Last week I shared part one of my query received back from Historic New England regarding preservation for my home. I’ve been a bit confused how to properly preserve and renovate my chimneys and fireboxes. It’s something that’s become a bone of contention—weather we should use stainless liners, thermocrete or nothing. I wanted to hear from the experts and get a preservation’s perspective.
We have four flues and three fireplaces that are exposed and one behind a wall. On the second floor of the house I would love to get two of the fireplaces working again. The den the room we’ve been working on for years now is my biggest priority. The second room will become a bedroom and I’m not as committed to having a functioning fireplace but the mortar in the firebox does need some work. Below tips from the experts as well as photos of each of our fireplaces.
Historic New England: Making old fireplaces functional is usually a matter of selecting which individual fireplaces are most desirable for active use since it is inadvisable from a safety and preservation perspective, and often cost prohibitive as well, to restore every flue and fireplace in a old or historic house to full operability. Regular use of fireplaces in an old chimney can stress the historic masonry and create opportunities for a fire. The safest approach involves adding a flue liner to the fireplaces that will be used and capping and/or sealing off the others.
Historic New England recommends use of a stainless steel “floating” liner in historic chimneys as floating liners do not alter or damage historic chimney masonry. A poured-in-place liner essentially replaces the historic chimney structure with a new solid masonry structure in which the old chimney is made redundant. While the poured in place liner is perfectly acceptable as a means of making an old chimney functional, the lining cannot be removed at a later date and thus violates the first principle of architectural conservation, that whatever modifications are made to an historic structure ought to be reversible without damaging or destroying the historic fabric of the building. Exceptions might be made to install a poured in place liner, for example, in a flue used for a heating system, which receives heavy use and requires a higher safety standard, or for a more recent or modern chimney, but if possible, introducing a poured in place liner into an old or historic chimney should be avoided.
Among the possible downside risks to the poured-in-place liner is the potential for the lining slurry to create pressure on or burst through a weak point in the masonry. The downsides of the floating stainless steel liner are 1) the need for a relatively straight shot for the steel liner (which cannot accommodate a significant angle in the flue) to run from the flue opening to the firebox and 2) the limited life span of the stainless material. These considerations may affect or limit the fireboxes selected for lining.
Within the fireplace opening, selective replacement of deteriorated brick may be necessary. A restoration mason should be able to replace individual brick in the firebox itself and repoint in an appropriate mortar. Traditionally, protected sections of 18th century chimneys were laid up using clay mortar, but the fireboxes and the exposed sections of the chimney at and above the roofline used a lime mortar as the lime mortar was more durable and weatherproof than clay. Care should be exercised not to introduce a modern Portland cement mortar into a historic masonry structure as the modern mortar will be stronger than the historic brick and cause cracking and deterioration. A high lime mortar, with very little cement and a high proportion of sand, is typical of historic lime mortars: one part cement, 4 parts lime and 12-15 parts sand may be used as a guideline. In historic masonry, the mortar is considered a “sacrificial” material, intended to give way, to flex within the structure, to absorb and release moisture from within the structure, and periodically to be replaced by digging out the old mortar and pointing with new.
Traditionally, the area around the fireplace opening inside the mantel, would have been parged, or coated, with a lime plaster, and then painted/stained black. Only high-style houses would have had tiled fireplace surrounds in the 18th century and many of those now present on older houses were likely to have been installed in the late 19th or early 20th century.
For more detailed information on the construction of fireplaces and brick manufacture, see A Building History of Northern New England, James L. Garvin, published by University Press of New England. Also useful is “Chimneys and Flues” by Russell Taylor:
For more detailed information on traditional mortars, see “Repointing Mortar Joints in Historic Masonry Buildings,” National Park Service Preservation Brief #2 available on the NPS website at nps.gov.
Capping unused flues at the roofline without a highly visible chimney cap may require a custom solution. Pop-up dampers, such as the “Chim-a-Lator” at Volko Supply, may have a sufficiently low profile (if painted black) not to obtrusive visually. If the chimney is not to be used at all, a bluestone cap can be mortared in over the chimney opening. Chimney “balloons” are available to place inside an unused flue but you can also fashion your own “balloon” using a “pillow” fashioned from house wrap (Tyvek, for example) stuffed with loose fill fiberglass insulation and stapled and duct taped at the seams. Extra caution must be taken, however, not to place anything in a chimney or flue that may be subject to heating or cross-over sparks from an adjacent flue.