wiring against the studs. New Jersey, New York and Connecticut each adopt the NEC on their own cycle, so the edition that governs is whichever one your state has adopted.

We do not do electrical work. A licensed electrician rewires the circuits, or confirms in writing which runs are dead, and then we insulate. Plenty of old houses have knob-and-tube abandoned in place decades ago; the job is proving it, not assuming it.

Vermiculite

Poured loose into attics and sometimes into wall cavities, vermiculite is, in the EPA’s description, “a pebble-like, pour-in product and is usually gray-brown or silver-gold in color.” The EPA says to assume it may be contaminated with asbestos, to leave it undisturbed in your attic or in your walls, and to hire a professional asbestos contractor for renovations that would disturb it. That is licensed abatement work and not ours; the first step is a test by an accredited lab. The same caution applies to old pipe and boiler wrap in the cellar.

Lead-safe work on pre-1978 paint

Every hole drilled in a painted surface in a house built before 1978 is a lead question. Under the EPA’s Renovation, Repair and Painting rule, firms performing work that disturbs lead-based paint in pre-1978 homes must be EPA- or state-certified and must use certified renovators who follow specific work practices. The EPA treats work disturbing six square feet or less of interior painted surface per room, or 20 square feet or less outside, as minor repair and maintenance outside the rule; a whole-house drill-and-fill is far past that line, so the rule applies to it. In practice that means containment, plastic and HEPA cleanup, and it is one more reason interior drilling is the second choice. Before anyone drills a pre-1978 wall, ask for the firm’s EPA or state certification and the name of the certified renovator who will be on the job — ours, and every other company you are quoting.

Rot, damp cellars and fouled insulation

Old houses hide water: sills rotted under a gutter that has run down the wall for years, joist ends powdery where they sit in a stone foundation, a chronic wet corner in the cellar. Insulating over rot makes it worse, because the wood gets colder and stays wetter. Structural repair belongs to a carpenter and standing water to a waterproofing contractor, and both come first; the Department of Energy also tells contractors not to air seal or insulate under a leaking roof. Attics fouled by squirrels, mice or bats we do take: removal and cleanup under containment into a HEPA insulation vacuum, then a fresh floor. Mold remediation is a separate licensed trade.

Brick, stone and timber-framed houses

A solid masonry wall has no cavity, so the only choices are the inside face or, rarely, the outside. The Department of Energy’s guidance is a genuine warning: interior insulation on a solid masonry wall “creates a serious risk of moisture damage,” because the bricks end up colder, and where winter temperatures go well below freezing, repeated freeze-thaw cycles can permanently damage the walls. Its researchers do not recommend fiber insulation in direct contact with masonry, and say to cut rain absorption first — gutters, flashing, repointing. For most brick and stone houses here the practical answer is: do the attic, the cellar and the air sealing, fix the water, leave the walls.

Brick veneer over a stud wall is a different animal and behaves like any framed wall, which Building America calls relatively safe to fill, on the assumption that “the brick veneer is properly ventilated (i.e., cavity is not filled with mortar).” In timber-framed houses, an infrared scan before drilling settles which bays actually exist.

Capes, colonials, knee walls and cathedral ceilings

A cape or colonial with finished rooms upstairs has a specific set of voids: a knee wall on each side, a small triangular attic behind it, the sloped ceiling over the rafters, and a flat ceiling in the middle. Insulating only the knee wall is the classic half-job, because the floor joists under it run straight out into the cold attic and carry air under the finished floor. The Department of Energy’s guidance is to plug those open joist cavities with rigid foam or insulation-stuffed bags, insulate and air seal the knee wall or cover it from the attic side with sealed rigid foam, and treat a knee wall door like an exterior door. The alternative is to carry the insulation up the roof line so the small attic comes inside.

The sloped ceiling above is the hard part: 2x6 or even 2x4 rafters with no room for R-49. Either the slope stays vented, with the 1-inch space between insulation and roof sheathing that Section R806.3 of the IRC requires, or it becomes an unvented assembly under Section R806.5 — the 2021 edition in Connecticut, the 2024 edition in New Jersey and New York — which permits that only under a list of conditions, among them that where air-impermeable insulation alone is used it be applied in direct contact with the underside of the roof sheathing. Our roof and cathedral ceiling page works through it rafter by rafter.

Historic districts and the outside of the house

In a local historic district or on the National Register, the exterior is regulated and the interior usually is not. Attic floors, cellars and rim joists never come up; drilling siding does. The National Park Service sorts energy retrofits into two groups in Preservation Brief 3, and its first group, the one that “requires minimal alteration,” is reducing air leakage, adding attic insulation, installing storm windows, insulating basements and crawlspaces, sealing and insulating ducts and pipes, and weatherstripping doors. Adding insulation to wood-frame walls and to masonry walls sits in the second group, which “requires more alteration.”

So do the invisible work first, and take any exterior drilling to your town’s historic district commission or architectural review board as a specific plan: which elevation, which course of siding, how many holes, plug material, finish. We write that plan up and supply the product information behind it; the application itself goes in under the owner’s name, because the approval is issued to the property. Storm windows are outside what we do, but they belong in the same submission — they address the most visible source of cold without touching the original sash.

What not to do in a 100-year-old house

  • Do not spray closed cell foam against old board sheathing. It shuts the wall’s inward drying and leaves the sheathing depending on siding and paint to dry outward. Vapor-open fill belongs in those cavities.
  • Do not blow insulation over live knob-and-tube. Rewire first, or have an electrician certify the runs are dead.
  • Do not disturb vermiculite. Have it tested; leave removal to licensed abatement.
  • Do not insulate under a leaking roof, over rotted framing, or above a bath fan that dumps into the attic.
  • Do not stuff fiberglass into the top of a balloon-framed stud bay and call it sealed. Air goes straight through it; the bay needs a solid cap sealed at the edges.
  • Do not put polyethylene on the inside of a masonry or board-sheathed wall. A wall with a barrier on both faces cannot dry either way.
  • Do not accept a quote that names a depth of insulation and never mentions air sealing. In a house with open stud bays, that is half a job.

A realistic order of work

  1. Look first. An energy assessment with an infrared camera, outlets opened to check for existing wall fill, and the wiring, roof and water history established. The energy audit is free.
  2. Clear the blockers. Roof leaks, rotted framing, active knob-and-tube, vermiculite, bath fans venting into the attic, a wet cellar corner. Some of that is ours; some belongs to an electrician, roofer, carpenter or waterproofing contractor.
  3. Remove what has to go. Rodent-fouled or water-damaged insulation comes out under containment into a HEPA vacuum. Dry, intact insulation usually stays.
  4. Air seal the ceiling plane and the cellar. Chimney chase, top plates, balloon bay tops and bottoms, plumbing chases, dropped soffits, the hatch, knee wall doors, the rim and sill.
  5. Insulate the attic floor to the R-value your state calls for, with baffles at the eaves so the soffit vents stay open.
  6. Insulate the cellar or crawl space — foundation walls and rim joist, or the ceiling in a dry, unheated, unused cellar.
  7. Then decide on the walls, with the first steps done and a heating season of evidence behind you.

Steps four and five are the least destructive work in the house and the part we would do first in any pre-war house. Stopping there for a season is a sound decision: the walls will still be there, and a winter of living with the change makes the wall call an easier one.

What insulation costs in an older home

Most of our projects run between $3,500 and $12,500. An attic floor with a chimney chase, a dozen open bay tops and a knee wall costs more than a plain attic of the same size, and it is worth more. Spray foam is the highest cost per inch and earns it at an irregular rim joist, a shallow cathedral slope or a rubble foundation wall; cellulose and fiberglass cost less and are the right call on an attic floor and in most closed walls. Our spray foam cost page and the materials page go further.

State and utility energy programs in New York, New Jersey and Connecticut offer incentives for insulation and air sealing, and eligibility usually depends on an assessment: see rebates in NY and NJ and rebates in Connecticut. The federal 25C credit ended for property placed in service after 31 December 2025.

Common questions about insulating an older home

What is the best insulation for an old house?

By location, not by product. Blown cellulose or fiberglass on the attic floor, over sealed top plates and a sealed chimney chase. Closed cell foam or sealed rigid board at the rim joist and on a rubble foundation wall. Dense-pack cellulose or injection foam in closed wall cavities that can take it. The material to avoid in an old wall is closed cell foam against original board sheathing: it shuts the wall’s inward drying and leaves the sheathing depending on the siding to dry outward.

Can you blow insulation into the existing walls of an old house?

Usually, yes. We drill a 1-inch or 2-inch hole in each stud bay about 6 inches below the ceiling, plus holes below any blocking and above and below windows, dense-pack cellulose to 3.5 to 4.5 pounds per cubic foot or inject a slow-expanding foam, verify the fill with an infrared camera and patch. In a balloon-framed house the bays have to be blocked at the top and bottom of the walls first, or the material runs away into the cellar and the attic.

Is it better to drill from the inside or the outside?

Outside, in a plaster house. The Department of Energy says drill and fill is not recommended from the inside because it can damage the plaster, disturb lead paint and produce a lot of dust. From outside, a course of siding comes off, the sheathing is drilled and plugged, and the siding goes back with nothing showing. Interior drilling is the fallback for walls that cannot be reached from outside.

Can you insulate a house with knob and tube wiring?

Not while it is live. The Department of Energy’s Building America guidance points at the scope of the National Electrical Code, Article 394.1, and says the NEC does not allow insulation to be applied to or surround knob-and-tube wiring, and that active knob-and-tube must be replaced before insulation work begins. Many old houses have knob-and-tube abandoned years ago; a licensed electrician confirms which runs are dead, or rewires the live ones, and then the insulation goes in. We do not do electrical work.

Should I insulate the walls of my old house, or just the attic?

Attic first, every time, along with air sealing and the cellar. That is where the heat is going, and it is the least invasive work in the house. Walls are last, cost the most and carry the most risk, and in some houses — soft plaster, solid masonry, a wall with a water history — the right answer is to air seal and leave them.

How do you air seal a balloon-framed house?

By closing every stud bay at both ends. In the attic, old insulation is pulled back at the eaves and each open bay capped with rigid foam or wood sealed at the edges; in the cellar, the bays are blocked at the sill. The chimney chase is sealed with sheet metal and high-temperature caulk, keeping combustibles 3 inches off the flue. Then the hatch, the top plates, the plumbing chases and the dropped soffits.

Is spray foam a good idea in an old house?

In the right places. Closed cell foam is the best answer for an out-of-square rim joist, a stone foundation wall and a shallow cathedral slope, where it seals and insulates in one pass. It is the wrong answer sprayed into a closed cavity behind original board sheathing, and a standard expanding foam in a plaster wall can push the wall off the lath. Injection foam made for existing cavities expands slowly for exactly that reason.

Do I have to remove the old insulation first?

Not usually. Dry, intact insulation can be left and topped up; ENERGY STAR notes that dirty-looking attic insulation is the result of filtering dusty house air and is still fine to use. It comes out when it is wet, when rodents have fouled it, or when the air sealing underneath cannot be reached any other way, and we handle that removal under containment with a HEPA vacuum. Vermiculite is the exception: it stays until a licensed professional tests it.

How much does it cost to insulate an older home?

Most of our projects run between $3,500 and $12,500. Where an old house lands depends on how much air sealing the framing needs, whether anything has to come out first, how many areas are treated, and how the walls are accessed and patched. The energy audit is free.

Have someone look at the house

We walk the attic, the cellar and the rooms in between, open an outlet or two to see whether the walls hold anything, scan with an infrared camera where it helps, and tell you what the house needs in the order it needs it — including the parts that are not ours to do. The Home Insulators is family-owned, based in New Rochelle, New York, working since 2015 with a crew of more than a hundred that we never subcontract, open 8am to 8pm seven days. Call 914-554-2289 or use the contact page. We cover all of New Jersey and Connecticut, New York’s Hudson Valley and Long Island; the service area page has the list.

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