One is that will reduce the average u-value of the floor. The second is that there could be slightly greater potential for mould, which should be addressed by ensuring a well ventilated sub floor area.
Luckily the sub floor area of our home was pretty dry to start with. I am doing the same job soon. There is no insulation and lots of gaps in the tongue and groove so letting in huge amounts of air conditioning at ground level. I thought of using omni fit knauf mm. I was planning on just getting the insulation, staple gun and netting then placing it under all gaps and fixing in place. Loads of ventilation down there so not an issue there.
What brand of vapour permeable membrane is needed please and what does it do? I thought actually any membrane albeit breathable would be the same as a netting if not worse as would create less air flow inside.
Is it more to do with lessening the temperature differences between floor and rockwool therefore reducing damp? Actis insulation make great products as an alternative to traditional pir and wool products and a lot easier to use. This may be a really stupid question — but is adding a layer of insulation OVER the existing suspended floorboards and under a new layer of plywood underlay topped with new floorboards possible??
Or would this somehow lead to damp? I do think the cons section for the PIR insulation should be updated to note that it is highly combustible and in combination with timber floors could present a significant fire risk. As another poster points out this could cause resale and building control issues further down the track. Hi James, Thanks for your comment. For clarity, it is also worth stating that this was in a completely different application, as part of a complex cladding system on a high rise block of flats.
Use in this high rise flat application is quite rightly no longer permitted. In that sense there would be no building control issues as it is an approved material. I have not spotted any reports in regard to fire in domestic houses or bungalows associated with PIR, despite its widespread use. I am in the middle of insulating the floor.
I had some over mm PIR board from another project but have found it to be difficult to cut at the board to get a good 90degree angle, i am cutting across the the short edge but its still a pain to get right as its so thick. I am now considering a 60mm PIR board and topping it with rockwool. Just after some advise if this is a viable solution Thanks. Also any damp from the external walls could spread through the rockwool with no means of escape.
See the left hand image below as an example. Hi Jon I am about to start getting the raised timber floor of our s house insulated, together with adding wet underfloor heating using 22mm chipboard pre-routed above the joists. I am keen for the maximum insulation level being achieved to minimise heat losses, and was considering using the flexible insulation detail you show above as I thought the rigid one could result in gaps where joists arent perfectly parallel or slightly unevenly spaced.
How did you go about this issue with the rigid insulation? Hi Andres, With rigid board you can measure the gap between the joists and cut the board accordingly. Not always perfect but you can fill any gaps with expanding foam and also tape to prevent any air movement. Nowadays if insulating from above I would probably use flexible insulation supported by a breathable membrane, primarily as it means the whole solution is breathable.
Great site. Please can you advise me on insulation below the floorboards of a bedroom that half sits above a long porch or loggia that is unglazed. The rest of the floor is above the hall. Our house was built in and we have access under the floorboards in the bedroom. Many thanks Esther. I We are looking at installing underfloor insulation under suspended timber floor of a bungalow and are engaging with a contractor as part of green homes grant.
If we go ahead, are we going to experience thermal bridge and damp issues because of different thermal protection? Is there any way to isolate kitchen by lifting central part of the floors and keeping cabinets in place? Brilliant article and very informative for a beginner.
I had insulation put under the suspended timbers of three rooms in July. It was insulated with PIR from beneath. I have just started to notice damp patches on the internal hallway walls.
Do you think this is likely due to the insulation or other human habits cooking, showering, breathing etc. Any suggestions would be appreciated! Hi Izzie, Thanks for the feedback.
The moisture is likely to be a combination of slightly less ventilation in the hall and water vapour from some of the activities you describe. Insulating the floors often significantly reduces the airflow that comes from under the floorboards which can change the balance.
It would be worth considering how to reduce moisture in the house, potentially with improved extraction in the kitchen, bathroom and utility room. Making it warmer can help — although potentially also move the problem elsewhere. It may also be worth looking at the airbricks to ensure there is adequate flow under the floor. Just the information I need as I wish to insulate under my draughty ground floor. I have a cellar under the house so access is fine — it is also vented quite well.
Everywhere else I read tells me to fit a VCL on top, but your diagram for fitting from underneath does not show one. If you put a breathable membrane under that it will support the insulation and stop draughts going through the material. The house is old, with stone walls so as you say I need a breathable option down there with a breathable membrane to hold it in place.
No option to fit a membrane above the the insulation though. How about using Spray Foam Underfloor Insulation? I have read that traditional insulation such as fibreglass, PIR board or rock wool does not produce an air seal, and It can also move and sag, causing drafts to appear and loosing its thermal resistance over time becoming ineffective.
Thanks for your comments. There are certainly other options for applying insulation. As mentioned in the article, do read Using Robotics for Underfloor Insulation of Suspended Timber Floors as this shows how spray on insulation can be accurately applied to avoid spraying items such as electrical cables and gas pipes when insulating from below. Fibre glass and rockwool can be great for floor insulation when used with a membrane to prevent air flow, See the diagrams above for how either a vapour control layer or a breathable membrane can be used.
Current thinking is that the breathable membrane can be used to provide the support for flexible insulation if the floors are insulated from above with a vapour control layer covering the insulation. Rigid insulation should certainly be taped to ensure there are no air gaps.
Many older houses with suspended timber floors have big draughts and inadequate floor insulation; this can cause cold feet as well as higher energy bills.
Installing under floor insulation can address both these problems. This article is really helped me a lot. Thanks a lot. One must check this Moisturebarrier. A brilliant piece of content with some common questions answered! It continues to amaze me that people often overlook both the quality and suitability of their insulation.
Always stick with the reputable brands — such as Celotex, Rockwool and Dritherm. Hi After reading this article 3 times I have asked for pro advice my local guy and the main problem is not the U-value, but draughts. Also FYI Kingspan can be installed only when you lift the floor than finish with foam tones of it if the floor is not even, fire proof pink foam. If you do it from under the floor use mineral wool.
I have done it this way and the difference is degrees in my house on the ground floor, Kingspan is to be used on the walls, it has to be covered with drywall to be safe for fire regulations, so when you will do Home report you may get a surprise, also may not be impressed when it comes to home insurance payout, Only mineral wool is fully non-combustible. I would suggest to speak to your local council before you do this. Quality mineral wool will stop draughts as you can push it near walls and will work really well in the cold days and it is much cheaper.
Remember to put a net and staple it to hold it nicely. It will also stop air, wind and high air pressure coming with storms and pushing this cold air all the way up to your house. Do u cover Peterborough at all pe2 8td bungalow with crawl space of 4ft under floor. House is very cold and requires a breathable solution for the floor. Darren sanchez.
Hi We insulated under our year old granite farmhouse boarded floor with garden netting and mineral wool when new flooring was put down.
Just recently — say 5 years on and we are getting damp floor boards in one gable end room. Any help appreciated plz before we attempt taking up the boards! I need to insualte my cellar roof, but the cutting is going to be long and difficult as the joists are different sizes and angles between each one. Would there be benefit stapling thermal foil to the bottom of the joists?
However this would leave a gap still. Good luck with the insulation. If access from the cellar is not a problem amd you have problematic angles on the joists it may be worth considering mineral wool with a breathable membrane to support it. Although rigid board is my preferred solution as it offers the best U-Value and it helps to stop air infiltration, it can sometimes be easier to work with something like mineral wool that you can stuff into difficult areas.
The membrane supports the wool as well as providing a reduction in draughts. It does create its own challenges in terms of handling and fixing. Using rigid board you do need to fill any big gaps with expanding foam as otherwise you will lose heat via air infiltration. I did use aluminium tape to cover between the joists and the rigid board and this would reduce air infiltration through small gaps.
Generally low emissivity materials such as foil are used to reduce radiant heat losses in unventilated airspaces such as a cavity or a small gap as described above.
According to CIBSE Guide A the bible as far as these sorts of things are concerned in a 25mm unventilated cavity it can increase thermal resistance from 0. However I have no reference data on ventilated airspaces so suspect the benefit would be marginal with the airflow being the biggest factor. Maybe another commenter with more expertise in this area would be able to offer further insight.
I would be slightly nervous of the foil preventing any moist joists from drying out from below but it depends on the dampness and airflow in your cellar.
Useful discussion. As you say, the simplest way is to batten out the joists and fit rigid insulating board. What are the pros and cons of using foam backed radiator reflector pinned to the battens to reduce the bridging effect of the joists? And would draft excluder on the battens or joists deal with most gaps up to about 7mm.
Hi Stuart. I would struggle to comment on the technical benefit of foam backed radiator reflector. My experience of heat calculations on low emissivity materials is when an air cavity is involved so not sure how to approach it when there is no air gap. I like the idea of draught excluder; that seems a good solution if you run a batten the full length of the insulation edge along the joist.
Thank you for a great article and insight to how to go about the job using different insulation materials and different methods. Also big thank you to Miles for pointing out issue of thermal bridging through the joists was bugging me for some time.
I was thinking of the solution to thermal bridging and below you will read how I would like to eliminate to certain extent thermal bridging. The floor to be insulated rests on 4 brick wall supports with gaps here and there to allow for air circulation. I would like to screw few metal brackets to the walls supporting joists and place PIR board on them.
Seal any gaps on the sides with expandable foam. PIR board is underneath and across the joists. There is still about 20cm of space after fitting PIR board for the air to circulate u see the floor.
Original wooden boards will go back on top and large gaps between the boards closed with pine sleepovers. Now thermal bridging occurs on much smaller scale and only in places where joists are resting on brick wall supports. After fitting PIR boards there is about 20cm gap for air to circulate underneath the insulation. Hi Jon, the entire ground floor of our y. They used chip board on brick piles for the new floor.. The gap below the raised floor is about 35cm. We want to lay reclaimed floor boards over the chip board to match the upstairs.
Any help would be very appreciated. Am i right in thinking to use a breather membrane above the insulation and below the new floorboards? Hi Paul I can appreciate your confusion. If the floor joists are enclosed, the contractor can spray foam insulation into the space.
Other options are also available. Because a homeowner needs to prevent moisture buildup, some forms of insulation, such as batts, may do more harm than good in this given situation.
A qualified contractor will be able to recommend the right material for the job. According to the experts at This Old House , underfloor insulation will help the house retain heat, but it will not necessarily make the floor warmer by itself.
Homeowners expecting to have a heated-floor effect after they insulate beneath the surface will be disappointed. Hot air rises, therefore, the cold air outside the house will actually make the insulation colder than the interior. This natural phenomenon will not make the floor colder, nor will it create a warming sensation.
Underfloor insulation will, however, prevent heat loss. Insulation impedes temperature change, meaning less energy is needed to maintain your chosen temperature. This percentage can be much higher in older homes with hardwood floors—or floors made of other materials that conduct heat and cold. By preventing this heat loss with underfloor insulation, homeowners will see an energy bill savings of at least 10 percent.
Wood floors are especially vulnerable to moisture. A layer of insulation between the floor and crawl space helps create a vapor barrier, preventing moisture from creeping in and soaking the wood floor which could eventually stain or warp the wood. Panels or insulation with rigid corners is the best option for creating a complete barrier. In some houses, drainage and ventilation in the crawlspace can also play a role in combating moisture.
Homeowners should consider hiring a professional to check this space for other moisture problems before investing in underfloor insulation. A vapor barrier could also benefit homes with damp basements.
Homeowners left to rely on sump pumps or dehumidifiers to keep their basements dry should consider installing a layer of insulation under the first floor. In homes with piping beneath the floor, insulation provides extra protection. In climates with freezing winters, pipes are prone to cracking and bursting if the water inside of them freezes. Since pipes often run near the floor, they are placed in areas that residents do not usually inspect. Sections of pipe could be located in incredibly cold areas near or under the floor, and the homeowner would never recognize the risk of pipes freezing.
In colder climates, another option if feasible , is to seal off the crawl space and insulate the exterior walls rather than the floor. While floor insulation is a great way to save money, it is by no means the first form of insulation you should look at around your home.
Loft insulation is cheap to buy, easy to install you can even do it yourself and can save you serious amounts of money over the years. Even if you already have loft insulation in place it may be worth checking whether you have the recommended levels installed. The recommended depth for blanket-style insulation is between and mm. If your loft insulation was installed some time ago there's a good chance it's less than that. And even though loft insulation is cheap to buy and install, it's also possible to receive grants towards your loft insulation.
While significantly more expensive to install than loft insulation, wall insulation could save you even more money over the long term. Wall insulation is typically divided into two types, depending on what type of home you have.
Cavity wall insulation, typically suitable for homes built after , consists of pumping insulation into the space between the outer and the inner walls. Solid wall insulation, for homes built earlier, insulates around the outer wall itself. If your home was built more recently still there's a good chance you will already have cavity wall insulation in place but now know about it.
The problem with this form of insulation is that you will usually need help getting it installed. For cavity wall for instance, holes have to be drilled into your wall through which the insulation material is pumped. Luckily there are plenty of energy efficiency grants that will help you pay for the costs of wall insulation.
If installing proper insulation sounds like too much work don't worry, there is a quick fix solution that could still save you money and energy and is cheap to buy and easy to do yourself. Draught proofing is all about securing the cracks and spaces around typically areas of heat loss around the home. The most common culprits are windows and doors and their frames , but you should also check for draughts around letterboxes and chimneys. Luckily it's also an easy problem to solve.
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