(C) Daniel Friedman Proper Insulation Location in Cape Cod building Attics & in Crawl Spaces

  • INSULATION LOCATION for CAPES, CRAWLSPACES - CONTENTS: What is the the best location to place building insulation for heating & cooling energy savings and construction cost savings. Cape-style building roof & attic knee wall insulation advice. Crawl space insulation placement advice, & basement insulation advice. Insulation advice for attached building spaces such as mud rooms & Insulation advice for sunspaces. Insulation advice for interior walls, stairs, plumbing fixtures. What is the proper insulation placement for basements, mudroom insulation, insulation for sunspaces, and insulation for "interior bumps" or discontinuities in buildings such as stairways, cabinets, kitchen soffits, or plumbing chaseways
  • POST a QUESTION or READ FAQs about the proper location or placement of insulation in buildings

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Building insulation location & placement:

This article describes the optimum placement of building insulation for various building designs, problem spots, and hard-to-insulate or hard-to-ventilate building spaces.

We include Cape-style building roof & attic knee wall insulation advice or for homes with no basement, crawl space insulation placement advice, & basement insulation advice

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Where to Insulate Buildings - just where should insulation be placed?

Mineral wool insulation in an atticThis article explains the best options for insulating Cape-style homes, showing the heating and energy savings that come from proper placement of the building insulation.

Next the article discusses the best placement for insulation in crawl spaces. We explain that the location of building insulation is as important as its quantity. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Insulation Advice for Cape Style Homes

In the Cape-style under-roof insulation and ventilation plan suggested in this article and illustrated at page top, insulation location shown as option (B) at the right side of the sketch is the preferred insulation placement for a Cape-Cod home because

  1. Less total insulation is required
  2. The installation is easier
  3. A larger area of heated floor space is provided.
  4. The building heating costs will be less because there is less surface area through which heat may escape to cold spaces or to outdoors. Building and insulating knee walls (option (A) at page top) requires a larger insulated area so it is actually more costly to heat a building with this design.

Reader Question: details needed for Cape Cod Roof & Ceiling Insulation Placement & Materials

13 September 2015 Wendy said:

Our home is a Cape Cod 1 1/2 stories with living space on both floors. Roof has a ridge vent and roof fan with thermostat. Ventilation from the eaves is probable not enough after reading your articles.

We currently have removed walls on the dormer window side of the house for remodeling. Where to put the insulation? Two article I have read on your site and I am still not sure which option is the best for this situation. We would also like to minimize air leak. Is placing batts or foam board insulation between the roof rafters or insulating the living space preferable. Your diagrams are clear, just not sure how to apply the information you have provided. Thank you!

Reply: schematic for well-vented, high-R insulation of a cape cod sloped ceiling section or for a cathedral ceiling

Cathedral ceiling insulation detail (C) S Bliss D FriedmanWendy

[Click to enlarge any image]

Sprayed foam insulation will minimize air leakage; if you're opting for fiberglass - that may be more economical especially on small insulation jobs - work carefully to not leave voids in the insulation and be sure that any openings in the interior drywall are sealed against air leakage.

I would prefer to assure good air movement under the roof between the rafters from eaves to ridge, insulating using the details of the article above. For portions of the roof that form a ceiling over the occupied space, if headroom space allows, you can use solid high-R foam boards nailed to the rafter underside (then covered with drywall) to assure a good air passage;

Alternatively if headroom is sparse, you can nail furring strips along the rafter sides about 2" away from the roof deck, then cut and fit foam board between every rafter pair - a more labor intensive task that would be expensive if for hire but reasonable for a DIY project.

Fit the foam snugly to the rafter sides to minimize air or moisture leaks. Fill the remaining rafter space on the room side with un-faced fiberglass b atts or chopped fiberglass loose-fill or blown-in cellulose between the solid foam insulation and the the drywall on the ceiling surface.

This cathedral ceiling or cape cod ceiling insulation approach maximizes headroom, maintains an air path, and gives a high-R ceiling. In our illustration as shown, the closed cell solid foam foil-faced High-R insulation is itself a vapor barrier with a perm rating close to zero. Be sure that the fill insulation for the remainder of the roof cavity (shown as pink in the sketch above) does not include an additional vapor barrier: multiple vapor barriers risk trapping moisture in the roof cavity.

Styrofoam roof vent baffles installed in the eaves of a roof to provide air intak (C) Daniel FriedmanYou could use styrofoam roof vent baffles stapled against the roof deck and then fill the same space with fiberglass for a simpler but lower total R-value ceiling or roof insulation plan. But in my experience that approach does not give adequate ventilation for these reasons:

  • The air space is often compressed to less than an inch when insulation is pushed into the space between the rafters
  • The air space does not extend the full width of the roof deck between the rafters as the wide flanges of the baffles contact the roof deck with zero air space

At left from our photograph of a typical styrofoam roof vent baffle photo you can see that depending on the rafter spacing the roof vent baffle will not cover the full width and that we lose more width of air flow because of the edge flanges of the baffle.

More details are at


and at

CATHEDRAL CEILING VENTILATION where you will also see an original version of the sketch I show at above left but using only fiberglass, without the High-R insulation: that design yields a lower R-value and also omits the benefit of the role of the solid foam High-R insulation as a rigid barrier to guarantee that the air space stays open. More sketches of insulation and ventilation placement are in the two articles I just cited.

But in any under-roof insulation scheme beware of these common Cape Cod insulation problems:

Insulation Advice for Full Basements

Mineral wool insulation in an attic

Mr. Bliss prefers insulation on the basement interior for the same reason that apply to crawl spaces, including avoiding frozen pipes.

He suggests using a section of rigid foam insulation to separate off hard-to-insulate basement locations such as steel bulkhead basement walkout doors.

Our photograph (left) shows styrofoam insulating foam board used on the inside of a basement foundation wall.

We prefer to use solid foam insulation in any below-grade location that is at risk of period high moisture, because our field and lab work have shown up frequent hidden toxic mold reservoirs in fiberglass insulation that has been used in those locations.


Readers considering adding insulation inside or outside a basement foundation wall should
and also

Frost damage at basement exits to the exterior is discussed

Insulation Advice for Crawl Spaces

Crawlspaces are common in homes in the southeastern U.S. as well as in some west coast cities such as Los Angeles. Crawl spaces are a breeding ground for wood decay and mold because they combine a mixture of moisture, wood, and warmth. As Mr. Bliss points out in part 1 of the article above, less insulation will be required to insulate the crawl space walls and building rim joist than to insulate under the floors.

In cold climates insulating the floor may also require extra insulation on plumbing to protect it from freezing in the colder crawl area. In warm weather insulating the floor loses the cooling effect of its location over the cooler ground surface.

Crawl space insulation (C) Daniel FriedmanIn the crawl space insulation detail (sketch at left), beware of:

  • Crawl space moisture problems: contemporary best building practices call for installation of a 6-mil poly vapor barrier over dirt floors of crawl spaces; run the poly at least part-way up the foundation wall sufficient to cover the portion of the wall that is below-grade.

    Crawl space ventilation does not necessarily reduce crawl space moisture levels and in some weather conditions it can actually increase crawl space moisture, condensation, and mold risks.

    That's why modern best construction practice converts crawl spaces from the old "vented" design to new "closed, conditioned space". Just be sure to also identify and correct roof drainage or surface runoff problems that are sending water into the crawl area.
  • Watch out: Hidden termite damage to the building - we have found termites and on occasion other wood destroying insects attacking the building sills, rim joist, and floor framing when the poly vapor barrier and insulation extend from the floor to the building wooden sills.

    Insulation provides a hidden path from soil top into the wood framing. In the sketch, the use of poly under the insulation reduces but does not eliminate this risk.
  • Hidden mold contamination in buildings can be severe when fiberglass insulation is used in damp locations such as crawl spaces.


  • See CRAWL SPACE INSULATION RETROFIT for advice on using spray foam insulation products in crawl areas. In that article we add cautions about mold, moisture traps, and termite damage in crawl spaces depending on how insulation is installed.

Additional links to articles on good crawl space design and solving crawl space problems are provided below.

Insulation Advice for Mudrooms

In the articles above, the author suggests that attached spaces such as mudrooms that get frequent use should be inside the building's thermal envelope. If a mudroom is excluded from the building envelope of conditioned space (heated, dried, or seasonally cooled and dehumidified), there is an increased risk of mold since the rooms receive household moisture but little heat.

If the mudroom or airlock entry space is used rarely, such as an enclosed porch, you can go either way - insulating it or not, but if an attached room on a house is left unheated, be sure that it is also isolated from the home's moisture by proper placement of vapor barriers.

Insulation Advice for Sunspaces

Mr. Bliss points out that sunspaces are always insulated from the outdoors, but asks "... should they be insulated from the house?"

In climates that have frequent periods of cold and cloudy weather, it's a good idea to insulate and seal between the sunspace and the house - assuming that you are not trying to keep heat in the sunspace to keep plants alive through cold weather. If plants are being maintained in a sunspace you may need to heat that area as well.

Insulation between the house and the sunspace can be less than that in other house walls facing directly outdoors because of the buffering effect of the sunspace. But the air space between the sunspace and the house should be tight to keep greenhouse moisture from entering and causing mold or other problems in the main building.

In sunny climates that can keep the sunspace mass warm all winter, uninsulated walls between the sunspace and the main building are fine. Night insulation is a good idea for these sunspaces since the sunspace glass is part of the thermal envelope.

See SLAB INSULATION, PASSIVE SOLAR - slab insulation & vapor barrier placement in heated floor slabs.

Insulation Tips for Stairwells and Other Interior Discontinuities or "Bumps" in a House

Uninsulated attic stairs (C) Daniel FriedmanInterior walls, built-in cabinets, stairs, and plumbing fixtures can disrupt the continuity of the building's thermal envelope or shell.

Good planning during construction can avoid these problems by making sure that insulation and vapor barriers are properly placed. In retrofits to older buildings it's not so easy, Mr. Bliss notes.

For insulation retrofit on older buildings don't forget to watch for, evaluate, and if appropriate open and insulate these areas.

A common example that we see on older homes is the failure to insulate below attic stairs whose ceiling is exposed to the otherwise heated and insulated main area of the home.

In our attic stairwell photo (left) the house exterior wall and the area underneath the stairs themselves were uninsulated, leaking heat out of this home.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

What about Insect Damage or Mold Contamination in Insulation?

Readers concerned about termite damage associated with foam, fiberglass, or other building insulation materials should
see Insects & Foam Insulation


Readers interested in the mold resistance properties of foam insulation should

Original Solar Age Magazine Article on Building Insulation for Various Building Designs

"Where to insulate: the location of building insulation is as important as its quantity" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.

  • Where to Insulate in Cape Cod style homes & where to Insulate in crawl spaces - PDF form, use your browser's back button to return to this page
  • Where to Insulate - PDF form, part 2 addresses basement insulation, mudroom insulation, insulation for sunspaces, and insulation for "interior bumps" or discontinuities in buildings such as stairways, cabinets, kitchen soffits, or plumbing chaseways.


Continue reading at BASEMENT WALKOUTS & COVERS or select a topic from the More Reading links shown below.



Suggested citation for this web page

INSULATION LOCATION for CAPES, CRAWLSPACES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.


OR use the Search Box found below at Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References