Fibre Glass Insulation

[Merchiston]

At the end of the 1970\\\'s fibre glass insulation - the type which comes in rolls - was used in the loft of the house of a relative. The house was built in the 1870\\\'s and the insulating material was placed hard against the sarking boards. I think this was done so that the loft space itself, used for storage, and floored in part, could enjoy the benfit of the insulation. The alternative would have been just to place the insulation material in rolls, in the spaces between the joists. This was also done, but it is the insulation placed against the sarking which is now being adversely commented upon.

Some slating work has been needed following water penetration being noted. The slater expressed the view that placing the fibre glass insulation up against the sarking would contribute to long term rot of the roof sarking. Any views? There is good ventilation in the roof space and, indeed, the fibre glass rather flaps about when the wind is strong. Despite this, I consider that it does contribute to making the property that bit warmer. I think that his views may have been prompted by finding some rotten sarking boards - but given the age of the property and the fact that it is still the original roof I think that this may have arisen for reasons not related to the presence of the insulating material.

 

[nece]

Current advice is to have 50mm space between the insulation and the roof so that air can move and prevent condensation and so rot.

The fibreglass rolls are not good for this as it is difficult to control the gap but you can get rockwool slabs or other solid insulation material that is cut to fit the gap then wedged between rafters.

Nigel

 

[masona]

Agreed,you should also have roof ridge vent at the top every approx 2m otherwise you get trap air at the top

 

[Giblets46]

Hi,
I know you require an air gap of 5cm, if I have rafters in my roof, can I have the wool touching the rafters, and then with the space between the rafters (with the felt) being 5cm, is this ok,or does it have to be 5cm away from the rafters too?
Cheers
Edward

 

[masona]

Where you see the blue arrow,that where the air-flow should be and the insulation touching on both side of the rafter.

 

[Giblets46]

thanks for that! So I can the 'wool tochingt he rafters as long as air can cirulate next to the felt.
Many thanks
Edward

 

[masona]

Yes,but make sure the airflow can circulate round to the other side of the roof,if not then put a roof ridge air-vent.

 

[Giblets46]

I was just making sure, as I have a sloped roof, and have put in an extra 17cm of insulation over the rafters (there was about 10cm before between the celing rafters). And near the edges they touch the roof rafters (as it slopes towards the edge), so it is only a small area where there is contact. But there is space for air to flow between.
Again, many thanks
Edward

 

[GwaiLo]

Giblets,
If I understand correctly what you are saying is that you have insulation between the top floor ceiling joists. You also have insulation between the rafters (the sloping timbers).

If that is the case you may indeed have a problem.

The key reason for having ventilation is to avoid condensation, which can cause roof timbers to rot. Condensation occurs when warm moist air from the house rises to the cooler roof space, cooler air being able to retain a smaller proportion of water vapour than warmer air.

In a typical house without a loft conversion that ventilation is provided at the eaves (the edge of the roof) and allows air to circulate throughout the roof space. In this case the appropriate place for the insulation is between the veiling joists. The air circulation in the roof space removes excess moisture and prevents condensation from occuring. This design ensures that not only the rafters, but all of the timbers in the cooler roof space are adequately ventilated.

When a loft is converted it is normal to heat the previously unheated roofspace and to provide insulation between the rafters. Because the space is heated condensation can no longer occur and so the heated space no longer requires ventilation to avoid condensation. In this design ventilaion is provided between rafters on the cool side of the insulant.

However, what you appear to be describing is neither one or the other, but a hybrid. You have a conventional roof space that is unheated, but insulation has been added between the rafters. Additional insulation has also been added to the joists.

The problem with that is that the space remains unheated, is therefore cooler than the rest of the house, and therefore condensation can occur. All well and good if the eaves ventilation is adequate and still ventilates the whole of the roof space, but I think it unlikely that is the case as this would defeat the purpose of insulating between the rafters.

If I am correct you run the risk of condensation occuring and rot to roof timbers. You either need to restore ventilation to the entire roof space (in which case you might just as well get rid of the insulation between the rafters as it will be serving no purpose) or you need to heat the roof space. You can heat the roof space either by removing the insulation from between the joists (in which case heat will be allowed to rise from the rooms below) or by installing a heater of some kind (probably a radiator assuming you have central heating.

In essence you should comply with part F2 of the Building Regulations. The approved document which provides a starting point can be found here

There is also an excellent and easy to understand guide published by the Building Research Establishment which you can buy here

If you can't sleep at night the approved document to Part L1 of the Building Regulations is also helpful, but a bit more difficult for a layman to decipher I should warn you. You can read that here

 

[Giblets46]

Sorry there, I am causing confusion due to my lack of knoledge between joists and rafters! Apologies for that.
I originally had some insulation between the ceiling joists (blown fibre), and have topped it up with a mineral wool roll. At the edge of the roof, Where the rafters slope down to meet the joists at the eaves, the mineral wool touches the rafters, (but leaves a gap between them), so that there is a gap of about 5cm between the wool, and the felt lining, hope this is a better description,
many thanks, and apologies for any confusion,
Edward

 

[masona]

What GwaiLo is saying ,you can either have the roof insulation or ceiling insulation but not both,that the way I understand it,I'm sure Gwailo will correct me if I'm wrong Also I have found a useful site with pictures for all difference type of insulation,see this, which I got this picture from.

Gwailo,
While we're on this subject I understand new building regulations come in April 2002 regarding the roof ceiling now required 250mm (10")insulation.
1,I would've thought that the ceiling joists need to breathe which now can't ?
2,I can't see a great deal of saving ????
3,Could cause problems storing items in the loft as you cannot walk on it anymore.

 

[Giblets46]

Thanks for that, the site (actually the manufacturers of the insulation I have!) explained in great details, and showed the advantages of each type, and "Maintain an airspace between insulation and sarking felt/sarking board at the eaves." rather than the rafters which is good!
Due to get cavity insulation next month too, so looking forward to be piping hot!

On to a new subject, if its OK.
Downstairs I have polished wooden floorboards on joists, is it possible to fill the joists these are on with wool, or do I need to leave them open for ventilation?
Many thanks
Edward

 

[masona]

I have polished wooden floorboards on joists, is it possible to fill the joists these are on with wool, or do I need to leave them open for ventilation?

No need to,you have ventilation in the room and via air-brick vents outside which should go thru' under the floorboards.

 

[GwaiLo]

While we're on this subject I understand new building regulations come in April 2002 regarding the roof ceiling now required 250mm (10")insulation.
1,I would've thought that the ceiling joists need to breathe which now can't ?
2,I can't see a great deal of saving ????
3,Could cause problems storing items in the loft as you cannot walk on it anymore.

I hate Part L questions because it is very complex.

Firstly the regulations are mainly aimed at new build. Items 1 and 2 are easily overcome with deeper joists or, as is becoming more common these days, the use of composite I beams.

Secondly, relevant Regulation L1a merely states that "Reasonable provision shall be made for the conservation of fuel and power in dwellings by; limiting heat loss through the fabric." In that sense the Regs have never actually changed since Part L was first introduced. What has changed over the years is what is considered "Reasonable" and that is where the approved documents come in.

There are three ways of demostrating compliance. The elemental method, the target U value method and the carbon index method. Between them these methods give plenty of scope for a variety of designs.

As far as existing dwellings go there is no expectation that they will have to be upgraded and you are correct in thinking that there is a diminishing return in adding ever better insulation. Nontheless these increased requirements are just the beginning if you read here you will see that the requirements will be changing again and are going to get progressively tougher and more complex.

In answer to questions 1 and 3, if you do do work that requires upgrading insulation in an existing roof space it is worth looking at the ranges of products available these days (I know most builders tend to think in terms of rockwool) as there are more efficient products that don't require the same depth of insulation. You should also remember that it is the U Value of the entire structure that matters, so improvements can be made elsewhere. Finally, I think the "builders" solution (the one that requires little in the way of design or calculation) will probably be to thicken the joists or to adopt alternative products.[/quote]

 

[diy60]

can somebody help me.
my dad is converting the loft, and wants to know how to properly support the joists.
if someone could provide some diagrams that would be extremely helpful

thanx
phwynski