Insulating rafters AND joists - good idea or not?

[kpainton]

Hi,

I'm new to this forum, and I didn't see this exact question asked in a search. If it's already been posted, please point me in the right direction. I've recently bought my first house, and the loft insulation is well below recommended depths. It's only about 1 inch thick! So I wish to get it properly insulated before this winter.

The building is built in the 70s and has a fairly low roof, and the joists and rafters are both 100mm deep. I would like to use the loft space for storage primarily - it will not be a livable area. I would like to install loft boards in the central area, where I'd like to be able to walk in order to store things up there.

The issue I have is that if I install lots of insulation at the floor level I will lose so much height in the loft that I won't be able to walk around in there, so I was wondering if it would be recommended to put insulation on both the joists and the rafters?

I was considering putting a vapour barrier and 100mm insulation between the joists, then laying 100mm planks of wood 90 degrees across the original joists at 400mm spacing with another 100mm depth of insulation between those new joists (giving 200mm total on the floor). I was then going to put down loft boards on top.

For the rafters I was going to attach 50mm battens to the existing rafters to increase the depth to 150mm. I was then going to put in 100mm depth of blanket insulation (leaving a gap of 50mm between insulation and felt that's already installed against the tiles). Then I was going to staple a vapour barrier to the battens and some thermal insulation foil roll to reflect any radiated heat back downwards.

My thoughts are that this will keep most heat in the house where it's needed most, any that does escape there will then be kept in the loft to help prevent anything stored up there from freezing.

So my questions are:

- Is this worthwhile insulating both the rafters and the joists?
- Am I likely to introduce issues with damp (or anything else) based on how I was planning to do it?
- Am I likely to have issues with the strength of the loft "floor" with the extra wood and loft panels I'm putting there (There is a supporting wall near the centre of the loft)

Many thanks for your help!

 

[mointainwalker]

If you are putting 200 mm on the ceiling there is no point in insulating the rafters/

You will be spending lots of cash with no benefit.

 

[kpainton]

Thanks very much for the reply mointainwalker. So is 200mm enough then? Current recomendation is 270mm (I think), so I assume that if I have less than that there must still be some heat escaping. Why would insulating the rafters not help with keeping that in? I'm confused!

Please forgive me if this is a dumb question, as I know very little about insulating houses! What I do know is from what I've read in a DIY book, and what I've seen on the Internet.

 

[freddiemercurystwin]

In order to prevent condensation and ultimately timber decay to the trusses/rafters etc lofts need to be ventilated. This means that the insulation bubble must be at ceiling level. So add a layer of insulation between your existing ceiling joists, add some 100mm cross battens perpendicular to those with another layer of insulation between those. Board on top of the cross battens. In an ideal world you would have 270mm overall. Don't store anything that is very susceptible to the cold in the loft and crouch a bit when you're up there. Simples! If you were really concerned about losing height you could use a rigid insulation between the existing ceiling joists and a thin layer of continuous rigid insulation above the top of the joists and then board on top.

If you were to add insulation at rafter level, to be effective, this would need to join the ceiling level insulation to form a bubble of insulation if you like. You would need to retain a route for ventilation from the eaves up to the ridge by adding continuous ridge ventilation or several tile vents near the ridge. This loft void would still be quite cold though as any heat from the house is largely being prevented by the insulation at ceiling level.

Anything contrary to this you have read in Books or on the web is incorrect.

Frankly it would be cheaper to rent a storage unit somewhere than adding rigid insulation/extra battens/ridge ventilation etc to the existing pitched roof if its that much of a problem.

 

[mointainwalker]

Example.

Let's say your house has a ceiling area of 35 m2 ( average newish 3 bed-house ), is heated @ 17C in the bedrooms, it's 0 C outside and maybe 5C in the loft.

The formula for heat loss is Watts per m2 per degree K ( = degree of temperature difference between two areas )

In the above case with 200 mm of fibre wool the result is as follows ( ignoring small differences like plasterboard):

0.2 ( heat loss in watts per m2 with 200 mm ) x 12 ( K = difference between 17 and 5 ) x 35 ( ceiling area in m2) = 84 W per hour

Your hourly heat loss is therefore the equivalent of an 80 W bulb ( remember those ). The heating effect of this bulb over a loft area of maybe 70 m3 ( guesstimate) is tiny. That is the same as trying to heat two good-sized lounges/sitting-rooms with the same bulb - not very much is it ?

So with so little heat in the loft it is in my view, extremely foolish to spend a lot of money to insulate the rafters to save a very small percentage of the heat generated by a light-bulb because you will only save max 5 %, but that is only a wild, unsubstantiated guess on my part.

Rafter insulation after ceiling insulation therefore wildly uneconomic also giving no benefit.

The above formula shows you the heat loss for 200 mm so this is it for 270 mm below.


0.15 ( heat loss in watts with 270 mm ) x 12 ( difference between 17 and 5 ) x 35 ( ceiling area ) = 62 W per hour

Saving 20 W/hr , maybe 1600 W ( cost 16 p max )per week for a working family.

Balancing extra cost vs loss of easy usage without expensive modification to loft joists gives me a clear benefit for using the 200 mm.

On reflection this is rather heavy on numbers but I have tried to sum up and explain each conclusion to make it more digestible. If anything still incomprehensible, just ask

MW

Edit

I was only trying to address the thermal losses. Everything Freddy has written is completely valid and gives additional reasons not to double-insulate

 

[kpainton]

Thanks very much for the replies Mointainwalker and freddymercurystwin. I really appreciate you both taking the time. I understand what you have both said, and I've decided that I will just stick to 200mm on the floor and board on top (I guess the loft panels will also be adding some inulsation as well). I'll probably lay 300mm near the edges where the roof is so low that I won't be storing stuff there.

 

[john4703]

I found this when searching for loft insulation.
I have about 125mm of fibreglass insulation between the rafters and then chipboard over that. I am thinking of putting something like this http://www.screwfix.com/p/airtec-double-insulation-1-05-x-25m/21112#rating_link_anchor
Airtec Double Insulation stapled to the underside of the roof timbers. The roof is slate, nailed to sarking boards with no felt and the amount of dirt that covers everything stored in the loft is my main concern. Are there any problems with putting this type of insulation under the roof rafters? Is there a better way to stop all the dust and dirt coming into the loft?

 

[SimonH2]

... 84 W per hour

... 62 W per hour

Saving 20 W/hr ...

Call me pedantic if you like, but watts are a measure of power (energy/time). Therefore what you should have written is just "W".

It's a common mistake and a common source of confusion.

 

[mointainwalker]

I also think that language is valuable and should be used with precision , however in this case I do not know if I agree with you as I am not deeply involved with electrical engineering.

Perhaps I should point out though that the formula for U-values gives heat loss - and therefore W - at an hourly rate.

 

[freddiemercurystwin]

mointainwalker, glad to see you're still kicking around, how's that old barn of yours?

 

[SimonH2]

I also think that language is valuable and should be used with precision , however in this case I do not know if I agree with you as I am not deeply involved with electrical engineering.

One doesn't need to be involved with electrical engineering. A watt is a unit of power, not just electrical power but any power. This isn't just about correct language, it is about correctly passing technical information - using the correct units is an important part of that.

Perhaps I should point out though that the formula for U-values gives heat loss - and therefore W - at an hourly rate.

Got a reference ? Normal U values will be (in SI units) W/(m^2 K) so by the time you've multiplied by temperature difference and area, K and m^2 disappear leaving just W.


But lets not get into too much of an argument Like I say, it's a common source of confusion and one of those things that I both feel strongly about, and find irritating when I see it.