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How Do You Insulate A Flat Roof

What Are The Benefits Of Xps Insulation

How to install Roll insulation on flat roofs.

When XPS insulation material is totally immersed in water, its water absorption rate is 0.3 percent. The closed-cell structure of XPS insulation makes it highly resistant to moisture. XPS roofing insulation complies with ASTM C578, a primary reference standard for polystyrene insulations.

XPS insulation is often utilized under a variety of single ply roofing membranes. In such instances, cover boards must be installed to prevent exposure to solar heat. Due to heat absorption and the maximum temperatures XPS insulation can endure, this insulation type is discouraged under black metal roofing.

Additional benefits of XPS include its reusability after roof leaks are repaired. When insulation is considered reusable, the amount of tear-off materials that go into landfills is reduced. The XPS insulation also retains its R-value and energy saving capacity even after exposure to water.

What If There Is Already Some Insulation In My Loft

If you do have existing roof insulation, the first thing to do is tocheck the depth. If there is only 25mm of insulating material, then this is likely to date back to the 1970s, as this type of insulation was subsidised then. In fact, any insulation that is less then 100mm is likely to be old and is worth disposing of it is also difficult to top-up older insulation to the recommended depth as modern insulation is sold in standard depths of 100mm and 170mm.

Most typically, especially where insulation has been put in since the 1980s, this will fill the approximately 100mm depth of the joists, and this is easy to top-up. The commonly available, low cost and subsidised products tend to come in either 100mm depth, to go between the joists, and 170mm to be laid over and across the joists, to take the depth up to the recommended 270 mm.

How Much Does It Cost To Insulate A Flat Roof

For the most up to date pricing on flat roofing insulation, follow the links in the section below.

In most cases, flat roofing insulation is included in the costings put together by installation companies when quoting for a new roof. Alternatively, existing insulation material may be reused when a new roof is installed if it’s still in good enough condition.

Professional firms will be able to supply a specific cost for insulation materials if that is your only requirement. You will also find it detailed on the quote sheet as part of the job.

Powerdeck F Insulation Board

If you are thinking of installing insulation into a warm flat roof yourself, the Powerdeck U Insulation Board is specifically designed for this purpose. It is suitable for use with any hot-applied waterproofing system and any bitumen compatible single-ply membrane system.

Another option is Powerdeck F Insulation Board – a durable, market leading PIR insulation board for built-up felt, mastic asphalt and single ply membrane waterproofing systems.

Both these options offer industry-leading insulation solutions for flat roofing at a manageable price point.

Also Check: Where To Buy Hydrostop Roofing

Unvented Roofs Are Easier

In many ways, its easier to build an unvented low-slope roof, and there are several ways to do it.

If you install a combination of closed-cell foam and vapor-permeable insulation, you also need to be sure that the foam is thick enough to keep the roof sheathing above the dew point during the winter. The International Residential Code specifies the following minimum R-values for the foam layer of these hybrid insulation sandwiches: R-5 in zones 1-3, R-10 in zone 4C, R-15 in zones 4A and 4B, R-20 in zone 5, R-25 in zone 6, R-30 in zone 7, and R-35 in zone 8.

While vented roof assemblies are designed to dry to the exterior, unvented roof assemblies are designed to dry to the interior. Thats why an unvented roof assembly should never include interior polyethylene. If a building inspector insists that you install some type of interior vapor retarder, you can always install a smart vapor retarder such as CertainTeeds MemBrain to satisfy your inspector.

Insulation Mistakes To Avoid

Techniques on How to Insulate a Flat Roof from the Outside ...

Fiberglass batts should never be used for insulating a flat roof. Unlike using them in walls or in an attic, there is a strong possibility of condensation forming and making these batts wet. That moisture will compress the insulation, destroying its effectiveness and possibly even causing it to fall out away from the roof, increasing the chance of more condensation forming in the future.

The old way of insulating a flat roof is referred to as cold roof insulation. This requires cutting insulation to fit into the spaces between the roof supports. A space is left between the insulation and the roof itself, which makes it possible for condensation to form in that space, attacking the roof sheathing and support beams. Using cold roof insulation techniques will result in rotted roof sheathing and beams, requiring major repairs in just a few short years.

Some roofing contractors will recommend a combined roofing system, where a thinner layer of insulation is applied on the top of the roof, with additional insulation below it. But this is no different than cold roof insulation and will result in the same problems.

Also Check: How Much For A Patio Roof

Prevent Rotting Of The Sheathing And Roof Deck

Across the US, the majority of flat roofs either have no ventilation at all, or are poorly vented. This is due to faulty original architectural design that did not take proper venting into account.

As a result, if you open up most flat roofs, especially those located in places like Arizona, you will see a lot of moisture, rot and mold as a result.

This problem also occurs in roofs that have fiberglass batt insulation and no venting. Sometimes, inspectors find that it is completely wet there is severe rot and mold growing in the sheathing.

To mitigate venting issues, and still have all the benefits insulation offers, its ideal to install high quality foam board products, such Polyiso or XPS. They are not susceptible to moisture infiltration and will keep the roof dry, even without a good ventilation system.

You just need to make sure that you put in enough insulation to keep the sheathing above dew point temperature in the cold months. For best protection, professional contractors recommend at least 6 inches of foam insulation board.

Roof Insulation Mistakes To Avoid When Installing Flat Roofing

Roofing insulation is one of the most important and misunderstood building components. There are many different types and installation methods out there, along with a lot misinformation about which products offer the greatest energy savings.

Here, we share the most common mistakes people make when installing flat roofing insulation.

We will also offer our professional take on what insulation actually works, how much it costs and the benefits you get as a homeowner.

Average Flat Roof Cost:

See Costs in Your Area! Start Here – Enter Your Zip Code

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What Is Polyiso Insulation

Polyiso is a commercial roofing insulation. The insulation boards, consisting of closed cell foam, come in thicknesses ranging from one inch to four inches. The boards typically are sized to be four feet by four feet or four feet by eight feet. Polyiso is installed under the roofing membrane.

Polyiso boards can be either tapered or flat. The tapered Polyiso boards are sloped. The boards angles direct water toward a drain, thereby efficiently preventing rainwater from standing along the roof or its corners. Flat Polyiso boards are installed on low slope roofs and direct water toward the gutter.

With The Thermano Thermal Insulation Installed

How to Insulate a Flat Roof , Floor , Stud Walls & Calculating U Values
  • You will have a roof with excellent thermal protection, and one of the best materials available on the market to compare performance, Thermanos lambda parameter reaches the level of 0.023 . This means Thermano has almost-twofold better insulation.

  • You will secure your roof for many years. Thermano is extremely durable, resistant to pressure and moisture, and does not lose its thermal insulation properties over the years.

  • You will immediately notice that less energy consumption is needed to heat your home, which will translate into significant savings.

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How Does A Warm Roof Solution Work

For a warm roof solution you will need toinstall insulation in the plane of the roof pitch, that is, immediately below the sloping roof. This will help to avoid your roof-space becoming excessively hot in summer or cold in winter, and will reduce heat losses from your roof. Assuming you don’t have a room in the roof, and it is just a ‘loft’, then you should also implement all the ‘cold-roof’ insulation measures described above, including any raised boarding as appropriate.

One important point to remember with a warm roof solution is the need forventilation immediately below the roof tiles. This is to prevent any condensation build up, or water getting in through or around the tiles. You need to create a space for air to flow parallel to the rafters and immediately under the tiles from one side of the roof to the apex and down to the other side, otherwise the roof structure may begin to rot.

Insulating A Roof: All You Need To Know

ByTim Pullen27 May 2020

Insulating a roof is one of the best energy saving home improvements you can make, says Tim Pullen

When it comes to insulating a roof, your choices will depend on:

  • your roof type
  • its structure
  • your willingness to invest

In most cases, extra investment made in beating the Building Regulations on insulation is money well spent it gives a lot back in returns.

Insulating a roof is perhaps the easiest of all the energy-saving home improvements. Mineral wool rolls are ubiquitous and cheap, meaning a whole roof on a standard home could be insulated to the recommended 270mm with a spend of little more than £200.

Its also within the capability of most DIYers, although not the most pleasant of tasks. And the effect it will have on your energy bills depends on what youre replacing and how big your roof is. Its also much more important to do if you dont have any loft insulation at all.

Read Also: How Many Screws Per Square For Metal Roofing

Insulating A Flat Roof

I am currently a licensed contractor performing remodeling services for residential and commercial customers in Wisconsin.

Read my step-by-step guide to insulating a flat roof.

Flat roofs have been a daunting task for contractors and homeowners alike since they were invented. They are hard to waterproof, hard to maintain, and hard to insulate. For today’s purposes, we will stick with the insulation issue.

In order to properly insulate a flat roof area, you need to be very careful on the material and the methods used. I will break these into sections for organizational purposes:

Insulating The Exterior Of The Roof

Flat Roof Joist Calculator

This is one of the most popular insulation methods for flat roofs. By insulating the exterior of the roof, professionals give themselves more space and flexibility to work. Contractors can use both spray foam and foam boards for this method. When using foam boards, contractors must ensure the materials are waterproof or have a waterproof membrane on top of them to prevent damage from the elements. Spray foam is a more convenient material for exterior roof insulation because it acts as a single, consistent layer across the entire building.

Read Also: How To Waterproof A Flat Concrete Roof

Five Ways To Insulate An Unvented Low

Of the various methods to insulate a shallow roof, applying insulation to the exterior is easier than working in a space with limited access. Whatever method is used, the amounts of insulation must achieve R-values mandated by local codes.

1. Exterior rigid foam

Install a thick layer of rigid-foam insulation above the roof sheathing.

Cut Panels On The Roof

Before you get through with any roofing work, you need to bring up the insulation up to the roof. In order to place it quickly, you may also need to cut them precisely according to the size needed.

However, it will be much more effective if you do it when you are up on the roof. It will allow you to bring the insulation in one or two times to go up and down by using the ladder.

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How To Improve Insulation Levels Of Flat Roofs

Homes with flat roofs require a different approach to insulation than the standard approach used for homes with pitched roofs. Because most flat roofs don’t have an attic, crawl space or venting, the insulation must go underneath the roof’s covering on the exterior of the home or be placed on the inside of the home. If there is a small crawl space available, you can add insulation there. For the homeowner with a flat or low-sloped roof, there are multiple solutions to improve insulation levels.

How Do You Insulate An Enclosed Porch

When should I Insulate my flat roof?
  • Use a vapor barrier to keep your floor above free from condensation caused by the new insulated area.
  • Roll out the fiberglass insulation sheets between the joists, facing side up, making sure it is snugly in place.
  • Cut styrofoam insulation to the correct size and layer it below the fiberglass already in place.
  • Recommended Reading: Where To Get Roof Trusses

    Insulating A Sloping Roof

    It’s worth insulating the sloping insides of a pitched roof in order to conserve heat if the space is to be used for anything except simple storage.

    It must be remembered that just because a roof space is insulated and the floor boarded, it does not mean that the room can be considered a ‘habitable’ room – for a full conversion of a roof space, building regulations must be met which includes suitable fixed access , lighting etc.

    Before fitting any insulation to the sides of a pitched roof space, any repairs to the roof should be completed – once the insulation is in place it will be harder to spot any leaks inside the roof and any leaks will soak the insulation itself.

    It is important that air is allowed to circulate between the roof covering and the insulation to avoid condensation – a gap of at least 50mm must be left between the insulation and the inside of the roof covering. This gap must be ventilated at the top and also at the bottom to allow the air to move.

    The insulation may consist of 2 or 3 layers :

  • Sheet insulation fitted between the rafters – the thicker the better but the size of rafter may limit the thickness possible once an allowance has been made for the required 50mm air gap.
  • A sheet vapour barrier fitted to the inside of the rafters – not required if the insulated plasterboard incorporates a vapour barrier.
  • Can You Insulate A Flat Roof From The Inside

    Can a flat roof be insulated from inside , from the ceiling of the area below? Insulating a roof from inside is technically feasible its whats called the cold roof principle. This system was often used in the past, but is not recommended nowadays. In fact, it may cause damage in the long term.

    Recommended Reading: How Much Should A Shingle Roof Cost

    Building Science Article: Ventilated Or Not Protection Against Condensation In Wooden Flat Roofs

    The New Quadriga is a German wood-frame construction magazine. The article gives an in-depth description of the development of the building science for wood flat roof construction. It notes that the ventilation requirement was based on the research of Prof. Liersch at the end of the 70s. His research showed that the ventilation could get rid of the humidity that was the result of diffusion into the insulation . However Prof Liersch’s study did not include humidity brought into the insulated flat roof cavity by convection . The article notes that the ventilation cannot overcome the significant humidity drive into the flat roof structure caused by small air leaks. Consequently the article concludes that the concept of venting a flat roof is no longer safe, and instead don’t vent the flat roof and install a smart vapor retarder inboard.

    Read the translated article here.

    Pipes Water Tank And Loft Hatch

    Flat Roof Construction

    Insulating between the joists of your loft will keep your house warmer, but make the roof space above colder.

    This means pipes and water tanks in the loft space could be more likely to freeze, so you will need to insulate them. If your water tank is some distance from the loft hatch, you will also need something to walk on for safe access.

    The cooler air in your insulated loft could mean that cold draughts come through the loft hatch. To prevent this, fit an insulated loft hatch and put strips of draught-excluding material around the hatch edges.

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    What Is Flat Roof Insulation

    A flat roof is defined as any roof that has a pitch of less than 10 degrees. In practice a flat roof is pretty easy to spot because essentially it is horizontal. Also flat roofs dont tend to have the loft space that you normally associate with pitched roofs, so on the whole they wont have the loft hatch in the ceiling.

    Therefore in insulated flat roofs, the insulating materials are laid between your ceiling and the roof membrane to prevent heat from escaping or coming into your property.

    What Thickness Of Roof Insulation To Use In Concrete Deck Warm Roof Construction

    The thickness of insulation required will be dependent on the U-Values you wish to achieve. In this warm deck construction, the number and type of mechanical fixings can change the thickness of insulation required. The below U-Value tables show the thickness of insulation required dependent on which insulation board youre using, for the detail outlined. These U-Values are based on telescopic tube fasteners with a thermal conductivity of 1.00W/mK or less, the effect of which is insignificant. To get U-Values specific to a different detail, use our free U-Value calculator.


    U-value results above based upon flat roof construction shown:

    Plasterboard on battens, 150mm concrete deck 50mm sand/cement screed AVCL bonded to screed with hot bitumen Therm Flat Roof insulation waterproofing layer . Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges

    *Where different thicknesses of insulation are used, the thicker Therm board should be placed outermost.

    When calculating U-values to BS EN ISO 6946, the type of mechanical fixing used may change the thickness ofinsulation required. The U-values in these tables were calculated using telescopic tube fasteners with a thermal conductivity of 1.00W/m.K or less, the effect of which is insignificant

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